Predation by Gambusia holbrooki - The Plague Minnow

Approved NSW Threat Abatement Plan 

Predation by Gambusia holbrooki 

- The Plague Minnow 

August 2003 

N S W 

NATIONAL 

PARKS AND 

WILDLIFE 

SERVICE

© NSW National Parks and Wildlife Service, 2003. 

This work is copyright. However, material presented in this plan may be copied for personal 

use or published for educational purposes, providing that any extracts are fully acknowledged. 

Apart from any use as permitted under the Copyright Act 1968, no part may be reproduced 

without prior written permission from NPWS. 

NSW National Parks and Wildlife Service 

43 Bridge Street 

(PO Box 1967) 

Hurstville NSW 2220 

Tel: (02) 95856444 

www.nationalparks.nsw.gov.au 

For further information contact 

Biodiversity Management Unit 

Biodiversity Research and Management Division 

NSW National Parks and Wildlife Service 

PO Box 1967 

Hurstville NSW 2220 

Tel: (02) 9585-6426 

Email 

Cover illustrations: Gambusia holbrooki – The plague minnow 

Illustrator: Judy Denby 

This plan should be cited as follows: 

NSW National Parks and Wildlife Service (2003). NSW Threat Abatement Plan. Predation by 

Gambusia holbrooki – The Plague Minnow. NPWS. Hurstville, NSW. 

ISBN 0 7313 6671 9

Threat Abatement Plan Predation by Gambusia holbrooki 

Executive Summary 

This document constitutes the NSW National Parks and Wildlife Service (NPWS), threat 

abatement plan for the listed key threatening process Predation by Gambusia holbrooki – the 

Plague Minnow, and as such considers the known impacts and management actions necessary 

to abate this threat on native fauna, in particular threatened frogs. 

Predation by Gambusia holbrooki, hereafter referred to as gambusia, was listed in January 

1999 as a key threatening process on Schedule 3 of the NSW Threatened Species 

Conservation Act 1995 (TSC Act). The NSW Scientific Committee determined that predation 

by gambusia is a serious threat to the survival of threatened species such as the green and 

golden bell frog (Litoria aurea) and New England bell frog (Litoria castanea) and could 

cause other native frog species to become threatened. The NPWS is required to prepare a 

Threat Abatement Plan (TAP) to manage this key threatening process, so as to abate, 

ameliorate or eliminate the adverse impacts of gambusia predation. 

Since their introduction into Australia in 1925 for the purpose of mosquito control, gambusia 

have become widespread in NSW, especially modified waterways, and are considered to be a 

contributing factor to the decline of frogs (threatened or otherwise) as well as other native 

species such as freshwater fishes and macro-invertebrates. 

This threat abatement plan provides a strategy for the management of gambusia in NSW. 

Given the widespread distribution of gambusia and the difficulties posed by removing the 

species from the environment, this plan identifies those frog species considered most at risk 

from predation by gambusia in order to make the most effective use of management 

resources. 

The plan seeks to minimise ongoing human dispersal of gambusia through a program of 

education and awareness of the risks associated with introducing the species into the 

environment, particularly habitats of key threatened frog species. 

In addition, the plan seeks to reduce the impacts of gambusia at sites where control is most 

critical. The plan proposes to achieve this by undertaking a program of gambusia control at 

key habitats for high priority threatened frog species. At sites where gambusia removal is not 

considered feasible, opportunities for the creation of gambusia-free supplementary habitat 

will be evaluated. Sites will be monitored on an ongoing basis to assess the effectiveness of 

the gambusia control program. 

A number of research actions are recommended in order to clarify aspects of the ecology of 

gambusia and its impacts on frog species. Additional information is also required on the 

efficacy of proposed control methods and their impact on non-target species. Outcomes from 

this research will assist in the future management of gambusia. 

i

Predation by Gambusia holbrooki Threat Abatement Plan 

Although this threat abatement plan targets the impact of gambusia on threatened frogs. It 

also documents the potential effects of this on non-threatened frog species, freshwater fishes 

and other aquatic organisms such as macro-invertebrates. The plan recognises that effective 

long-term control of gambusia across the landscape will only be achieved in partnership with 

programs that endeavour to restore aquatic ecosystems. This plan therefore links with other 

broad-scale water reform processes that seek to address aspects of habitat modification 

favoured by gambusia. 

The NPWS will coordinate the implementation of this plan over a five-year period. 

Brian Gilligan Bob Debus 

Director-General Minister for the Environment 

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Acknowledgements 

This threat abatement plan has been prepared by Ron Haering (NPWS). Background material 

(sections 1 to 8 of this plan) has been sourced from a review of the literature prepared by the 

Arthur Rylah Institute for Environmental Research under contract to the NPWS (McKay et al. 

2001). 

Ross Wellington, Nick Sheppard, (both NPWS) John Pursey, David Pollard and Jamie Knight 

(NSW Fisheries), Roger Dekeyzer (Environment Protection Authority), Michael Mahony 

(University of Newcastle) and Tony Miskiewiez (Wollongong City Council) assisted with the 

preparation of this plan. Thanks to Angela Arthington (Griffith University), Jack Baker, 

Andrew Leys, Rodney James, Melanie Bannerman, Paul Downey, Paul Mahon, and Joanne 

Edney (all NPWS) and Marion Anstis for their comment and/or editorial input. 

Special thanks are due to Michael Mahony and Marion Anstis whose expertise enabled the 

preparation of Appendix 3. 

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TABLE OF CONTENTS 

1. INTRODUCTION .........................................................................................................................1 

2. LEGISLATIVE FRAMEWORK .................................................................................................1 

2.1 COMMONWEALTH LEGISLATION ..................................................................................................1 

2.2 NSW LEGISLATION......................................................................................................................2 

3. INDUSTRY FRAMEWORK........................................................................................................6 

4. DESCRIPTION..............................................................................................................................6 

4.1 TAXONOMY AND MORPHOLOGY ..................................................................................................6 

4.2 DISTINGUISHING CHARACTERISTICS ............................................................................................7 

5. HISTORY OF INTRODUCTION................................................................................................9 

5.1 SPECIES ORIGIN AND ENTRY INTO AUSTRALIA ............................................................................9 

5.2 MOSQUITO CONTROL – SUCCESS OR FAILURE? ..........................................................................9 

5.3 DISPERSAL OF GAMBUSIA ...........................................................................................................10 

6. BIOLOGY AND ECOLOGY OF GAMBUSIA.........................................................................11 

6.1 DISTRIBUTION ............................................................................................................................11 

6.1.1 Factors influencing distribution .......................................................................................12 

6.2 HABITAT PREFERENCES .............................................................................................................13 

6.2.1 Use of modified habitats by Gambusia .............................................................................14 

6.3 BREEDING BIOLOGY, SOCIAL ORGANISATION AND DISPERSAL IN NATURE ...............................15 

Social organisation including behavioural characteristics ...........................................................16 

6.4 DIET AND FACTORS INFLUENCING DIETARY PREFERENCES .......................................................16 

6.5 KNOWN AND POTENTIAL DISEASES, PREDATORS AND COMPETITORS........................................16 

7. IMPACTS OF GAMBUSIA ON NATIVE PLANTS AND ANIMALS ...................................17 

7.1 IMPACTS ON NATIVE VEGETATION AND RIVER HEALTH............................................................17 

7.2 IMPACTS ON MACRO-INVERTEBRATES .......................................................................................18 

7.3 IMPACTS ON NATIVE FISH ..........................................................................................................18 

7.4 IMPACTS ON NATIVE FROGS.......................................................................................................21 

7.5 BENEFITS TO NATIVE PLANTS AND ANIMALS.............................................................................27 

8. CONTROL OF GAMBUSIA......................................................................................................27 

9. PROPOSED MANAGEMENT OF GAMBUSIA ......................................................................29 

9.1 INTRODUCTION...........................................................................................................................29 

9.2 THREAT ABATEMENT ACTIONS..................................................................................................29 

10. ECONOMIC AND SOCIAL IMPACTS OF THE PLAN ...................................................36 

11. REVIEW DATE ......................................................................................................................36 

12. REFERENCES AND PERSONAL COMMUNICATIONS ................................................37 

v


APPENDIX 1: NSW SCIENTIFIC COMMITTEE FINAL DETERMINATION .........................50 

APPENDIX 2: NSW RIVERS SURVEY RECORDS OF GAMBUSIA...........................................51 

APPENDIX 3: DEVELOPMENT OF A RANK SCORING SYSTEM TO PREDICT 

GAMBUSIA IMPACT ON NATIVE FROG SPECIES....................................................................53 

APPENDIX 4: THREAT ABATEMENT PLAN COST TABLE.....................................................62 

vi


Introduction 

Gambusia has been colloquially described as the ‘animal weed’ of our aquatic environment, because of its 

ability to rapidly reproduce, disperse widely and occupy diverse habitats, to the detriment of native 

species. This small, introduced fish is also highly aggressive and predatory. 

Originally introduced in 1925 from the USA into the Royal Botanic Gardens Sydney, for the purpose of 

mosquito control. Gambusia are now common and widespread, occurring in most freshwater habitats in 

south-eastern Australia, as well as the coastal drainages of Queensland, and some parts of the Northern 

Territory and Western Australia. It has been an extremely successful invader assisted by human dispersal 

and facilitated by its high reproductive potential, fast maturation rate, flexible behaviour and broad 

environmental tolerances. In five months, a population of gambusia can increase to over 100,000 fish 

after natural mortalities. 

The effectiveness of gambusia to control mosquitoes has generally failed internationally and the World 

Health Organisation no longer recommends its use for malaria control programs primarily due to its 

harmful impact on native fish. What remains however, is the legacy of another introduced species 

establishing itself in Australia and impacting on native species including frogs, fish and macroinvertebrates. 

Predation by gambusia is now listed as a key threatening process in NSW. 

There are presently no effective and specific methods to control gambusia. Once introduced, it is almost 

impossible to eradicate from the environment, particularly from connected waterways such as creeks, 

rivers and streams, and large permanent water bodies. A number of physical, chemical and biological 

approaches have been trialled with varying degrees of success and inherent risks. An integrated targeted 

strategy is proposed in this plan, which combines public education, gambusia control (where feasible) and 

ecological rehabilitation. Actions identified in the plan are targeted predominantly towards ameliorating 

the impacts of gambusia on frogs, particularly threatened frog species. 

1. Legislative Framework 

2.1 Commonwealth Legislation 

Wildlife Protection (Regulation of Exports and Imports) Act 1982 

Regulations concerning the import of exotic species and export of native species are provided under the 

Wildlife Protection (Regulation of Exports and Imports) Act 1982. Schedule 6 of the Act includes a list of 

fish, which can be imported into Australia. Gambusia are not included on this list. Persons wishing to 

import gambusia would need to have the species approved for inclusion on this Schedule. 

Given the widespread availability of the species in Australian waterways and its lack of value as an 

ornamental fish (see section 3), it is unlikely that gambusia would knowingly be imported into Australia. 

Environment Protection and Biodiversity Conservation Act 1999 

The impacts of gambusia are not listed as a key threatening process under this Act. 

1


2.2 NSW Legislation 

Threatened Species Conservation Act 1995 

The NSW Threatened Species Conservation Act 1995 (TSC Act) aims to conserve biological diversity, 

prevent extinction and promote the recovery of listed species, populations and ecological communities. 

The ultimate goal of the TSC Act is to recover threatened species, populations and ecological 

communities, so that their long-term survival in nature can be assured. This involves eliminating or 

managing processes that threaten the survival or evolutionary development of such species. The key 

mechanisms provided in the TSC Act to achieve this goal are the preparation and implementation of 

recovery plans and threat abatement plans. 

The TSC Act provides for the listing of key threatening processes. A threatening process is eligible to be 

listed if, in the opinion of the Scientific Committee it: 

a) adversely affects two or more threatened species, populations or ecological communities, or 

b) could cause species, populations or ecological communities that are not threatened to become 

threatened 

Predation by Gambusia holbrooki the Plague Minnow was gazetted as a key threatening process under the 

TSC Act in January 1999. The final determination of the NSW Scientific Committee is provided in 

Appendix 1. The rationale for this determination is based on the view that predation by gambusia is a 

serious threat to the survival of threatened species such as the green and golden bell frog (L. aurea) and 

New England bell frog (L. castanea) and other native frog species. The NPWS is therefore required to 

prepare a TAP to manage this key threatening process, so as to abate, ameliorate or eliminate the adverse 

impacts of gambusia predation on threatened species. 

This plan constitutes the final approved TAP for this listed key threatening process. The TSC Act requires 

Ministers and public authorities to take any appropriate action available to them to implement the 

measures included in the plan for which they are responsible. Furthermore, they must not make decisions 

that are inconsistent with the provisions of the plan. A public authority identified in a plan as responsible 

for the implementation of particular measures must report to Parliament on actions taken to implement 

those measures. The NPWS has been identified as the implementation authority for this TAP. 

The TSC Act requires the final approved recovery plan to include a summary of advice given in the NSW 

Scientific Committee submission to the draft plan and a reason for any departure from that advice. In 

regard to this plan, the NSW Scientific Committee commented that overall, the plan is logical and well 

focused. The Scientific Committee also noted that the frog ranking system provided in Appendix 3, 

should be periodically updated as ongoing research clarifies the uncertainty about which native frog 

species are most at risk from predation by gambusia. 

Fisheries Management Act 1994 

The Fisheries Management Act 1994 (FM Act) aims to conserve, develop and share the fishery resources 

of NSW for the benefit of present and future generations. The objectives of this Act relate to conserving 

fish stocks and protecting key habitats, conserving threatened species, populations and communities, 

2


promoting ecologically sustainable development, promoting viable commercial fishing and aquaculture 

industries, promoting quality recreational fishing opportunities and sharing fisheries resources between 

users. 

NSW Fisheries are responsible for the management of fish resources in NSW and has management 

responsibility for all aquatic animals (with the exception of aquatic mammals, birds, reptiles and 

amphibians which are managed by NSW NPWS) including fish and their habitat in all waters of the State 

(including private and public waters and all permanent and intermittent waters). Through amendments to 

the FM Act, NSW Fisheries are responsible for threatened fish species, populations and ecological 

communities and for the conservation of biodiversity of all fish and marine vegetation. 

Like the TSC Act, the FM Act includes provision for the listing of threatened species, populations and 

ecological communities and key threatening processes and includes provisions for the preparation of 

recovery plans and threat abatement plans. The NSW Fisheries Scientific Committee has made a final 

recommendation to list the “Introduction of fish to fresh waters within a river catchment outside their 

natural range” as a key threatening process. This listing cites the impacts of gambusia as part of this 

threatening process. 

Noxious fish and noxious marine vegetation 

The FM Act, Part 7, Division 6 (Sections 209 to 213) provides for the declaration of different categories 

of noxious fish (which represent the different levels of threat they pose to the aquatic environment) and 

includes penalties for their sale and possession. It is a defence against prosecution under Section 211 if a 

person neither introduced nor maintained a noxious fish in those waters. Conditions may be included in 

aquaculture permits for the destruction or control of noxious fish. Section 213 enables the destruction of 

noxious fish. 

Gambusia is not currently declared a noxious fish in NSW. 

Clarke et al. (2000) reviewed the status of gambusia in other Australian states. It is classified as a pest in 

Queensland under the Fisheries Act 1994, where the species may not be released into waters or held in 

captivity. In Western Australia gambusia must not be returned to the water. In Victoria, it is listed as a 

noxious species under the Fisheries Act 1995. 

Release or importation of fish 

Part 7, Division 7 (Section 216) of the FM Act prohibits the release of any live fish (except under a 

permit) into any waters. Section 217 prohibits any person bringing live fish into NSW not taken from 

NSW waters except under permit. A person who sells or buys or has possession of a fish knowing it has 

been brought into the State is guilty of an offence. 

This section of the FM Act applies only to the release of fish into the sea, river, creek or other naturally 

flowing stream or water or into a lake. This excludes other waterbodies such as farm dams, outdoor ponds 

or other forms of aquaria. 

3


NSW Fisheries – Introduction and Translocation Policy 

This policy provides background on the introduction of native and non-native fish species, translocations, 

aquaculture, impact of introductions/translocations and legislation. The policy states: 

7.1 All stockings of fish into NSW waters require a permit from NSW Fisheries. 

7.2 NSW Fisheries will not permit any further introductions or translocations of native or nonnative 

species into NSW waters, except as permitted elsewhere in this policy. 

NSW Fisheries distinguishes between exotic, alien, introduced and translocated species. Gambusia are 

classified as alien, ie a species which was brought into Australia from a foreign country and has 

established wild populations. This policy notes that species such as gambusia have been introduced into 

the State either accidentally or deliberately. 

Environmental Planning and Assessment Act 1979 

Land use within NSW is primarily regulated by the Environmental Planning and Assessment Act 1979 

(EP&A Act). The EP&A Act seeks to encourage, inter alia, ecologically sustainable development by 

managing the development process and the effects of development on the environment. 

When evaluating a proposed development or activity, consideration by a consent or determining authority 

should be given to the potential of that proposed development or activity, resulting in the introduction of 

gambusia into the natural environment, including its effects on threatened species (Section 79C and 

Section 111 of the EP&A Act). 

In addition, Section 5A of the EP&A Act sets out eight factors to be considered when deciding whether 

there is likely to be a significant effect on threatened species, populations and ecological communities and 

hence, if a Species Impact Statement (SIS) is required. Part (g) of this ‘eight part test’ includes the 

following factor for consideration - whether the development or activity proposed is of a class of 

development or activity that is recognised as a threatening process. This part would be relevant if, for 

example, a proposed development was likely to result in the introduction of gambusia into an area. 

It is a requirement of the NPWS that all proposed activities (including pest control) on NPWS land are 

assessed under Part 5 of the EP&A Act. This involves an examination of whether the activity is likely to 

significantly affect the environment, including threatened species, populations and ecological 

communities and their habitats. The mechanism to undertake this assessment is generally regarded as a 

Review of Environmental Factors. Where a significant effect is likely, the EP&A Act requires the 

preparation of an environmental impact statement, and in the case of a significant effect on threatened 

species, populations or ecological communities, a SIS. 

Pesticides Act 1999 

The Pesticides Act regulates and controls the use of pesticides within NSW. Under the Act, it is illegal to 

possess, prepare for use, or use a pesticide in NSW unless it is registered by the National Registration 

Authority for agricultural and veterinary chemicals (NRA) or covered by an NRA permit issued under the 

Commonwealth Agricultural and Veterinary Chemical Code Act 1994. The Pesticides Act requires strict 

adherence to label instructions, as set out by the NRA, when using a registered pesticide. 

4


The Pesticides Act also makes it an offence to use a pesticide in a way that harms any non-target animal 

or plant. A defence against prosecution is provided where a person takes all reasonable precautions and 

exercises all due diligence when using the pesticide and the offence was due to causes beyond the 

person’s control. 

Rotenone for example, is a registered pesticide commonly used in various formulations such as ‘Derris 

Dust’ for agricultural pest control purposes. Rotenone is also an effective broad- spectrum piscicide that 

is toxic to most fish and has been used to kill pest fish species such as carp and gambusia (Hall 1988; 

Sanger and Koehn 1997; Koehn et al 2000; Willis and Ling 2000). Its application has generally been 

limited to small closed water bodies such as ponds or farm dams. 

Rotenone is not currently registered as a piscicide in Australia. The application of Rotenone to remove 

gambusia would therefore require its registration as a piscicide with the NRA and approval for its use 

from the NSW Environment Protection Authority and NSW Fisheries. Before registering any product, the 

NRA is required to conduct a rigorous assessment of its potential impacts on the environment, human 

health and trade and of its likely effectiveness for its proposed uses. Alternatively, the NRA can consider 

issuing either an ‘off-label’ permit for unregistered/registered products for minor or emergency uses, or a 

trial permit for research purposes to determine its efficacy data and assess non target impacts. 

Water Management Act 2000 

The Water Management Act 2000 (WM Act) is the principal piece of legislation controlling water 

management across NSW and is administered by the NSW Department of Land and Water Conservation 

(DLWC). The WM Act provides for the development of water sharing plans and water management plans 

by community based Water Management Committees (which includes NPWS representation). The WM 

Act may allocate the volume of water to be used for various purposes, including irrigation and 

environmental flows, identify the timing of water extraction for various purposes and discuss the natural 

flow regimes of a catchment or subcatchment. 

Water plans must be consistent with government advice and policy, including the Interim State Water 

Management Operating Plan, which sets the overarching policy context, targets and strategic outcomes 

for the States’s water resources. 

Catchment Management Amendment Bill 2001 (not yet passed) and the Catchment Management 

Act 1989 

The amendments to the Catchment Management Act 1989 (CM Act) will provide that certain plans 

prepared under the WM Act must be consistent with any relevant catchment management plan. The 

Catchment Management Plans (also called "blueprints") are expected to drive regional investment 

priorities under the National Action Plan for Salinity and Water Quality and the Natural Heritage Trust 

Mark 2. The establishment of Catchment Management Boards on which NPWS is represented will drive 

the development of these catchment management plans. 

NSW Weirs Policy (1995) 

The NSW Weirs Policy is a component of the State Rivers and Estuaries Policy (1991) being 

implemented by DLWC. The goal of the State Weirs Policy is to halt and where possible reduce and 

5


remediate the environmental impacts of weirs. The review of weirs will assist with the development of 

operational and structural changes needed to achieve river flow and water quality objectives. 

NSW Wetlands Management Policy (1996) 

The NSW Wetlands Management Policy adopts nine principles that aim to minimise any further loss or 

degradation of wetlands and, where possible, restore degraded wetlands. Principle six states that “Natural 

wetlands should not be destroyed, but when social or economic imperatives require it, the rehabilitation or 

construction of a wetland should be required”. This policy is consistent with the objectives of the WM 

Act. This policy is also being implemented by DLWC. 

3. Industry Framework 

Gambusia are unlikely to play an important role in the aquarium industry and due, to its ubiquitous 

distribution is unlikely to be imported specifically as an ornamental fish for the aquarium trade. In NSW, 

aquarium retail outlets are currently permitted to trade in gambusia supplied from local sources. It is 

principally traded as a fish to be fed to other aquarium fish, being readily available from local sources 

(Jared Patrick pers. comm.). The number of gambusia traded in this manner is difficult to estimate. As 

they are sourced locally and are relatively inexpensive, it is unlikely that any significant numbers are sold. 

4. Description 

4.1 Taxonomy and Morphology 

Gambusia holbrooki (Girard 1859) 

Order: Cyprinodontiformes 

Family: Poeciliidae 

Common name: Eastern Gambusia, Mosquito Fish, Plague Minnow 

The history of introduction(s) and subsequent spread of gambusia are poorly documented and confusion 

exists regarding which subspecies is actually present in Australia (Lloyd and Tomasov 1985). There is 

confusion about the correct species name in Australia (Howe 1995). The following summary of taxonomy 

should clarify this. 

There are approximately 30 species within the genus gambusia most of which are rare and restricted in 

range (Rivas 1963; Rosen and Bailey 1963). Prior to studies by Lloyd and Tomasov (1985), authors 

named the Australian introduction as Gambusia affinis, which is originally distributed from across southeastern 

USA to Texas (Lloyd and Tomasov 1985). Two subspecies of G. affinis were subsequently 

identified in south-eastern USA: the eastern form G. affinis holbrooki (Krumholz 1948) and the western 

form, G. affinis affinis (Baird and Girard 1853). Lloyd and Tomasov (1985) confirmed Wilson’s (1960) 

interpretation that the subspecies G. a. holbrooki is the taxon introduced into Australia. Wooten et al. 

(1988) later reinstated the original species G. holbrooki (Girard 1859) and G. affinis (Baird and Girard 

1853) into two species. The species found in Australia is presently named G. holbrooki (Lloyd and 

Tomasov 1985; Arthington et al. 1999). 

6


In about 1905, “mosquitofish” was adopted as the common name for gambusia (Lloyd 1990a), replacing 

the previous (American) name “topminnow” (Krumholz 1948; Lloyd 1990). The name mosquitofish 

implied that gambusia was as an effective mosquito control agent, making it the “logical” choice for the 

solution to mosquito control problems without first considering the use of native fish as potential control 

agents (Lloyd 1990a). Gambusia is now commonly referred to as the plague minnow, in light of its 

proliferation in Australian waters. 

The following description of the morphology of gambusia is adapted from McDowall (1996) and 

Cadwallader and Backhouse (1983): 

Body: A tiny, stout fish with a deep rounded belly and a flattened upper surface, especially the head 

Eyes: large, positioned near dorsal profile 

Mouth: small, upturned and protrusible, lower jaw a little longer than upper. Bands of minute teeth on 

both jaws 

Scales: head and body covered with large cycloid scales (28-32, usually 30-31 laterally) 

Dorsal fin: single, soft rayed (6-8 rays, usually 7), short based, high, rounded, and situated posteriorly 

Anal fin: (9-11 rays, usually 10) rounded or elongate, pointed 

Pectoral fins: short, rounded, positioned high on sides near top of gill openings 

Pelvic fins: abdominal, tiny rounded; bases close together. Caudal fin large, rounded 

Lateral Line: no lateral line 

Vertebrae: 31-33 

Gill rakers: 13-15 stout gill rakers of moderate length 

Gambusia is sexually dimorphic, with females much larger bodied than males (maximum standard 

lengths of 35 mm and 60 mm respectively) (McDowall 1996; Cadwallader and Backhouse 1983). Males 

cease growing when they reach maturity, but females continue to grow until they die (Cadwallader and 

Backhouse 1983; Vargas and de Sostoa 1996). 

Gambusia are generally green olive to brown on the back, the sides are grey with a bluish sheen, and the 

belly is silvery-white (Cadwallader and Backhouse 1983; McDowall 1996). The lower jaw is steel blue 

and often has a dark, diagonal stripe below the eye. Fins are colourless, except for the dorsal and caudal 

fins, which may bear numerous fine black spots, sometimes forming indistinct rows. Individuals of some 

populations of gambusia have brownish-black spots on the sides (Cadwallader and Backhouse 1983). 

Females have a distinct black blotch surrounded by a golden patch just above the vent. Males have a 

highly modified anal fin, the third, fourth and fifth rays of which are elongated and thickened with very 

small hooks at the tip. These form the gonopodium or intromittent organ, used to facilitate internal 

fertilisation of eggs in the female (Cadwallader and Backhouse 1983; McDowall 1996). 

4.2 Distinguishing Characteristics 

There are no other species of gambusia present in NSW. However, there are several native and introduced 

species, which may potentially be confused with gambusia (Table 1). 

7


Table 1. Fish species occurring in NSW, which could potentially be confused with gambusia 

Common name and scientific name Distinguishing features Distribution Habitat conditions 

Darling River Hardyhead 

Protrusible mouth and thin lips. Scales almost Upper tributaries of Darling Gently flowing, shallow, clear water 

Craterocephalus amniculus 

circular and barely overlapping. 

River 

or in aquatic vegetation at the edges of 

(Small individuals can be confused with gambusia) 

such waters. 

Murray Hardyhead 

Mouth restricted by a labial ligament from 1/3 too Once abundant in southern A highly mobile schooling fish that 

Craterocephalus fluviatilis 

halfway along the thin lips. Scales on top of head waters of inland NSW. No often found over very sandy shallow 

robust and large, with a single large interorbital records in the past 10 years flats. 


scale reaching as far as the anterior margin. 

Marjorie’s Hardyhead 

Craterocephalus marjoriae 


Flyspecked Hardyhead 

Craterocephalus stercusmuscarum fulvus 


Pacific blue-eye 

Pseudomugil signifer 


Guppy 

Poecilia reticulata* 


Swordtail 

Xiphophorus helleri* 


Platy 

Xiphophorus maculatus* 

(Small females may be confused with female gambusia) 

8 

Head usually blunt, slightly flattened and sloping 

towards snout. Mouth protrusible, with small, 

sharp, inwardly pointing teeth 

Small fish, more slender than most hardyheads; 

head in larger specimens tending to slope 

downwards toward snout. Lips moderately thick; a 

small protrusible mouth. 

Small fish, semi-transparent body which can vary 

in colour from pale olive, yellow and blueish. The 

iris of the pacific blue eye is blue. There is often a 

line of pearly spots along the side of the body. 

Males can grow to 88 mm, females to 63 mm in 

length. 

Single dorsal fin a little behind middle of body. 

Males brightly coloured with irregular markings of 

green, turquoise, blue, red, orange and yellow. 

Modest size, dorsal fin high on arching back. Tail 

truncated, lower margin elongated to form a long 

sword in male. Aquarium fish are bright orange on 

body and fins; wild populations olive brown with 

orange-red midlateral stripe. 

Very deep bodied and much compressed, with 

dorsal fin high on arching back. 

Clarence River in northeastern 

NSW 

Previously present in most 

parts of the Murray-Darling 

drainage system in NSW 

East coast of Australia, from 

Cooktown in Queensland to 

Narooma in NSW. 

Occurring in coastal 

drainages of northern NSW 





(Ivantsoff and Crowley 1996; Arthington and McKenzie 1997; Allen 1989; McDowall 1996; Australian Museum 2002). * Introduced species. 

Often found in large schools in 

shallow water with a gravelly or 

sandy bottom though also frequents 

weedy sections of streams. 

Usually schools in still or gently 

flowing water over sand, gravel or 

mud. 

Prefers clear fast flowing streams and 

mangrove regions of estuaries. 

Still or gently flowing waters. Around 

margins and edges of aquatic 

vegetation. Prefers water above 15C. 

Gently flowing streams with sparse 

vegetation over gravelly substrates. 

Prefers still warm water above 20C.


5. History of Introduction 

5.1 Species Origin and Entry into Australia 

Gambusia are native to southern USA and Northern Mexico (Lloyd and Tomasov 1985; 

Clarke et al. 2000). The native range of gambusia is the area from central Alabama, east into 

Florida and throughout the Atlantic coastal drainages northwards to New Jersey (Rivas 1963; 

Wooten et al. 1988). 

Gambusia was introduced into Australia in 1925 from Georgia (USA) (Wilson 1960; Myers 

1965; Bayly and Williams 1973; McKay 1984), to control mosquitoes, and was first released 

into the Botanical Gardens in Sydney (Bayly and Williams 1973; McKay 1984). In 1926, the 

Chief Health Inspector of the City of Sydney established wild populations from specimens 

imported from another introduced location, Italy (Wilson 1960; Clarke et al. 2000). Gambusia 

was introduced to other parts of NSW from 1927 onwards until World War II, when it was 

widely established in the state. Until about 1930, city and Municipal Councils distributed fish 

in the Newcastle and northern coastal regions (Wilson 1960). In 1940, gambusia were flown 

to Darwin. During World War II they were spread through military camps in many parts of 

Australia (Myers 1965; Boulton and Brock 1999). Gambusia are reported to have been 

released into some parts of NSW for the purpose of mosquito control, eg in the Illawarra and 

Central Coast areas as late as the 1960s (Ross Wellington pers. comm.). 

5.2 Mosquito Control – Success or Failure? 

During the early 1900s, after it was discovered that mosquitoes transmit both malaria and the 

deadly yellow fever, public health officials and doctors worldwide began to show an interest 

in reducing or eradicating those diseases by attacking mosquito at their larval stages (Myers 

1965; Boulton and Brock 1999). Many attempts have been made to reduce the problems 

caused by mosquitoes, with many mosquito control options suggested, including physical and 

chemical methods. The search for a natural control method for mosquitoes led to the concept 

of biological control (Lloyd 1990a). 

Gambusia was first used in 1905 as a mosquito control agent, when specimens from Texas 

were released in Hawaii (Krumholz 1948; Wilson 1960). Public health authorities were 

delighted by the hardiness of the so-called ‘mosquitofish’ and the ease with which it spread 

(Boulton and Brock 1999). American research on mosquito-destroying fishes was thus 

concentrated mostly on the mosquitofish, which gradually became known throughout the 

world as THE fish to introduce in the fight against mosquito-transmitted diseases (Myers 

1965). 

Wilson (1960) emphasises the general opinion of many people at the time in Australia that 

gambusia was successful in controlling mosquitoes. He stated that gambusia was of distinct 

value as a mosquito control agent, which exerts good control of mosquitoes in permanent 

pond habitats. However, Wilson (1960) also considered that the use of gambusia in Newcastle 

and the north coast of NSW was unsuccessful. 

9


Since 1982, the World Health Organisation (WHO) has no longer recommended the use of 

gambusia for malaria control programs and indicates that it should not be introduced into new 

areas, primarily because of its apparent harmful impact on native fish species (Legner 1996). 

Views on the effectiveness of gambusia as a mosquito control agent vary. Lake (1971), for 

instance, stated "I believe their effect on mosquitoes has been negligible". Grant (1978) noted 

that it was arguable whether gambusia offers better mosquito control than some native fish. 

Studies in Australia indicate that gambusia is not an effective mosquito predator, with 

mosquitoes only making up a small part of its diet (Lloyd 1984; Lloyd 1986). In another study 

in the lower River Murray, only 10% of the diet of gambusia consisted of mosquito larvae, 

whereas four endemic fish species consumed more mosquitoes (Lloyd 1986; Arthington and 

Lloyd 1989). Reddy and Pandian (1972) found heavy mortalities of gambusia reared on a diet 

restricted to mosquito larvae, and the few survivors showed poor growth and delayed 

maturation. 

Several authors have observed that gambusia may actually encourage mosquito populations 

by preying on their invertebrate predators (Stephanides 1964; Hoy et al. 1972; Hurlbert et al. 

1972; Hurlbert and Mulla 1981). Gambusia are inappropriate for mosquito control in certain 

habitats such as temporary ponds, waters with dense vegetation and running waters. It is also 

unlikely to be effective as a predator on cool cloudy days, where drops in temperature and 

oxygen occur (Lloyd 1986). Where good mosquito larvae control has been reported, the 

evidence was largely anecdotal or derived from poorly designed experiments (Courtenay and 

Meffe 1989; Rupp 1996). 

5.3 Dispersal of Gambusia 

Gambusia can be spread either directly or indirectly by humans, naturally through floods, or 

perhaps by other animals such as birds feeding on and regurgitating small fish, dropping them 

in flight, or transporting fish on mud adhered to their plumage and feet. Lloyd (1987) states 

that humans were the major dispersal agents of gambusia. McKay (1984) also observed that 

past records of gambusia around Alice Springs were probably a result of the release of 

aquarium fish. 

The NPWS is not aware of any local councils or health authorities in NSW currently 

advocating the introduction of gambusia as a mosquito control agent. However, individuals 

from the public, without realising the impact that the species has on the environment, may 

still move these fish around, believing that gambusia are efficient at removing mosquito 

larvae from waterways or simply to dispose of unwanted fish. Some argue that gambusia are 

safe when used in the “contained” and artificial environments in which they are often stocked, 

and will not escape to the wild (Meffe 1996). It is also likely that some people move 

gambusia around believing them to be native species (Ross Wellington pers. comm.). 

10


Flooding of waterways is another potential mechanism for the dispersal of gambusia. One 

example, is the Brisbane flood of January 1974, which caused widespread flooding of 

suburban creeks and a number of outdoor ponds and indoor aquaria, which resulted in fish 

being liberated (McKay 1984). Most outdoor ponds, which are thought to be secure 

environments, are not fitted with an outlet screen to prevent the discharge of aquarium fish 

into storm water drains (McKay 1984). Down-stream ponds in particular are prone to 

reinfection by gambusia sourced upstream during flooding events. 

Gambusia has been known to spread from overcrowded pools into wheel ruts or puddles after 

heavy rainfall (Serventy and Raymond 1980; Wager 1995a). In one reported case a number of 

fish were found 275 m from the parent lake, swimming along the tiny stream formed by a 

wheel rut (Serventy and Raymond 1980). 

Lloyd (1987) observed that the distribution of gambusia in isolated waterbodies in central 

Australia was almost certainly a result of flooding. Gambusia are known to be present in 

artificial water bodies on the Nullabor Plain (Serventy and Raymond 1980; Boulton and 

Brock 1999). Irrigation channels could be a potential source of infestation of gambusia into 

nearby creeks and rivers. Wager (1995a) found gambusia to be common in artificial habitats 

such as bore drains and the wetlands associated with some flowing bores in the Diamantina 

River Catchment in western Queensland. These artificial habitats probably act as point 

sources for the ongoing infestation of surrounding water bodies (Wager 1995a). Gambusia 

has been recorded in several floodplain waterholes in the Cooper Creek drainage (Angela 

Arthington, pers. obs.). 

Lloyd (1984) indicated that adults do not move beyond their home range although they can 

undertake small scale thermal migrations within this area, while it has been noted that 

juveniles tend to migrate away from adult populations (Lloyd 1986). 

6. Biology and Ecology of Gambusia 

6.1 Distribution 

In Australia, gambusia are found in at least eight of the eleven major drainage divisions 

(Merrick and Schmida, 1984). The species is considered to be widespread and common 

throughout NSW, South Australia and Victoria in both inland and coastal drainages. It is 

common in coastal drainages of Queensland, is present in rivers draining into Lake Eyre, in 

parts of the Northern Territory and Western Australia, but has not been recorded from 

Tasmania (Allen 1989; McDowall 1996; Arthington et al. 1999). Arthington et al. (1999) 

observed that the species occurs in most aquatic habitat types in south-eastern Australia. 

Although there have been no systematic targeted surveys for gambusia in NSW, there have 

been a number of surveys, which have identified its presence. The most comprehensive fish 

survey undertaken has been the NSW Rivers Survey, during which 80 sites were surveyed 

over four periods from 1994 to 1996. Gambusia were recorded at 27 sites (see Appendix 2) 

11


with six sites having captures or observations of more than 50 individuals (Faragher and 

Lintermans 1997). They were most widely distributed in the Darling and North Coast regions. 

Gambusia were also recorded at four sites in the South Coast and two sites in the Murray 

regions (Faragher and Lintermans 1997). They were also found at altitudes of 20 to 1120m, 

although the majority of sites were below 300 m (Faragher and Lintermans 1997). 

Other surveys of fish in NSW include records of gambusia, at the Darling River anabranch 

(Callanan 1984) and in the Darling River (Callanan 1985), Wingecarribee River (Burchmore 

et al. 1990), Pindari Dam enlargement, Severn and Macintyre Rivers (Swales and Curran 

1995), the Macquarie River and its tributaries in the area of the Macquarie Marshes (Swales 

and Curran 1995a), the Cudgegong River (Swales et al. 1993), Gol Gol Swamp (Brown 

1994), the Hawkesbury-Nepean River system (Gehrke et al. 1999) and lower Balonne 

floodplain and Narran River (Mottell 1995). Lewis and Goldingay (1999) recorded gambusia 

at nine of fifteen coastal sites surveyed between Red Rock (40 km north of Coffs Harbour) 

and Ocean Shores (15 km north of Byron Bay). Gambusia has also been observed in 

numerous areas at Lake Macquarie on the Central Coast and in the Illawarra catchment and 

Sydney basin (Ross Wellington pers. comm.; Arthur White pers. comm.; Goldingay and 

Lewis 1999). 

While records of gambusia in the south coast area of NSW are limited, this does not 

necessarily mean that the species does not occur there. The presence of gambusia in the 

Snowy River catchment in Victoria was recorded during a survey of the lower Snowy River 

(Raadik et al. 2001). Recent surveys by Daly and Senior (2001) for the green and golden bell 

frog on the far south coast of NSW, between Batemans Bay and Eden, detected gambusia at 

15 of the 115 sites surveyed. 

6.1.1 Factors influencing distribution 

Factors influencing the distribution of gambusia in NSW include those directly and indirectly 

associated with humans, and also natural events such as dispersal through floods, via other 

animals, and those biological features of the species, which assist its dispersal in nature. 

Courtenay and Meffe (1989) suggested that gambusia fit seven of the criteria of a successful 

invader (identified by Ehrlich 1986) as follows: 

are abundant in original range 

are polyphagous 

have a short generation time 

a single female can colonise a new site 

have a broad physical tolerance 

are closely associated with humans, and 

have high genetic variability 

12


Courtenay and Meffe (1989) proposed two additional criteria for success: 

specialised reproduction (ie high fecundity, highly developed young, reproduce numerous 

times per year, young are independent of adults after birth, the species is tolerant of broad 

range of temperature and day-lengths) 

females are extremely aggressive often causing the death of other species (Meffe 1985) 

6.2 Habitat Preferences 

Gambusia inhabit rivers, creeks, lakes, swamps and drains and occurs in both clear and 

muddy water (Cadwallader and Backhouse 1983). It has been able to invade a wide range of 

habitats including turbid, silty lower reaches of rivers, swamps, lakes, (including salt lakes 

and dystrophic systems of very low productivity in coastal dunes), billabongs, thermal 

springs, farm dams, the cooling pondage of a power station and ornamental ponds in many 

urban parks (Lloyd 1984; Lloyd et al. 1986; Arthington and Marshall 1999). 

Undisturbed lotic (ie flowing systems) with naturally variable discharge regimes are not 

favoured by gambusia. High river discharges almost eliminate populations (Meffe 1984; 

Arthington et al. 1990; Galat and Robertson 1992), perhaps because predatory efficiency is 

low, and long-term survival impossible (Reddy and Pandian 1974; Mees 1977; Lloyd et al. 

1986). Reddy and Pandian (1974) for example observed gambusia to be less efficient at 

preying on mosquito larvae in flowing waters. 

Gambusia has an ability to withstand adverse conditions, sometimes far beyond their normal 

tolerances. Lloyd (1984) noted that this enables gambusia to persist (though perhaps without 

breeding) in an unfavourable habitat before colonising other habitats. In laboratory 

experiments using gambusia, calm water was found to be the most important habitat variable, 

followed by submerged vegetation cover, which provides concealment from predators. Dense 

surface vegetation appears less favourable as it obstructs access to surface water where it 

forages. These laboratory preferences indicate that gambusia actively seeks a suitable habitat 

in which it can compete successfully and be protected from predatory birds and fishes 

(Casterlin and Reynolds 1977; Lloyd et al. 1986). 

Gambusia are abundant in warm slow-flowing or still waters amongst aquatic vegetation at 

the edge of waterbodies in water depths of 10 cm or less (Merrick and Schmida 1984; 

McDowall 1996; Faragher and Lintermans 1997; Arthington et al. 1999). They generally 

prefer warm water temperatures (>25C) (Lloyd 1984; Clarke et al. 2000), showing a thermal 

preference for water of 31C and thermoregulate during the day by moving from deep to 

shallow water (Winkler 1979). Populations are able to withstand wide temperature ranges 

from just above freezing (0.5C), to a critical thermal maximum of 38 (Lloyd 1984; Lloyd et 

al. 1986; Clarke et al. 2000). Populations have been known to survive for short periods of 

time in water temperatures as high as 44C (Lloyd 1984). Young gambusia are more resistant 

than adults to high temperature allowing them to colonise and exploit warm patches in the 

environment (Lloyd 1984). Females also appear to show more resistance to high water 

temperatures than males (Winkler 1975 cited in Lloyd et al. 1986). 

13


Luna (2001) noted that the species tolerated a pH range of between 6.0 and 8.8, whereas 

Swanson et al. (1996) noted that a broader pH range of between 4.46 to at least 10.2 is 

tolerated based on both laboratory tolerance experiments and field observations. Additionally, 

Knight (2000) has observed gambusia occupying waters of pH 3.93. 

6.2.1 Use of modified habitats by Gambusia 

Lloyd (1984) suggested that modified habitats are particularly susceptible to invasion by 

gambusia due to these areas having relatively abundant sources of food, and low species 

richness because of harsh physical conditions. Arthington et al. (1990) developed this idea 

with examples from around Australia. The following summary identifies those aspects of 

habitat modification likely to be favoured by gambusia. 

River Impoundment 

Impoundment of water by dams or weirs can lead to reduced water discharge and slower 

flows. The subsequent development of shallow littoral zones, pools and areas of lentic habitat 

can facilitate growth of fringing vegetation. Such areas can provide very favourable habitat 

for gambusia. Arthington et al. (1983) observed that the proliferation of gambusia in the 

waterways of urban Brisbane corresponded to human induced changes, including construction 

of water-supply dams and flood retention basins, diversion of stream channels for flood 

mitigation, excavation of sand and gravel to form lentic habitat. 

Bank, riparian and channel alterations 

Degradation of riparian areas through agricultural and pastoral practices may lead to loss of 

riparian vegetation, bank erosion and collapse, sedimentation and river-bed alterations. These 

are conditions which gambusia can tolerate to the disadvantage of native species (Arthington 

et al. 1990). 

Water quality and pollution 

Arthington et al. (1990) argue that the ability of gambusia to tolerate low dissolved oxygen 

concentrations has probably enabled the species to survive in areas such as stagnant urban 

drains, enriched ponds and eutrophic impoundments. Gambusia are able to utilise oxygen-rich 

surface layers of water, enabling them to survive in anoxic situations due to their dorsallyoriented 

mouth and flattened head (Lloyd 1984). 

Lloyd (1987) reviewed the tolerance of gambusia to pollutants and observed that the species 

was resistant to a wide range of pollutants, including organic wastes, phenols, pesticides, 

heavy metals and radiation. He observed that the species tolerance and resistance to pesticides 

is well known as it had been used in combination with pesticides to control mosquito larvae in 

rice fields in the United States. 

Gambusia are tolerant of a wide range of salinities, from very low salinity fresh water to 

marine conditions (McDowall 1996; Arthington and Lloyd 1989). Gambusia generally 

tolerate salinities of 25g/L in the field in the United States, but have been recorded in 

14


Australian salt lakes with salinities of 30g/L. Under laboratory conditions, most gambusia can 

tolerate salinities of 50g/L and some can survive 80g/L salinity conditions for short periods 

(seawater has a salinity of 35g/L) (Lloyd 1984). 

6.3 Breeding Biology, Social Organisation and Dispersal in Nature 

Sexual maturity 

Gambusia grow rapidly, becoming sexually mature in less than two months (McDowall 

1996). Immature male gambusia are sexually active well before their copulatory organ 

(gonopodium) has completely developed and before they are able to transfer sperm (Bisazza 

et al 1996). 

Fecundity 

Sexually mature females have been recorded as having up to nine broods a year from about 

August to April (Milton and Arthington 1983; McDowall 1996). On average, females have 

two or three broods per season, and store sperm between breeding seasons (Howe 1995; Lund 

1999a). Female gambusia may store sperm for up to eight broods or eight months and may 

nourish the live sperm within their reproductive tracts (Constantz 1989). Peak reproductive 

activity occurred in October in a population of gambusia studied near Brisbane, with 94% of 

females being pregnant at that time (Milton and Arthington 1983). The reproductive cycle is 

primarily governed by photoperiod, with reproduction ceasing once day length falls below 

12.5-13 hours, even when water temperature remains favourable (Lloyd 1986; Milton and 

Arthington, 1983; Pen and Potter 1991). Gambusia are live bearers (i.e viviparous), with 

fertilisation occurring internally and the embryos developing within the female (Cadwallader 

and Backhouse 1983; McDowall 1996). The gestation period is between 21 and 28 days, with 

about 50 young being produced on average, though broods may often exceed 100, with more 

than 300 having been reported in a single brood (Cadwallader and Backhouse 1983; Milton 

and Arthington 1983; McDowall 1996). 

Survivorship 

While the approximate sex ratio of young gambusia is initially 1:1, the subsequent higher 

mortality in males results in females usually dominating the adult population (Cadwallader 

and Backhouse 1983; Vargas and de Sostoa 1996). Males tend to disappear from populations, 

which may be due to their reproductive efforts and male fish being more susceptible to 

overcrowding and temperature stress than the females (Krumholz 1948 cited in Vargas and de 

Sostoa 1996). When female gambusia are pregnant their morphology changes and slower 

movements may render them more visible and therefore more susceptible to predators, 

skewing the sex-ratio in favour of males at this time (Vargas and de Sostoa 1996). The 

maximum life span of up to two years occurs in females that do not mature until their second 

summer (Cadwallader and Backhouse 1983), although most will perish during winter (Lund 

1999a). 

15


Social organisation including behavioural characteristics 

Gambusia display a wide range of behaviours which enable them to adapt to a variety of 

situations (Lloyd 1984). In a study of the impacts of gambusia on the southern blue-eye 

(Pseudomugil signifer), no documented evidence of territoriality was found for gambusia. 

Territorial behaviour has not been observed in the family Poeciliidae of which gambusia is a 

member (Howe 1995). However, gambusia is known to show aggressive behaviour towards 

other fish species such as the southern blue-eye (P. signifer) (Howe 1995). Bisazza et al. 

(1996) found in experimental trials that adult males showed aggressive behaviour towards 

another male attempting copulation, irrespective of the maturity of the latter. The aggressive 

behaviour of gambusia toward fish species is discussed in detail in section 7.3. 

6.4 Diet and Factors Influencing Dietary Preferences 

Gambusia is an opportunistic omnivore with a preference for animal food (Rosen and 

Mendelson 1960; Al-Daham et al. 1977; Farley 1980 cited in Lloyd et al. 1986). Gambusia 

select their prey according to size, colour, movement (Bence and Murdoch 1986; Lloyd et al. 

1986) position in the water column (Arthington and Marshall 1999) and availability (Lloyd 

1984). Arthington (1989) found that gambusia preferred small prey, a finding consistent with 

that of Bence and Murdoch (1986) who investigated size-selective predation by gambusia. 

These two studies conflict with field studies by Wurtsbaugh et al. (1980) who suggested that 

gambusia generally select the largest prey they can successfully capture. 

Gambusia feeds on a diverse range of terrestrial insects such as ants and flies that fall onto the 

water’s surface, as well as aquatic invertebrates including bugs, beetles, fly larvae and also 

zooplankton (Lloyd et al. 1986; Arthington 1989; McDowall 1996). It is an adaptable 

generalist predator, able to vary its diet according to prey availability (Arthington 1989; 

McDowall 1996). Gambusia are diurnal visual feeders that feed during daylight hours and 

rely on sight to detect, track and attack prey (Swanson et al. 1996). In a study by Arthington 

and Marshall (1999) they found the diet of gambusia was composed of aquatic invertebrates, 

filamentous algae, terrestrial insects, arachnids, fragments of fruit and other plant tissues. 

More than 50% of the diet comprised items found at the water’s surface, such as chironomid 

pupae (midges), arachnids and terrestrial insects. 

There is no direct evidence that frog eggs or tadpoles form a natural component of the diet of 

gambusia (Reynolds 1995), although the impact of gambusia on native frogs has been studied 

(eg Reynolds 1995; Morgan and Buttemer 1996; Webb and Joss 1997; Healey 1998; Gillespie 

and Hero 1999; Komak and Crossland 2000; Pyke and White 2000). Refer to section 7.4 for 

more information regarding the impacts of gambusia on frogs. 

6.5 Known and Potential Diseases, Predators and Competitors 

Disease and Parasites 

Gambusia has relatively few parasites in Australia when compared to North America (Lloyd 

1990). Swanson et al. (1996) has prepared a list of 23 of the most common and important 

parasites and pathogens of gambusia in the U.S.A. The only parasite observed on gambusia in 

16


the lower Murray River (South Australia) is an exotic parasitic copepod, Lernaea cyprinacea 

(Lloyd 1984; Arthington and Lloyd 1989; Lloyd 1990). Another introduced parasite found in 

gambusia is a protozoan, Goussia piekarskii (Lom and Dykova 1995). Dove (1998) noted that 

11 parasites have been recorded in gambusia in Queensland. 

Predators 

In North America, predatory fish, wading birds, snakes and invertebrates have been found to 

prey on gambusia (Meffe and Snelson 1989). Swanson et al. (1996) list major predators of 

gambusia in the USA, as migratory and resident birds (herons, egrets, bitterns, grebes, ducks, 

kingfishers, terns, crows and blackbirds), piscivorous fishes (sunfish, catfish, bass), bullfrogs, 

and some aquatic insects including notonectids (backswimmers), corixids (water boatmen), 

dytiscids (predaceous diving beetles), and larval anisopterans (dragon flies). 

Predators of gambusia in Australia probably include birds, fish and even spiders (Lloyd 

1984). Many of the major predators of gambusia in the USA listed in Swanson et al. (1996) 

are also possible predators in Australia. Lloyd et al. (1986) noted important fish predators to 

include species of Anguilla, Mogurnda, Gobiomorphus, Leiopotherapon and Glossamia, 

although their impacts on gambusia are not known. Lloyd (1987) noted that water rats 

Hydromys chrysogaster and the fish eating bat Myotis adversus also apparently fed on 

gambusia. There have been very few studies of predators of gambusia in Australia, but there 

have been suggestions as to the likely cause of low predation levels (Lloyd 1984). Both native 

and exotic fish predators avoid gambusia as prey when given a choice (Lloyd 1984; Lloyd 

1990). Reports suggest gambusia are considered unpalatable and that native predators may 

not have evolved behaviours appropriate to the capture of gambusia (Lloyd 1984). In inland 

lakes of NSW, little black cormorants (Phalacrocorax sulcirostris) feed mainly on exotic 

fishes such as carp and gambusia (Boulton and Brock 1999). Lloyd (1984) noted that, when 

exposed to predators, gambusia can rapidly develop complex escape and avoidance 

behaviours. 

Competitors 

Native fish species may compete with gambusia for food or other resources. Section 7.3 

includes discussion of the theory of competition in relation to gambusia and other species. 

Extensive dietary overlap occurs between gambusia and a number of native species. 

7. Impacts of Gambusia on Native Plants and Animals 

7.1 Impacts on Native Vegetation and River Health 

No forms of direct physical disturbance to aquatic environments, including to aquatic 

vegetation, appear to have been attributed to gambusia. The species does not exhibit obvious 

behaviours (such as carp disturbing aquatic vegetation while feeding), which may lead to 

habitat degradation. 

17


7.2 Impacts on Macro-invertebrates 

Few studies have been undertaken concerning the impact of gambusia on invertebrates and 

the impact on threatened Australian invertebrates is not currently known. There is some 

evidence that gambusia can cause reductions in populations of invertebrates such as rotifers, 

cladocerans, ostracods, copepods, mayflies, beetles, dragonflies and molluscs (Hurlbert et al. 

1972; Lloyd 1990a; Lund 1999b; Anstis 2002). 

Declines in some invertebrates may cause an increase in phytoplankton populations (Lloyd 

1990a). Stephanides (1964 cited in Hurlbert et al. 1972) observed a dramatic top-down effect 

of introducing gambusia to a small lake where fish had previously been absent; the 

elimination of zooplankton by gambusia caused a tenfold increase in phytoplankton 

populations. Gambusia may change invertebrate assemblages of ponds by differential 

predation, which can make a system unstable (Lloyd 1984). Lund (1999b) argued that these 

fish could potentially reduce water quality and could also increase the amount of algae in the 

water through excretion of nutrients. He suggested that in more pristine environments 

gambusia may eliminate rare taxa. In relation to mosquito control, Lloyd et al. (1986) argued 

that at low densities gambusia may actually encourage mosquito larvae by eating their 

invertebrate predators in preference to mosquitoes. 

Ecological attributes of macroinvertebrates that would make them susceptible to population 

level impacts from gambusia may include their method of reproduction, dispersal and 

migratory habits. Aquatic insects that have a terrestrial stage may be susceptible to predation 

by gambusia when undergoing the emergence stage of their life cycle (eg chironomid pupae - 

Arthington and Marshall 1999). Insects that deposit eggs on the water's surface may be 

susceptible to predation by gambusia if this occurs during times of the year when gambusia 

are at peak abundances. 

7.3 Impacts on Native Fish 

Worldwide Impacts 

Some thirty-five fish species worldwide have declined in abundance or range as a result of 

interactions with gambusia (Lloyd 1990). Lloyd (1987) provides a list of these species. 

Arthington and Lloyd (1989) considered that gambusia have been implicated in the extinction 

of small fish species in the USA, Asia and Africa and in the reduction in range or abundance 

of twenty-five species in the worldwide. In the USA, the replacement of a native fish species, 

the Sonoran topminnow (Poeciliopsis occidentalis) by gambusia is well documented (Galat 

and Robertson 1992). This native species was once widespread and abundant in desert areas 

and is now considered threatened due to habitat loss and predation by gambusia. 

Impacts on Australian Species 

While no local extinctions of fish species have been attributed to gambusia in Australia, this 

species may influence the distribution and abundance of particular native fish in areas where 

they co-occur (Arthington and Lloyd 1989). Howe et al. (1997) concluded that the ubiquity of 

gambusia has major implications for the conservation of several smaller species of fish in 

18


Australia. In association with a range of environmental alterations, gambusia are thought to 

have played a role in the decline of fish species from six genera in Australia: Mogurnda, 

Ambassis, Melanotaenia, Pseudomugil, Craterocephalus and Retropinna (Arthington et al. 

1983; Lloyd 1990). However, Lloyd (1990) cautioned that much of the evidence was 

circumstantial and patchy. 

Lloyd (1984) argued that there is extensive overlap in requirements for food and space where 

populations of native fish and gambusia co-occur, and Arthington et al. (1983) demonstrated 

this in streams. Extensive dietary overlap has been documented between gambusia and at least 

seven native fish species, while two native species also show shifts in feeding niches, through 

expanding their feeding preferences, when living in association with gambusia (Lloyd 1990). 

Arthington and Marshall (1999) observed high dietary overlap between gambusia and the 

native ornate rainbowfish (Rhadinocentrus ornatus), and moderate dietary overlap with the 

native fire-tailed gudgeon (Hypseleotris galii). At some times of the year, gambusia switched 

its diet to feed on aquatic invertebrates usually eaten by these gudgeons, which increased the 

dietary overlap between the two species. 

In some areas of eastern Queensland, gambusia can dominate fish assemblages and may 

reduce the abundance and diversity of native species. McKay (1984) observed that in 

Queensland coastal streams where gambusia, guppies and swordtails occurred, native surface 

feeding or mosquito eating fish such as Melanotaenia, Pseudomugil, Craterocephalus and 

Retropinna were usually rare or absent. He noted the example of a significant decline in the 

southern blue-eye (P. signifer) at a site in the Brisbane River five years after gambusia and 

guppies had invaded. 

In creeks in the Brisbane area, Arthington et al. (1983) observed that large numbers of 

gambusia were correlated with small numbers of the native fire-tailed gudgeon (H. galii) and 

crimson-spotted rainbowfish (Melanotaenia fluviatilis), and that where they occurred together 

encounters were very likely. Arthington et al. (1983) noted that while the small populations of 

crimson-spotted rainbowfish (M. fluviatilis) may have been caused by habitat disturbance, the 

relationship between gambusia and fire-tailed gudgeon (H. galii) may have been due to 

interactions between these two species. They suggested that the importance of interactions 

such as competition and predation may vary with the size structure of populations. 

There is some evidence of predation and aggression by gambusia on native species. 

Investigation of gut contents of wild gambusia in NSW identified juveniles of Australian 

smelt, Duboulay's rainbowfish (Melanotaenia duboulayi), ornate rainbowfish (R. ornatus), 

southern blue-eye (P. signifer) and fire-tailed gudgeon (H. galii) (Ivantsoff and Aarn 1999). 

Fish remains were only identifiable up to 4 hours after feeding, which suggests that gut 

analyses may often underestimate many food items. Ivantsoff and Aarn (1999) noted that their 

study could not identify the significance of predation on native species by gambusia as newly 

hatched fish may be an important food source for fish species. In tank experiments, Howe 

(1995) observed gambusia to actively hunt and eat the young of southern blue-eye. Aarn 

(1998) suggested that in marginal eutrophic habitats where gambusia breed during the warmer 

19


months, high levels of predation of eggs and larvae may lead to the extinction of 

melanotaeniids (rainbow fishes). During monitoring of populations in January, in springs in 

central western Queensland, Wager (1995) suggested that the absence of juvenile red-finned 

blue-eyes (Scaturiginichthys vermeilipinnis) was probably due to predation of their eggs or 

fry by gambusia. 

Gambusia can exhibit aggressive behaviour towards other fish, including those much larger 

than themselves. This behaviour includes harassment such as chasing and fin-nipping, which 

may be so severe that fins are entirely removed (McKay 1984; Unmack and Brumley 1991; 

Steve Saddlier pers. obs.). In some instances, aggressive behaviour may also lead to 

secondary bacterial or fungal infections and eventual death (Faragher and Lintermans 1997; 

Knight 1999). Stress caused by aggression may also reduce breeding success as well as 

feeding and metabolic processes (Howe et al. 1997). 

Howe et al. (1997) argued that fin nipping may only occur in crowded conditions, such as 

with receding tides or prolonged drought conditions, where large numbers of fish occur in 

small ponds and waterholes. Howe et al. (1997) also suggested that the aggressive 

interactions observed may be a form of interspecific competition for space. Knight (1999) 

investigated interference competition between southern blue-eye (P. signifer) and gambusia. 

He found that under captive conditions, interference competition was density dependent and 

that gambusia inflicted stress and physical damage to the blue-eyes. Variations in aggressive 

behaviour were observed between individual fish, and males tended to attack more often than 

females. Further experiments under captive conditions have shown gambusia to exhibit 

aggressive behaviour towards ornate rainbowfish, (R. ornatus) Duboulay’s rainbowfish (M. 

duboulayi) and firetailed gudgeon (H. galii) (Jamie Knight pers. comm.). 

Lloyd (1990) indicated that there was some evidence that gambusia could affect growth rates 

of native fish species. In tank experiments, gambusia significantly affected both the growth 

and reproductive characteristics of southern blue-eye (P. signifer)(Howe et al. 1997). They 

found that fish did not gain weight or grow in total length, their ovarian weight and fecundity 

were greatly reduced, and their ovaries were morphologically undeveloped. These results 

clearly indicate that gambusia has the potential to significantly affect reproductive success as 

well as survival of this native species in confined habitats (Howe et al. 1997). 

Koster (1997) undertook tank experiments to determine whether gambusia affected the 

growth of native southern pygmy perch. His results indicated that under controlled conditions 

where food was not a limiting factor, gambusia did not affect the growth of the native species. 

In these tanks, gambusia nipped the fins of the southern pygmy perch as well as those of 

dwarf galaxias. However, this did not result in any infections or deaths of the native fish. 

Breen et al. (1989) also noted a total overlap in the diets of gambusia and dwarf galaxias. 

In some countries, hybridisation has occurred between gambusia species (Lloyd 1986). Howe 

(1995) points out that hybridisation between gambusia and native fish species is not possible 

as gambusia are live-bearers rather than egg laying fish. 

20


Impacts on Threatened Fish Species in NSW 

The NSW Fisheries Scientific Committee has identified gambusia as a possible cause of 

decline for the following threatened fish: 

Oxleyan pygmy perch (Nannoperca oxleyana) (Endangered species) 

Murray hardyhead (Craterocephalus fluviatilis) (Endangered species) 

Silver perch (Bidyanus bidyanus) (Vulnerable species) 

Western population of southern purple-spotted gudgeon (Mogurnda adspersa) 

(Endangered population) 

Western population of olive perchlet (Ambassis agassizii) (Endangered population) 

7.4 Impacts on Native Frogs 

Predation is generally recognised as a major factor regulating the distribution of amphibian 

tadpoles (eg Calef 1973; Heyer et al. 1975; Duellman 1978; Scott and Limerick 1983; Smith 

1983; Woodward 1983; Wilbur 1984; Hayes and Jennings 1986; Kats et al. 1988; Gillespie 

2001). Predatory fish are important determinants of tadpole species-composition (ie 

proportions of species present) and tadpole species richness (ie total number of species 

present) in both temperate (Hecnar and M’Closkey 1997) and tropical systems (reviewed by 

Gillespie and Hero 1999). Many frog species avoid predation from fish by reproducing in 

habitats inaccessible to them, such as ephemeral pools or terrestrial microhabitats. Amphibian 

species that do reproduce in habitats naturally containing predatory fish invariably possess 

one or a combination of survival strategies to evade predation (Kats et al. 1988), such as 

cryptic colouration (Wasserug 1971), behavioural responses such as use of refugia (Sih et al. 

1988), schooling (Waldman 1982; Kruse and Stone 1984) and chemical defences (Liem 1961; 

Wasserug 1971; Brodie et al. 1978; Kruse and Stone 1984; Kats et al. 1988; Werner and 

McPeek 1994). 

Survival strategies tend to be predator specific and are unlikely to be effective against all 

predators. Palatability of a species of tadpole is not constant, varying according to the 

predatory fish species (Hero 1991; Holomuzki 1995; Gillespie 2001). Therefore, the 

distribution of each tadpole species is related to the survival strategies it possesses and is 

strongly influenced by the distribution of predatory fish (Gillespie and Hero 1999). 

Tadpoles of native species may be able to evade predators with which they naturally coexist 

but may be unable to evade an introduced or new predator. These species may not identify 

such fish as predators and hence fail to use the appropriate survival strategies (temporal or 

spatial isolation), or may not have the necessary anti-predator defences (eg cryptic 

colouration, behavioural responses such as refugia, schooling and chemical defences) that 

allow them to coexist with introduced fish species (Gillespie and Hero 1999; Gillespie 2001; 

Hamer et al. 2002). The introduction of a predatory fish species may result in the local or total 

extinction of some native prey species and a shift in the species composition of native frogs to 

21


those species, which have the survival-strategies that allow them to coexist with the 

introduced predator (Gillespie and Hero 1999). 

The consequences of introducing fish into breeding habitats for frogs have been well 

documented overseas. A number of studies in Europe, North and South America have 

implicated or demonstrated that introductions of predatory fish are responsible for the decline 

or extinction of some frog species (see Gillespie and Hero 1999). 

Several studies in USA have demonstrated predatory impacts of gambusia on amphibians. 

Gamradt and Kats (1996) found that gambusia contributed to localised population declines of 

California newts (Taricha torosa) through predation on their larvae. Goodsell and Kats (1999) 

found that gambusia preyed heavily on tadpoles of Pacific tree frogs (Hyla regilla), both in 

natural streams and laboratory experiments, despite the presence of high densities of mosquito 

larvae as alternative prey. 

Gambusia may impact upon amphibian populations directly and indirectly by: 

predation of eggs and hatchlings 

predation of tadpoles 

aggressive behaviour causing tadpole tail damage or loss, which can result in increased 

risk of disease, increased risk of predation by other predators because of loss of mobility 

and reduced energy reserves and growth rates resulting in poorer post-metamorphic 

fitness 

aggressive behaviour causing changes in microhabitat use and activity patterns of tadpole, 

resulting in increased risk of predation by other predators or reduced growth rates 

The broad distribution and wide range of habitats occupied by gambusia in Australia means 

that this species may potentially impact upon many lentic and lotic frog populations over a 

large region. Gillespie and Hero (1999) reviewed the literature on interactions between 

gambusia and Australian amphibians. The following discussion is taken from this review, 

with the addition of more recently published information: 

A number of studies have identified negative associations between the presence of gambusia 

and frog species. Dankers (1977) found that tadpole numbers of several species were 

drastically reduced in ponds containing gambusia after early December in NSW, coinciding 

with a seasonal increase in fish biomass. McGilp (1994) found a negative correlation between 

the occurrence of the brown tree frog (Litoria ewingii) and that of gambusia in waterbodies 

along the Yarra River in Melbourne, Victoria. 

Reynolds (1995) found eggs of the sign-bearing froglet (Crinia insignifera) and Glauert's 

froglet (Crinia glauerti) to be unpalatable to gambusia. Preliminary trials also suggested that 

eggs of the slender tree frog (Litoria adelaidensis), Moore's frog (Litoria moorei) and 

Tschudi's froglet (Crinia georgiana) may also be unpalatable (Reynolds 1995). However, 

several studies have shown experimentally that gambusia are capable of preying on hatchlings 

22


and small tadpoles of a number of Australian frog species, such as the spotted grass frog 

(Limnodynastes tasmaniensis), Leseuer's frog (Litoria leseueuri) and bleating tree frog 

(Litoria dentata) (Harris 1995), sign-bearing froglet (C. insignifera) and Glauert's froglet (C. 

glaureti) (Reynolds 1995), green and golden bell frog (L. aurea) and bleating tree frog (L. 

dentata) (Morgan and Buttemer 1996), striped marsh frog Limnodynastes peronii and signbearing 

froglet (C. insignifera) (Webb and Joss 1997). 

Blyth (1994) compared survival and recruitment of tadpoles of three species of Western 

Australian frogs, Glauert's froglet (C. glaureti), sign-bearing froglet (C. insignifera) and 

moaning frog (Heleioporus eyrei), in the presence and absence of gambusia in experimental 

field enclosures. Tadpole survival of all species was significantly lower in the presence of 

gambusia at the end of the experimental period. However, the design of the enclosures 

allowed access for breeding by frogs from local frog populations, as evidenced by increases in 

numbers of frogs in some enclosures. Other potential predators of tadpoles also had access, 

such as invertebrates and birds. Furthermore, no species/fish treatments were replicated. 

These factors limit interpretation of the results of this study. 

Webb and Joss (1997) examined frog species richness and abundance in relation to gambusia 

density and cover of emergent aquatic vegetation in ten ponds near Sydney. They found a 

significant negative relationship between fish density and frog abundance but no relationship 

with frog species richness. The descriptions provided for each waterbody indicate high 

variability in habitat among pond sites. Unfortunately, additional factors such as pool size and 

native vegetation cover, which may strongly affect frog abundance, were not considered in 

their analyses. Tadpole density is easier to sample systematically than adult frog density in 

pond habitats (Heyer et al. 1994). Given that tadpoles are one of the life stages upon which 

gambusia potentially preys, a measure of the relative abundance of tadpoles, rather than that 

of frogs, may have provided a more reliable indicator of the impact of gambusia. 

Reynolds (1995) examined the occurrence of six frog species with gambusia in water bodies 

near Perth, Western Australia. In contrast to the above studies, he found no relationship 

between the presence/absence of gambusia and individual frog species, with one exception, 

the sign-bearing froglet (C. insignifera), which was found only infrequently together with 

gambusia. However, he observed that most of the sites used by the sign-bearing froglet (C. 

insignifera) were ephemeral and unsuitable for gambusia. Frog species richness was generally 

lower at sites occupied by gambusia, but many of these sites were also degraded, contributing 

to their unsuitability as frog breeding habitats. 

Reynolds (1995) also experimentally examined predation by gambusia on several tadpole 

species in Western Australia. Trials with tadpoles indicated that gambusia were able to attack 

and kill tadpoles of the slender tree frog (L. adelaidensis), Tschudi's froglet (C. georgiana) 

and moaning frog (H. eyrei). Controlled palatability experiments showed that survival of 

Moore's frog tadpoles was significantly reduced in the presence of gambusia. However, 

gambusia showed a strong preference for invertebrate prey (Daphnia sp. or mosquito larvae). 

Both groups were consistently consumed completely before tadpoles in all trials. In a field 

23


enclosure experiment, in which tadpoles were also exposed to invertebrate predators, 

Reynolds (1995) found no significant difference in survival in the presence or absence of 

gambusia. These results, in conjunction with his field survey data, suggest that the impact of 

gambusia upon populations of these frog species is influenced by several factors, and under 

natural conditions may be limited. 

Webb and Joss (1997) conducted predation experiments examining the impact upon survival 

of different size classes of sign-bearing froglet (C. insignifera) and striped marsh frog (L. 

peronii) tadpoles by hungry and pre-fed gambusia. They found significant differences 

between predation rates related to tadpole size class and hunger status of fish. Tadpoles of 

species which are able to rapidly attain moderate to large sizes, may therefore be less predated 

upon than tadpoles of other species (Caldwell et al. 1980; Crump 1984). 

Several studies have reported damage to the tails of larger tadpoles from gambusia attack 

(Dankers 1977; Blyth 1994; Harris 1995). This damage could result in reduced survival of 

larger tadpoles due to reduced mobility and feeding, inability to escape other predators, or 

reduced metamorphic fitness. However, some tadpole species have been found to survive tail 

loss (Harris 1995). Wilbur and Semlitsch (1990) reported tail regeneration by tadpoles of the 

American bullfrog (Rana catesbeiana) even after considerable loss, and suggest that this may 

be a general mechanism to reduce the impact of predation. 

Concerns for the role of gambusia in the decline of some frog species, particularly members 

of the bell-frog complex, have been expressed by several authors (Mahony 1993, 1999; Daly 

1995; Morgan and Buttemer 1996; White and Pyke 1996; White and Ehmann 1997; van de 

Mortel and Goldingay 1998; Goldingay and Lewis 1999; Lewis and Goldingay 1999; 

Biosphere Consultants Pty Ltd 2001; Daly and Senior 2001; Hamer et al. 2002). However, 

direct evidence linking gambusia to declines of frog populations in the ‘bell frogs’ is limited, 

due in part to conflicting findings and methodological limitations of some studies. For 

example, Morgan and Buttemer (1996) conducted controlled predation experiments 

examining the impact upon survival of tadpoles of green and golden bell frog (L. aurea) and 

bleating tree frog (L. dentata) by gambusia. The influence of aquatic vegetation on the 

predatory impact of gambusia was also examined. They found that in the absence of aquatic 

vegetation, gambusia were able to significantly reduce tadpole survival of both species within 

24 hours. In the presence of aquatic vegetation, the effect was substantially reduced, and no 

significant impact of gambusia could be detected on the green and golden bell frog (L. aurea) 

after three days. However, survival of the bleating tree frog (L. dentata) was still significantly 

reduced after two days. These findings indicate that presence of gambusia may significantly 

influence the survival of tadpoles, but that this is likely to be strongly influenced by habitat 

structure and tadpole behaviour. Green and golden bell frog (L. aurea) tadpoles have also 

been found occurring with native predatory fish (Graeme Gillespie pers. obs). In the absence 

of comparative data on the impact of these natural predators upon larval survival, it is difficult 

to assess the relative ecological significance of gambusia predation. 

24


Pyke and White (1996) surveyed waterbodies in the Sydney region for green and golden bell 

frogs (L. aurea), and examined associations between evidence of breeding, occurrence of 

introduced fish, and habitat. They found that breeding was most strongly associated with 

ephemeral rather than permanent or ‘fluctuating’ ponds, followed by the absence of 

introduced fish, primarily gambusia, and speculated that this fish was a major cause of decline 

of green and golden bell frogs. However, examination of their data reveals that pond 

permanency and occurrence of gambusia were highly correlated and so the results could also 

be explained in terms of unmeasured features of pond permanency and also abundance of 

other predators. 

Hamer et al (2002) however, has experimentally demonstrated that the growth of green and 

golden bell frog tadpoles is more favourable in permanent, rather than ephemeral water bodies 

and found that tadpoles did not respond to the presence of gambusia, making them more 

vulnerable to predation. The authors conclude that predation from gambusia may have 

reduced the suitability of permanent water bodies as optimal breeding habitat for green and 

golden bell frogs and that the long-term use of less favourable ephemeral habitats may have 

contributed to the decline of this species. 

White and Ehmann (1997) suggested that gambusia was also implicated in the decline of the 

yellow-spotted tree frog (Litoria flavipunctata), a closely related species to the green and 

golden bell frog. However, Osborne et al. (1996) point out that many of the sites from which 

this species has disappeared do not contain gambusia. Furthermore, both the green and golden 

bell frog (L. aurea) and southern bell frog (L. raniformis) (an ecologically similar species 

which occasionally hybridizes with the green and golden bell frog (L. aurea)) (Watson and 

Littlejohn 1985), have been recorded in abundance at some sites containing gambusia (van de 

Mortel and Goldingay 1998; Graeme Gillespie pers. obs.; Ross Wellington pers. obs.). 

In the USA, introduced fish have been implicated in the decline of the California red-legged 

frog (Rana aurora draytonii) (Hayes and Jennings 1986). However, Lawler et al. (1999) 

experimentally demonstrated that despite the widespread occurrence of gambusia throughout 

the former habitat of the California red-legged frog (R. aurora draytonii), predation from 

introduced American bull frogs (Rana catesbeiana) was likely to be a more substantial 

contributor to the decline of this species. In eastern Australia, gambusia are widespread and 

most abundant in disturbed habitats. To assess the role of gambusia in amphibian declines 

necessitates uncoupling of the impact of gambusia from other potentially threatening 

processes. 

Christy (2001) experimentally examined the cumulative effects of salinity and gambusia on 

the survival of tadpoles of the green and golden bell frog (L. aurea). She found an interaction 

between the effects of salinity and gambusia, which was far more detrimental to tadpoles than 

either factor in isolation. This study demonstrates significant synergistic effects between 

habitat quality and gambusia, which may have contributed to the decline of the green and 

golden bell frog (L. aurea) and related species such as the southern bell frog (L. raniformis). 

25


In summary, gambusia has been shown to kill or injure tadpoles, predate on frog eggs and 

exert some influence over frog habitat selection. However, conclusive evidence for gambusia 

having population level impacts on the abundance of Australian amphibians is yet to be 

clearly determined and is probably complicated by the cumulative effects of other threatening 

processes occurring at that time. Thus no absolute measure of the impact of gambusia on 

threatened frogs exists which may be used for prioritising management actions. So for this 

plan, an objective method for comparing the likelihood of impact between species was 

derived (Appendix 3). This model will then act as a means for prioritising the allocation of 

resources to gambusia control. 

The likelihood of impact on NSW frogs was modelled by comparing factors related to the 

susceptibility of a frog species to gambusia predation. These factors were derived from those 

ecological attributes of frogs, particularly the egg and tadpole stages of the life cycle, that 

make them potentially vulnerable to impacts from gambusia. Factors included in the model 

were, habitat use, diet, fecundity, exposure/protection of eggs, length of larval period and 

anti-predator adaptations. These factors were scored for all threatened and native frogs. 

Species with high scores are considered to be at higher risk of population level impacts from 

gambusia. 

The model identified four threatened species as most likely to be at risk from gambusia 

predation (Appendix 3). They are the endangered green and golden bell frog (L. aurea), 

southern bell frog (L. raniformis) and yellow-spotted bell frog (L. castanea) and the 

vulnerable wallum froglet (Crinia tinnula). The ‘bell frog’ group of species have been 

identified by the NSW Scientific Committee determination process as well as by the scientific 

literature as species likely to be impacted by gambusia. Habitats containing key populations 

of these species will be considered as high priority for management intervention, including 

gambusia removal where feasible. 

Other threatened species such as the vulnerable olongurra frog (Litoria olongburensis) and the 

Nandewar and New England Bioregion endangered population of tusked frog (Adelotus 

brevis) had lower scores and are considered medium priority for gambusia control. The 

remaining threatened species had a very low, or zero score from the model and are 

considered, at this stage, to be a low priority for management. 

A number of non-threatened native frog species were also identified as being at higher risk 

from gambusia predation. They include species in the genera Litoria, Paracrinia and 

Limnodynastes, in particular the brown tree frog (Litoria ewingii), eastern dwarf tree frog 

(Litoria fallax), Perons tree frog (Litoria peronii) and Litoria tyleri, which are generally 

regarded as common and widespread in NSW (Appendix 3). For the purposes of this plan, 

these species will be considered of lower priority for targeted management intervention (ie 

actions that seek to remove gambusia from key sites) as they are less likely to experience 

population level impacts from gambusia predation than high risk threatened species. 

Nevertheless, these species are likely to benefit from the threat abatement actions proposed in 

the plan. 

26


7.5 Benefits to Native Plants and Animals 

The presence of gambusia has not been shown to provide any clear benefits to native plants 

and animals. However, some larger native fishes and birds may occasionally utilise them for 

food. 

8. Control of Gambusia 

Very few documented control programs have been specifically targeted at gambusia, due 

mainly to the absence of control methods, which are both effective and specific for gambusia. 

The only effective control methods kill all fish species present and often other fauna species 

as well as gambusia. Chemical, biological and physical control measures trialed on gambusia, 

and other introduced fish such as carp, have been comprehensively reviewed by McKay et al. 

(2001). A brief summary is provided. 

Chemical Control 

Rotenone is a broad spectrum registered pesticide which is used as a garden insecticide. It is 

produced from the roots of several different plants, most commonly derris root, giving rise to 

the name ‘Derris Dust’. Rotenone has been used to control fish such as gambusia and carp. It 

can be applied to fish by suspension in water, by injection or by ingestion of an oral bait. In 

suspended form, rotenone enters the fish through the gills as the fish respire. It is carried 

through the entire body of the fish and causes the fish to suffocate because oxygen in the 

blood is not released to the tissues. 

Rotenone is not registered as a fish poison in Australia. It is toxic to most fishes and likely to 

impact on other species such as macro-invertebrates and possibly frogs, particularly at the egg 

and tadpole stages. It does break down into harmless by-products relatively quickly and has 

been used successfully in small, enclosed water bodies such as dams, farm dams and ponds 

(Meronek et al. 1996; Koehn et al. 2000). Rotenone has potential to be utilised in the creation 

of gambusia free supplementary habitat for certain frog species such as the green and golden 

bell frog (L. aurea), which utilises small permanent and ephemeral water bodies such as farm 

dams and ponds as breeding habitat. 

Lime and chlorine have also been used to control gambusia. However, neither are registered 

as fish poisons and both are also non-specific, killing most aquatic organisms. Dose rates and 

other impacts on non-target species require clarification. 

Biological Control 

Predation of gambusia by larger species is a control method, which is considered as one with 

high potential in the long-term control of gambusia. There is, however, little (if any) 

quantitative data available to support this technique under natural circumstances. White 

(2001) suggested that mouth almighty (Glossamia aprion) may be a useful adjunct to fish 

poisons in eradicating gambusia from warm temperate ponds. Mouth Almighty (G. aprion) is 

a species native to coastal drainages of (possibly) northern NSW, Queensland, Northern 

27


Territory, northern Western Australia and the southern rivers of Papua New Guinea 

(McDowall 1996). Appropriate stocking levels and potential impacts on tadpoles are not 

known and there would be significant risks associated with translocating this (and other) 

predatory species to locations outside of their normal ranges. 

Other biological control mechanisms such as the use of parasites, pathogens, bacteria and 

viruses have been proposed. However, more research is required to assess their effectiveness 

and impacts on native fauna. Research into molecular biology and biotechnology techniques 

to produce immuno-contraceptives or artificially enhanced pathogens that either kill or 

disable target species via the blocking of reproductive mechanisms may provide an effective 

future control method for gambusia. The Murray Darling Basin Commission is currently 

funding research into manipulating the genetic structure of carp to produce an inheritable 

‘daughterless carp’ (Murray-Darling Basin Commission 2002). The intent of the research is to 

reduce the long-term population of carp by restricting offspring to males. This research will 

initially be trialled on gambusia and therefore may have future benefits to the control of this 

species. 

Physical Control 

The most effective physical control method is likely to be the draining and drying of isolated 

habitats of specific frog species and the reduction of water levels to prevent access by 

gambusia to all or parts of this habitat. This technique is feasible, particularly if the water 

level in the waterbody or wetland can be easily manipulated and the potential for 

reintroduction from either upstream or downstream can be controlled. These practicalities will 

limit the size of waterbodies and wetlands that can be treated. The size and number of 

watercourses entering the waterbody will also be a significant constraint. Some of these 

waterbodies and wetlands may also rely on periodic inundation from nearby watercourses, 

and the feasibility of restricting re-introductions of gambusia from these sources may be 

limited. The draining of these wetland areas has been used to successfully control gambusia in 

isolated ponds found in Alice Springs (McKay et al. 2001). Although it has been determined 

that the area must be dried out entirely as gambusia are able to survive relatively harsh 

conditions with little water for some time. 

Restoration of fully functioning ecosystems and ecological processes may be regarded as an 

indirect method of control. Human induced processes that modify or degrade natural 

environments can favour the establishment and subsequent domination of introduced species. 

The reversal of these processes is likely to benefit many native species (Arthington et al. 

1990). Known habitat preferences of gambusia appear to support this hypothesis. The 

rehabilitation of ecosystem attributes such as habitat structure, stream bed contours, substrate 

type, flow regime, water quality, aquatic plants, riparian vegetation and connectivity between 

habitats should make habitat less favourable to gambusia and more favourable to native 

fishes. Many species of native fish would therefore be able to compete more effectively with 

gambusia. Ross Wellington (pers. comm.) reports that as part of a development approval 

process, artificial green and golden bell frog habitat were designed and installed with draining 

features to allow removal of water and thus facilitate on-going eradication of gambusia. 

28


Public education can be focussed on publicising the impacts on the environment and native 

frog and fish species and reporting the presence of gambusia. The objectives of the public 

education would be substantial reductions in translocations and re-introductions of gambusia 

and increased knowledge of the existing locations of gambusia. 

9. Proposed Management of Gambusia 

9.1 Introduction 

History suggests that the eradication of introduced fish is often impractical and almost always 

unsuccessful (Kailola 1990). This is likely to be the case with gambusia in Australia, where it 

occupies long stretches of inter-connected waterways and many other types of habitats across 

most of its range, and for which effective and species-specific control measures are currently 

lacking. For this reason, an integrated, targeted management strategy is proposed in this threat 

abatement plan, which seeks to contain the spread of gambusia and, where feasible, 

ameliorate the impacts of predation on threatened frogs by: 

minimising further human dispersal of gambusia through implementing enhanced 

government regulation, public education and awareness campaigns 

removing gambusia, where practical, from areas occupied by key populations of priority 

frog species 

creating supplementary gambusia-free habitat, adjacent to gambusia-inhabited 

 

populations of priority frog species, in areas where gambusia removal is considered not 

practical 

collaborating with broader water reform processes that seek to rehabilitate aquatic 

ecosystems and 

informing land managers by undertaking research into the biology and ecology of 

gambusia, its impacts on frogs and the efficacy of proposed control measures 

9.2 Threat Abatement Actions 

The broad objective of this plan is to ameliorate the impact of gambusia on frogs, particularly 

threatened frogs. This will be achieved through implementation of the actions identified 

below. Proposed actions are focussed on threatened frog species most likely to be impacted 

by gambusia and are both landscape and local in scale. Management of gambusia will be 

integrated with other natural resource management programs. The overall key performance 

criteria of this plan will be the increased viability of populations of key threatened frog 

species impacted by gambusia. 

29


Objective 1: Minimise human dispersal of gambusia 

Humans have historically been considered a major mechanism for the spread of gambusia. 

Management actions then should be directed at minimising the influence humans have over 

increasing the range of this species. This may be achieved through increased regulation and 

enhanced public awareness of the ecological consequences of releasing gambusia into the 

environment. It is acknowledged that the dispersal of gambusia through natural events such as 

flooding or other animals, eg birds is unlikely to be prevented and impossible to manage. 

Action 1: Propose gambusia for declaration as a noxious fish in NSW 

The NPWS will liaise with NSW Fisheries to evaluate options and implications for the listing 

of gambusia as a noxious fish under the Fisheries Management Act 1994 (FM Act). A 

declaration of gambusia as noxious will prohibit the sale, possession and introduction of 

gambusia into any waters unless under the authority of a permit issued by NSW Fisheries. In 

addition, Fisheries Officers, under the FM Act will then have the power to seize, take 

possession of and destroy gambusia. 

A noxious declaration will also raise the profile of gambusia as a process threatening frogs in 

NSW and should benefit native frog species and other species such as freshwater fish and 

macro-invertebrates, which are also adversely impacted upon by the species. Media coverage 

and other public awareness actions will be implemented to accompany the proposed 

declaration. This action will be initiated within the first year of the plan. 

Action 2: Develop and disseminate education and awareness material 

The purpose of this action is to target those groups that may be responsible for, or have some 

association with, the maintenance and dispersal of gambusia in the environment. Groups to be 

targeted include: 

councils 

ornamental fish suppliers, keepers of aquarium fish and reptiles such as freshwater turtles 

property owners with farm dams 

school children and the 

general public 

NPWS will prepare and implement an education strategy that informs each target group about 

the poor record of gambusia as a mosquito control agent and the subsequent potential impacts 

of deliberately, or inadvertently dispersing gambusia into the environment. The strategy will 

also provide practical methods for landholders to restore frog habitat and where practical 

remove gambusia from a waterbody. Education and awareness resources for consideration 

include fact sheets, continued dissemination of diverse frog conservation information (such as 

the NPWS (2001a) ‘Helping Frogs Survive’ brochure), NPWS Internet displays and media 

interviews. 

30


NPWS will seek to collaborate with existing frog recovery projects and link with other 

government department initiatives (including those identified in Action 8 below) and interest 

group programs, which can assist with raising awareness about gambusia. They include: 

DLWC, NSW Waterwatch and Bugwatch programs, Sydney Water Streamwatch program, 

NSW Fisheries, Department of Education Field Study Centres, Pet Industry Joint Advisory 

Council, Frog and Tadpole Society and the Australia New Guinea Fishes Association. 

The dissemination of educational material will be focussed on areas containing priority frog 

species most likely to be impacted by gambusia (refer to Objective 2, Action 4). 

Action 3: Prepare environmental assessment guidelines for consultants and consent 

authorities 

The NPWS will prepare advice to assist consultants, Councils and government agencies 

responsible for making an assessment or determination of the likelihood of a development or 

activity, which may result in the introduction of gambusia into an area occupied by threatened 

frogs or a change to existing interactions between gambusia and threatened frogs. This action 

is relevant to part (g) of the ‘eight part test’ (ie Section 95 of the EP&A Act) when deciding if 

a proposed development is likely to have a significant impact on the survival of threatened 

species, populations and ecological communities. 

Performance Criteria: Within the first year of the commencement of this plan, options for 

declaring gambusia noxious in NSW will be evaluated. Education and awareness material will 

be prepared and disseminated in the first two years of the plan. Within five years of 

commencement of the plan, public awareness of the issue and measures to mitigate ongoing 

human dispersal will be known amongst the target groups listed in actions two and three. 

Objective 2: Reduce impacts of gambusia on threatened frog species at key sites 

This component of the plan is directed at ameliorating the impacts of gambusia at the local 

scale. Actions are targeted at those threatened frogs, (ie green and golden bell frog (L. aurea), 

southern bell frog (L. raniformis), wallum froglet (C. tinula), olongurra frog (L. 

olongburensis) and the endangered population of tusked frog (A. brevis)) identified as being 

of high to medium risk of impact from gambusia. The yellow-spotted bell frog (L. castanea) 

has not been recorded since 1973 (NPWS 2001b). Actions for this species would be triggered 

when extant populations are located. 

Action 4: Survey threatened frog habitats for gambusia 

Surveys for gambusia, within and adjacent to habitats of high and medium priority threatened 

frog species will be conducted. Information from these surveys will improve our knowledge 

of the local distribution of gambusia in these habitats, particularly areas that are currently 

gambusia-free. Education and awareness resources identified in Action 2 can then be targeted 

to these areas, in an effort to prevent further human dispersal of gambusia. 

31


NPWS will coordinate these surveys and where possible undertake them in conjunction with 

surveys proposed in future recovery plans for these threatened frog species. The support of 

volunteers or other groups collecting similar survey data will also be utilised where available. 

This action will be implemented within the first two years of the plan, with a follow up survey 

in year five. 

Action 5: Remove gambusia at key sites for high priority threatened species 

Total removal of gambusia from the environment is not feasible at this time. However, 

opportunities may exist to control gambusia at key sites for high priority threatened frog 

species. 

Selection of sites containing key populations of these species will be made in consultation 

with the relevant NPWS recovery plan coordinator. Each site will then be assessed to 

ascertain the practicality of removing gambusia. Sites that may be suitable for this action are 

small, enclosed waterbodies or isolated pools such as farm dams or drying creek beds where 

the likelihood of permanent gambusia removal is high and where the potential impacts to nontarget 

species from the control method(s) used are considered low. 

A pilot case study for each species will then be implemented to gauge the efficacy of the 

control technique and evaluate the response of the target species. If the control program is 

considered successful it will then be expanded to other suitable sites. Permission will be 

sought from land holders and relevant approval authorities prior to implementing any control 

measure such as draining a waterbody or applying a chemical treatment. 

NPWS will coordinate the implementation of this action. 

Action 6: Create gambusia-free supplementary habitat key sites for high priority threatened 

frog species 

At sites where the removal of gambusia is not considered practical, opportunities for the 

creation of supplementary frog breeding habitat will be evaluated. The intent of this action is 

to develop areas of gambusia-free habitat adjacent to existing gambusia-inhabited habitat in 

key population areas. Supplementary habitat may be ephemeral in nature or periodically 

drained to ensure it remains free from gambusia. 

Identification of suitable sites will be made in consultation with the relevant recovery plan 

coordinator and will be undertaken in conjunction with the evaluation of sites for removal of 

gambusia. 

NPWS will coordinate the implementation of this action. 

32


Action 7: Monitor the response of threatened frog species to the creation of gambusia-free 

habitat 

NPWS will establish a rigorous monitoring program to measure the response of key 

threatened frogs at sites where gambusia is removed and where supplementary gambusia-free 

habitat is created. NPWS will undertake this action on an ongoing annual basis following the 

implementation of Actions 5 and 6 above. 

Performance Criteria 

Within two years of commencement of the plan, the presence or absence of gambusia in 75% 

of all habitats for high and medium risk frog species will be determined. In addition, within 

the life of the plan, programmes for the removal of gambusia and the creation of 

supplementary habitat for high priority threatened frog species will be established. The 

response of threatened frogs to gambusia removal and supplementary habitat creation 

programmes will be measured at these sites and reported on at the completion of the plan. 

Objective 3: Integrate this threat abatement plan with other aquatic restoration 

programs 

Modified habitats are particularly susceptible to gambusia invasion and effective amelioration 

of the impacts of gambusia can only be achieved through the restoration of aquatic 

ecosystems at the landscape scale (see section 6.2). This threat abatement plan therefore needs 

to link with other broad-scale water reform processes that seek to address aspects of habitat 

disturbance favoured by gambusia such as river impoundment, declines in water quality, 

changes to natural discharge patterns, thermal regimes and bank and channel alterations. 

Action 8: Link this threat abatement plan to broad-scale water reform and river health 

programs 

Current government legislation and programs relevant to this action are described in Section 2 

of the plan and include: 

Water Management Act 2000 

Catchment Management Amendment Bill 2001 (not yet passed) and the Catchment 

Management Act 1989 

NSW Weirs Policy 1995 

The NSW Wetlands Management Policy 1996 

There is no statutory requirement for threat abatement plans prepared under the TSC Act to be 

incorporated in, or implemented through the various legislative instruments described above. 

However, the NPWS is represented on various committees committed to implementing water 

reform programs and will advocate the importance of restoring aquatic ecosystems for the 

purpose of reducing the impacts of gambusia on frog species. Copies of this threat abatement 

plan will be provided to all relevant government agencies and committees responsible for 

river health and aquatic restoration programs. 

33


The NPWS will also seek to liaise with NSW Fisheries during the preparation of a future 

threat abatement plan for the listed key threatening process Introduction of fish to fresh waters 

within a river catchment outside their natural range as required by the FM Act. 

Performance Criteria: 

All relevant national and state government agencies and committees will receive copies of the 

plan in the first year of commencement. Contact with NPWS representatives participating on 

various water reform committees will be made and annual updates on the progress of the plan 

provided. Also information from these committees will be used to better implement this plan 

where possible. 

Objective 4: Increase knowledge of the general ecology of gambusia, its impact on native 

frog species and mechanisms for its control. 

Research is required to address gaps in the knowledge of the biology and ecology of 

gambusia, its impact on native species and the efficacy and impacts of proposed control 

measures. Information derived from these studies will assist in refining current management 

practices and/or develop new approaches to the control or removal of gambusia from the 

environment. Recommended areas of research are identified in the actions below. It is 

proposed that each of these actions be undertaken by, or in partnership with an academic 

institution or government research organisation eg NSW Fisheries. 

Action 9: Conduct investigations into factors limiting the establishment of gambusia in 

nature 

Proposed topics for research include: 

factors limiting the distribution and abundance of gambusia. Proposed studies would 

assess habitat preferences of gambusia to better understand factors, which influence their 

establishment and govern their rate of increase at a site 

comparing the distribution and abundance of gambusia in undisturbed and modified 

habitats to gain a better understanding of what habitat conditions influence their presence 

and 

identifying mechanisms of gambusia dispersal, to better understand how gambusia 

invades different habitats, particularly those likely to support frogs at risk from gambusia 

predation 

Action 10: Investigate the impacts of gambusia on frog species 

Evidence linking gambusia to declines in native and threatened frogs is currently 

inconclusive. Research is required to clarify the impacts of gambusia on frogs and to ascertain 

the role of gambusia in the decline of frog species assemblages in synergy with other 

processes that also threaten their survival eg frog chytrid fungus. This plan has identified a 

number of threatened and native frog species most susceptible to population level impacts 

from predation by gambusia (Appendix 3). Further research is also recommended into the 

34


palatability of frog eggs and tadpoles to gambusia, and the effectiveness of tadpole survival 

strategies in avoiding negative interactions with gambusia would also clarify the likely impact 

of gambusia predation on frog species. 

Action 11: Trial Rotenone as a gambusia control technique 

Rotenone has potential in certain circumstances to be an effective gambusia control agent 

(refer to section 8 of this plan). Trials will be undertaken to identify suitable dose levels and 

assess potential impacts to non-target species. Potential sites will be selected in consultation 

with the EPA and NSW Fisheries, and will be confined to small, enclosed waterbodies. 

An application for a research permit for the trials will be made with the National Registration 

Authority once suitable sites have been identified. 

Action 12: Monitor ongoing research into the control of gambusia 

The NPWS threat abatement plan coordinator will monitor ongoing advances in approaches to 

the management of pest species that may, or are being adapted to the management of 

gambusia eg Murray Darling Basin Commission funding of CSIRO research into 

‘daughterless carp technology’. 


Throughout the life of this plan, NPWS will seek partnerships and encourage research into the 

ecology of gambusia, its impacts on frogs and the development and efficacy of potential 

gambusia control measures. Relevant research outcomes will be incorporated into 

management. 

Objective 5: Ensure effective implementation of the threat abatement plan 

Implementation of this plan will require ongoing statewide coordination. 

Action 13: Provide support for the implementation of this plan 

Successful implementation of this threat abatement plan will require ongoing coordination 

through continued liaison with threatened frog recovery program coordinators, consultation 

with stakeholders such as NSW Fisheries, EPA, Local Councils, DLWC and individual 

landowners. The threat abatement coordinator will be responsible for overall implementation 

of threat abatement actions and communication of results of the plan to land management 

agencies, landholders and the public. 


Each of the actions identified in the plan will be initiated by the threat abatement coordinator 

within the prescribed timeframes (assuming funds for implementation are available). Progress 

toward objectives assessed by the performance criteria will be reviewed and updated in year 

five of plan. 

35


10. Economic and Social Impacts of the Plan 

The total cost of implementing the plan is estimated to be approximately $220,000 over five 

years. A breakdown of costs per action per year is provided in Appendix 4. Expected costs are 

approximations, which may require revision once actions are initiated. Some costs, identified 

in the plan may be partly absorbed by other recovery plans for threatened frog species or met 

by funding programs such as the NSW Biodiversity Strategy and Commonwealth 

Government Natural Heritage Trust. Approximately half the projected costs are attributed to 

research actions. The estimated cost of these actions may also be substantially reduced 

through the acquisition of a funding grant eg ARC Linkage Grant. Other economic impacts 

associated with implementation of this plan are likely to be minimal. 

The major social benefit of ameliorating the impact of predation by gambusia will be meeting 

the desire of many persons in the community to protect native frogs, particularly threatened 

species. Implementation of the actions proposed in the plan may also benefit other groups of 

native species such as fish and macro-invertebrates. No impact on Aboriginal heritage is 

expected. There are unlikely to be any significant animal welfare issues related to this plan. 

Chemical control trials will be undertaken with the appropriate environment assessment, 

animal care and ethics and other relevant approvals to minimise non-target impacts. 

11. Review Date 

This threat abatement plan is to be formally reviewed by the NPWS in consultation with 

NSW Fisheries within five years of commencement of the plan. 

36


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Personal Communications 

Graham Gillespie - Department of Natural Resources and Environment, Victoria 

Jamie Knight – NSW Fisheries 

Jared Patrick - Pest Industry Joint Advisory Council 

Steve Saddlier - Department of Natural Resources and Environment, Victoria 

Ross Wellington - National Parks and Wildlife Service, NSW 

Arthur White – Frog and Tadpole Society of NSW 

Personal Observations 

Angela Arthington – Griffith University, Queensland 

Steve Saddlier - Department of Natural Resources and Environment, Victoria 

Graham Gillespie - Department of Natural Resources and Environment, Victoria 

49


Appendix 1: NSW Scientific Committee Final Determination 

The Scientific Committee, established by the Threatened Species Conservation Act, has made 

a Final Determination to list Predation by Gambusia holbrooki (Plague Minnow) as a KEY 

THREATENING PROCESS on Schedule 3 of the Act. Listing of Key Threatening Processes 

is provided for by Division 2 Part 2 of the Act. 

The Scientific Committee has found that: 

50 

1. Gambusia holbrooki Girard, 1859 (previously known as Gambusia affinis) (Plague 

Minnow, also known as Mosquito Fish) is a small freshwater fish originally 

introduced into Australia in the 1920s. The fish was imported as an aquarium fish but 

some were released into creeks around Sydney, Melbourne and Brisbane. 

2. During the Second World War a government sponsored campaign was initiated to 

spread Gambusia holbrooki into as many east coast waterways as possible, as a 

control agent for mosquitoes. 

3. Gambusia holbrooki is an aggressive and voracious predator. Overseas research has 

documented its impact on fish, invertebrates and frogs. (Grubb, J.C. 1972. American 

Midland Naturalist 88, 102-8; Hurlbert, S.H., Zedler, J. & Fairbanks, D. 1972. 

Science 175, 639-41) 

4. Recent research has documented that Gambusia holbrooki preys upon eggs and 

tadpoles of the green and golden bell frog, Litoria aurea (Morgan, L.A. & Buttermer, 

W.A. 1996. Australian Zoologist 30, 143-149, White, A.W. & Pyke, G.H. 1998 

unpublished manuscript submitted to Australian Zoologist). 

5. Other studies have demonstrated that Gambusia also preys upon Litoria dentata 

(Morgan & Buttermer op.cit), Litoria lesueuri (White & Pyke, op.cit) and 

Limnodynastes peronii (Webb, C. & Joss, J. 1997. Australian Zoologist 30, 316-26). 

6. Presence of Gambusia holbrooki has been linked to the decline of Litoria aurea, the 

New England Bell Frog Litoria castanea, Southern Bell Frog Litoria raniformis, and 

the Southern Tablelands Bell Frog (Litoria sp.) 

7. Breeding by Litoria aurea is almost completely restricted to water bodies lacking 

Gambusia holbrooki. 

8. In view of 3, 4, 5, 6 above the Scientific Committee is of the opinion that Predation 

by Gambusia holbrooki is a serious threat to the survival of Litoria aurea and Litoria 

castanea, both species listed as threatened under the Threatened Species 

Conservation Act, and to other species of frog, and that predation by Gambusia 

holbrooki is therefore eligible to be listed as a key threatening process because it 

adversely affects two or more threatened species and it could cause species that are 

not threatened to become threatened. 

Exhibition period: 29/1/99 - 12/3/99


Appendix 2: NSW rivers survey records of Gambusia 

Rivers and their catchments where gambusia have been recorded during the NSW Rivers 

Survey 1994-1996 (Faragher and Lintermans 1997). 

51


Macquarie River Catchment 

Turon 

Little 

Talbragar 

Macquarie 

Bogan 

Duckmaloi 

Fish 

Namoi River Catchment 

MacDonald 

Peel 

Cockburn 

Hawkesbury River Catchment 

Cox’s 

Mangrove Creek 

Hunter River Catchment 

Hunter 

Goulburn 

Macleay River Catchment 

Gara 

Macleay 

Gwydir River Catchment 

Horton 

Gwydir 

Shoalhaven River Catchment 

Shoalhaven 

Murrumbidgee River Catchment 

Yass 

Colombo Creek 

Lachlan River Catchment 

Retreat 

Clarence River Catchment 

Clarence 

Orara 

Richmond River Catchment 

Richmond 

Leycester Creek 

Manning River Catchment 

Gloucester 

52


Appendix 3: Development of a rank scoring system to predict 

gambusia impact on native frog species 

A model to rank the likelihood of population level impacts of gambusia predation on native 

frogs, including threatened species and endangered populations, has been prepared to guide 

management and better target threat abatement actions. Published scientific literature of the 

impacts of gambusia predation on frogs is relatively scarce, being predominantly restricted to 

a small number of species. However, it is acknowledged that frogs do possess ecological 

attributes that render them susceptible to predation and as a precaution, this plan advocates an 

adaptive approach to their management ie undertake some management intervention for those 

frogs species likely to have some population level impacts concurrent with ongoing research 

that seeks to clarify the existence and degree of impact. 

The model for the likelihood of impact is defined as: 

Sensitivity rating = (microhabitat score) x (dietary overlap + fecundity + exposure/protection 

of eggs + length of larval period + anti-predator avoidance) 

This model gives particular emphasis to microhabitat (multiplicative factor), so that those 

stages of the frog life cycle (ie their eggs and tadpoles) which occur in inaccessible habitats or 

habitats unlikely to be invaded by gambusia score zero. 

Threatened species comprise the first group of frogs in the accompanying table followed by 

the remaining native frog species. 

Frog Microhabitat Use 

This factor describes the accessibility of frog eggs and tadpoles to predation and/or 

interference competition from gambusia. Frog species, which breed in habitat unlikely to be 

accessed by gambusia score zero. 

0 - Eggs and tadpoles occur in habitats inaccessible to gambusia. That is, where reproduction 

is totally terrestrial for all stages of the life cycle, or where the life cycle is partially aquatic 

but associated with water bodies unlikely to be colonised by gambusia (eg isolated ephemeral 

pools or high discharge first or second order streams). 

1 - Eggs and tadpoles are partially aquatic with gambusia having potential to opportunistically 

occupy habitats such as billabongs, farm dams or ox-bows through flooding or human 

dispersal. 

1 - Eggs and tadpoles are aquatic with a minimal chance of gambusia being present because 

they are not connected to permanent streams or waterbodies (eg ephemeral clay pans or sand 

dune swales etc). 

2 - Eggs and tadpoles are aquatic and occur in slow or moderate discharge third and fourth 

order streams with a reasonable chance of gambusia being present, or 

53


2 – Eggs and tadpoles are aquatic and are associated with a broad range of waterbody types, 

including flooded areas, permanent ponds and slow moving streams where there is a 

reasonable chance of gambusia being present in some of these habitats. 

3 - Those frog species whose life cycle occurs predominantly in permanent ponds, lentic 

interconnecting pools or slow flowing low altitude streams which gambusia can easily access. 

Dietary Overlap 

This factor describes the potential impacts of gambusia on frog species (essentially their 

tadpole stage) which have similar dietary preferences. Competition for food may occur in 

areas where resources are limiting. 

0 - None or minor overlap in diet ie tadpoles that are mostly herbivorous 

1 - High overlap in diet ie tadpoles that are predominantly macro-invertebrate or insect 

feeders 

Fecundity 

This factor describes the potential for frog species with a higher intrinsic rate of increase to 

compensate for mortality from gambusia predation on eggs. 

0 – High fecundity > 1000 eggs 

1 – Moderate fecundity of 500 to 1000 eggs 

2 – Low fecundity < 500 eggs 

Exposure/protection of eggs 

This factor describes the specific reproductive characteristics of frog spawn which place it at a 

higher risk of impact from predators. It assumes that frog species with foam egg masses are 

less vulnerable to predation than those with loose, simple egg masses. 

0 - Terrestrial egg mass not accessible to gambusia 

1 - Egg masses occur in aquatic habitat not easily accessible to gambusia eg in burrows, 

amongst litter, under rocks or on banks above water level. 

2 - Species with foam egg masses in aquatic habitats accessible to gambusia 

3 - Species with simple egg masses with loose eggs or clumps of eggs in exposed situations 

readily accessible to gambusia eg attached to submerged vegetation 

Length of larval period 

This factor describes the potential for frog species to be at higher risk of predation, if the 

aquatic stage of the life cycle occurs over a longer time frame. 

54


1 - Egg and tadpole periods less than 3 months 

2 – Egg and tadpole periods of approximately 3 months or greater 

Anti-predator adaptation 

This factor describes the potential for tadpoles to decrease the probability of predation 

through responses such as avoidance behaviour. Very little information is known on this 

factor, so scores are based on subjective opinion rather than known fact. 

0 – behaviour possibly effective in decreasing risk of predation eg schooling of tadpoles, or 

avoidance actions such as hiding amongst vegetation 

1 – no such behaviours currently known 

55


Scientific name Common 

name 

Endangered 

Frogs 

Litoria aurea Green and 

Golden 

Frog 

Bell 

Litoria castanea Yellow spotted 

Tree frog 

Litoria raniformis Southern Bell 

Frog 

Mixophyes iteratus Giant Barred 

Frog 

Litoria 

booroolongensis 

Neobatrachus 

pictus 

56 

Booroolong 

Frog 

Painted 

Burrowing Frog 

Microhabitat Dietary 

Overlap 

Fecundity Exposure 

protection of 

eggs 

Length of 

larval period 

Anti-predator 

avoidance 

Sensitivity 

Ranking 

3 1 0 3 1 1 18 

3 1 0 3 1 1 18 

3 1 0 3 1 1 18 

2 1 0 1 2 0 8 

2 0 0 1 1 1 6 

1 0 1 3 1 1 6 

Litoria spenceri Spotted Frog 0 0 1 1 2 1 0 

Mixophyes fleayi Fleay's Barred 

Frog 

Pseudophryne 

corroboree 

Endangered 

Populations 

Adelotus brevis 

Nandewar and New 

England Bioregions 

Southern 

Corroboree Frog 

0 1 2 1 2 1 0 

0 1 2 0 2 1 0 

Tusked Frog 2 1 2 1 2 0 14



name 


Overlap 


protection of 

eggs 

Length of 

larval period 

Anti-predator 

avoidance 

Sensitivity 

Ranking 

Vulnerable 

Frogs 

Crinia tinnula Wallum Froglet 3 0 2 2 1 1 18 

Litoria 

olongburensis 

Olongurra Frog 2 1 1 3 1 0 12 

Assa darlingtoni Pouched Frog 0 0 2 0 1 1 0 

Heleioporus 

australiacus 

Litoria 

brevipalmata 

Giant Burrowing 

Frog 

Green-thighed 

Frog 

Litoria littlejohni Littlejohn’s Tree 

Frog 

0 1 1 1 2 1 0 

0 0 2 3 1 1 0 

0 1 2 3 2 1 0 

Litoria piperata Peppered Frog 0 0 2 2 2 0 0 

Litoria 

subglandulosa 

Glandular Frog 0 0 2 2 2 0 0 

Mixophyes balbus Stuttering Frog 0 1 1 1 2 0 0 

Philoria 

kundagungan 

Mountain Frog 0 0 2 0 1 1 0 

Philoria loveridgei Loveridge's Frog 0 0 2 0 1 1 0 

Philoria 

sphagnicola 

Pseudophryne 

australis 

Pseudophryne 

pengilleyi 

Sphagnum Frog 0 0 2 0 1 1 0 

Red-crowned 

Toadlet 

Northern 

Corroboree Frog 

0 0 2 0 2 1 0 

0 0 2 0 2 1 0 

57



name 

Other native species 

58 

Litoria ewingii Brown Tree 

Frog 

Litoria fallax Eastern Dwarf 

tree Frog 

Litoria peronii Perons Tree 

Frog 


Overlap 


protection of 

eggs 

Length of 

larval period 

Anti-predator 

avoidance 

Sensitivity 

Ranking 

3 1 1 3 2 1 24 

3 1 1 3 1 1 21 

3 1 0 3 2 1 21 

Litoria tyleri 3 1 1 3 1 1 21 

Litoria verreauxii 3 1 1 3 1 0 18 

Paracrinia 

haswelli 

Haswells Frog 2 1 2 3 2 0 16 

Litoria freycinetti Freycinet’s frog 2 1 2 3 1 1 16 

Limnodynastes 

dumerilii 

Crinia 

parinsignifera 

Eastern Banjo 

Frog 

Crinia signifera Common 

Eastern froglet 

3 1 0 2 2 0 15 

2 0 2 3 1 1 14 

2 0 2 3 1 1 14 

Crinia sloanei 2 0 2 3 1 1 14 

Litoria gracilenta Dainty Green 

Tree Frog 

2 1 1 3 1 1 14 

Litoria latopalmata 2 1 1 3 1 1 14 

Litoria 

pearsoniana 

2 0 2 2 2 1 14 

Litoria jervisiensis Jervis Bay Tree 2 1 1 3 1 1 14



name 


Overlap 


protection of 

eggs 

Length of 

larval period 

Anti-predator 

avoidance 

Sensitivity 

Ranking 

Adelotus brevis 

Frog 

Tusked Frog 2 1 2 1 2 0 12 

Mixopheyes 

fasciolatus 

Great Barred 

Frog 

3 1 0 1 2 0 12 

Litoria nasuta Rocket Frog 2 1 0 3 1 1 12 

Litoria phyllochroa Leaf Green Tree 

Frog 

Litoria fletcheri Long-thumbed 

frog 

Litoria interioris Giant Banjo 

frog 

Litoria. peronii Brown-striped 

Frog 

Litoria salmini Salmon-striped 

Frog 

Litoria 

tasmaniensis 

Litoria 

terraereginae 

Spotted grass 

Frog 

Northern Banjo 

Frog 

2 0 2 3 1 0 12 

2 1 0 2 2 0 10 

2 1 0 2 2 0 10 

2 1 0 2 2 0 10 

2 1 0 2 2 0 10 

2 1 0 2 2 0 10 

2 1 0 2 2 0 10 

Uperoleia fusca 2 0 2 1 1 1 10 

Uperoleia 

capitulata 

Uperoleia 

laevigata 

2 0 2 1 1 1 10 

2 0 2 1 1 1 10 

Uperoleia tyleri 2 0 2 1 1 1 10 

59



name 

60 


Overlap 


protection of 

eggs 

Length of 

larval period 

Anti-predator 

avoidance 

Sensitivity 

Ranking 

Uperoleia rugosa 2 0 2 1 1 1 10 

Litoria lesueuri Leseurs Frog 3 1 0 1 1 0 9 

Litoria ornatus Ornate 


Litoria citropa Blue Mountains 

Tree Frog 

Lechriodus 

fletcheri 

Neobatrachus 

centralis 

Neobatrachus 

sudelli 

Litoria rubella Desert Tree 

Frog 

Geocrinia 

victoriana 

2 1 0 2 1 0 8 

2 0 0 3 1 0 8 

Fletchers frog 1 1 2 1 1 1 7 

Trilling Frog 1 1 1 3 1 1 7 

1 1 1 3 1 1 7 

1 1 1 3 1 1 7 

1 0 2 1 2 1 6 

Notaden bennettii Crucifix Toad 1 0 1 3 1 1 6 

Cyclorana 

verrucosa 

1 1 0 3 1 1 6 

Cyclorana brevipes 1 1 1 3 1 0 6 

Cyclorana 

novaehollandiae 

Cyclorana 

alboguttata 

Striped 


1 1 1 3 1 0 6 

1 1 0 3 1 1 6 

Litoria caerulea Green Tree Frog 1 1 0 3 1 1 6



name 

Litoria chloris Red-eyed tree 

frog 

Cyclorana 

platycephala 


Overlap 


protection of 

eggs 

Length of 

larval period 

Anti-predator 

avoidance 

Sensitivity 

Ranking 

1 1 0 3 1 1 6 

1 1 0 3 1 0 5 

Crinia deserticola 0 0 2 3 1 1 0 

Pseudophryne 

bibronii 

Pseudophryne 

coriacea 

Pseudophryne 

dendyi 

Brown toadlet 0 0 2 0 2 1 0 

Red-backed 

Toadlet 

Litoria dentata Bleating Tree 

Frog 

0 0 2 0 2 1 0 

0 0 2 0 2 1 0 

0 1 0 3 1 0 0 

Litoria revelata 0 1 2 1 1 1 0 

61


Appendix 4: Threat abatement plan cost table 

Estimated costs of implementing the actions identified in this threat abatement plan. 

Action 

No: 

62 

Action Title Priority Estimated Cost/yr Total 

Cost 

Yr 1 Yr 2 Yr 3 Yr 4 Yr 5 

1 Proposal to declare 

gambusia as ‘noxious’ 

2 Develop education and 

awareness tools 

3 Provide environmental 

assessment advice 

Responsible 

party/funding source 

1 $3500 $3500 NPWS $3500 

In-Kind 

Funds 

1 $3500 $7000 $3500 $14000 NPWS $7000 $7000 

1 $700 $700 NPWS $700 

4 Survey for gambusia 2 $2000 $3500 $3500 $9000 NPWS $9000 

5 Undertake targeted control 1 $6000 $5000 $5000 $16000 NPWS $10000 $6000 

6 Create supplementary 

habitat 

7 Monitor response of 

threatened frogs to 

gambusia removal 

8 Encourage and participate 

in broad scale river health 

programs 

9 Investigate factors limiting 

dispersal of gambusia 

10 Clarify impacts of gambusia 

on frogs 

11 Undertake chemical control 

trials 

12 Monitor progress of 

research 

1 $6000 $5000 $11000 NPWS $6000 $5000 

1 $8000 $8000 $8000 $24000 NPWS $15000 $9000 

2 $0 $0 $0 $0 $0 $0 NPWS 

2 $17000 $17000 $17000 $51000 NPWS, academic 

institution &/or other 

research institution 

2 $11000 $7000 $12000 $30000 NPWS, academic 



2 $22000 $22000 NPWS, academic 



2 $0 $0 $0 $0 $0 $0 NPWS 

13 Coordinate plan 1 $10500 $10500 $7000 $7000 $7000 $42000 NPWS $42,000 

Total $223,200 $93,200 $130,000 

Priority ratings are: 1- Action critical to meeting plan objectives, 2-Action contributing to meeting plan objectives. ‘In-Kind’ Funds represent salary component of permanent staff and current resources. 

‘Cash’ Funds represent the salary component for temporary staff and other costs such as travel and the purchasing of equipment. 

Recovery Plan Coordination includes all actions associated with ‘in-kind’ administration and general implementation of the recovery plan and is assumed to absorb costs associated with actions 8 and 12. 

Cash 

Funds 

$51,000 

$30,000 

$22,000

Predation by Gambusia holbrooki - The Plague Minnow

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