SCI. MAR., 68 (4): 597-603
SCIENTIA MARINA
2004
Food of sand smelt, Atherina boyeri Risso, 1810
(Pisces: Atherinidae) in the estuary of the Mala Neretva
River (middle-eastern Adriatic, Croatia)*
VLASTA BARTULOVIĆ 1, DAVOR LUČIĆ 2, ALEXIS CONIDES3,
BRANKO GLAMUZINA4, JAKOV DULČIĆ 4, DUBRAVKA HAFNER5
and MIRNA BATISTIĆ 2
1
Department of Aquaculture, University of Dubrovnik, Ćira Carića 4, 20000 Dubrovnik, Croatia. E-mail: vlasta@unidu.hr
2 Institute of Oceanography and Fisheries, Kneza Damjana Jude 12, Dubrovnik, Croatia.
3 Hellenic Centre for Marine Research, Agios Kosmas, Agios Kosmas, Hellinikon, 16604 Athens, Greece.
4 Institute of Oceanography and Fisheries, Split, Croatia.
5 Faculty of Education, University of Mostar, Mostar, Bosnia-Herzegovina.
SUMMARY: The feeding habits of sand smelt, Atherina boyeri Risso, 1810 in the mouth of the Mala Neretva River were
studied by examining the stomach contents of 1236 fishes collected from March 2001 to February 2002. Thirteen different
food categories were identified. The greatest diversity was recorded in winter, especially in January, when 11 categories
were present. The minimum (4) was in September, and was associated with the highest number of empty stomachs. Marine
and estuarine species represented the bulk of the prey, while typically fresh-water species (Daphnia spp. and Cyclops spp.)
were found only sporadically. Crustaceans were the most common prey and, of these, the most common were copepods
(45%), followed by gammarid amphipods (34%), cladocerans (13%), decapod larvae (12%) and mysids (8%). Insects represented 7%. The percentage number of prey showed high seasonal variations. Copepods dominated in autumn and winter,
cyclopoids in April, poecilostomatoids and harpacticoids in December, and calanoids in January and February. Decapod larvae dominated in March-May, and mysids in July-August. PCA and clustering analysis was performed on the prey data in
order to reveal associations between prey items or seasonal similarities. The opportunistic mode of feeding of the species
Atherina boyeri is also discussed.
Key words: sand smelt, food, Neretva River estuary, Croatia.
RESUMEN: ALIMENTACIÓN DEL PEJERREY, ATHERINA BOYERI RISSO, 1810 (PISCES: ATHERINIDAE) EN EL ESTUARIO DEL RÍO
NERETVA (ADRIATICO MEDIO-ESTE, CROACIA). – Los hábitos alimenticios del pejerrey, Atherina boyeri Risso, 1810 en la
boca del río Mala Neretva se estudiaron mediante el examen del contenido estomacal de 1236 peces recolectados desde
marzo de 2001 a febrero de 2002. Se identificaron 13 categorías diferentes de alimento. La mayor diversidad se observó en
invierno, especialmente en enero, en que estuvieron presentes 11 categorías.. El mínimo (4) se observó en septiembre, y se
asoció al mayor número de estómagos vacíos. Las especies marinas y estuáricas representaron el principal grupo de presas,
mientras que especies típicas de agua dulce (Daphnia spp. y Cyclops spp.) se encontraron sólo esporádicamente. Los crustáceos fueron las presas más comunes, siendo los copépodos los más frecuentes (45%), seguido de los amfípodos gammaridos (34%), cladóceros (13%), larvas de decápodos 12%, misidáceos 8%, e insectos 7%. El porcentaje de presas mostró
una alta variación estacional. En general, los copépodos dominaron en otoño e invierno: los Cyclopoides fueron más numerosos en abril, los poecilostomátidos y harpacticoides en diciembre y los calanoides en enero y febrero. Las larvas de decápodo dominaron en marzo-mayo y los misidáceos en julio-agosto. Con los datos de presas se realizaron análisis multivariantes para revelar asociaciones entre presas o similaridades estacionales. En este trabajo se discute, también, el modelo de
alimentación de tipo oportunista de la especie Atherina boyeri.
Palabras clave: pejerrey, alimentación, río Mala Neretva, Croacia.
*Received January 12, 2004. Accepted April 29, 2004.
FOOD OF ATHERINA BOYERI IN THE ESTUARY 597
INTRODUCTION
The sand smelt, Atherina boyeri [Risso 1810], is
common in the Mediterranean and its adjacent seas,
and is also found in the northeast Atlantic, from the
Azores to the northwest coast of Scotland. It is a
small, short-lived, euryhaline (Quignard and Pras,
1986) fish that inhabits mainly coastal and estuarine
waters and, more rarely, inland waters. The species
has also been introduced into freshwater lakes and
reservoirs for stock enhancement purposes (Economidis et al., 2000).
Sand smelt are opportunistic feeders. In typical
coastal ecosystems they prey on zooplankton, while
in lagoons and estuaries they feed on benthic organisms (Kiener and Spillman, 1969; Trabelsi et al.,
1994). Some authors have found that sand smelt
feed mainly on zooplankton and only secondarily on
benthic organisms (Rosecchi and Crivelli, 1992),
while others have found a preference for benthic
prey (Scipiloti, 1998). Research based on the ratios
of stable carbon and nitrogen isotopes also suggests
a clear feeding preference for benthic organisms,
mainly Isopoda and Mysidacea (Vizzini and Mazzola, 2002). Some of the differences in conclusions
drawn from gut and isotope analyses may relate to
differences in ingested versus assimilated food
(Scipiloti, 1998; Vizzini and Mazzola, 2002). Other
studies have identified changes in food selection
related to seasons (Trabelsi et al., 1994; Scipiloti,
1998) and fish size (Ferrari and Rossi, 1983-84;
Rosecchi and Crivelli, 1992; Scipiloti, 1998).
Besides its direct commercial importance, the
sand smelt plays an important role in the estuarine
food webs. For example, it is one of the most
important links between planktonic, benthic and
organisms that regularly, although only temporarily, enter estuarine food webs, such as insects, and
carnivorous fish species in the estuary. Sand smelt
largely dominated (49.7%) in the ichthyofauna
composition of the wider area of the Mala Neretva
estuary (Srš en, 1995) and seem to efficiently use
several properties of this complex ecosystem.
However, sand smelt are also the prey of highpriced carnivorous species, such as sea bass,
Dicentrarchus labrax. It is noteworthy that the
recent construction of levees in the upper parts of
the Neretva River estuary, which inadvertently
modified the local environment in a way that has
led to an increase in the sand smelt population, has
been accompanied by a significant increase in the
catch of D. labrax (Glamuzina, unpublished data).
598 V. BARTULOVIĆ et al.
These artificial changes of natural ecosystems in
the Neretva River estuary significantly affect its
biological and economical (mainly fishery) properties, creating new fishery resources and contributing to the welfare of small local communities. In
order to quantify the potential benefits and threats
of these public works, research into the fish community response and changes was initiated. This
paper presents data on the food composition of
sand smelt from the Mala Neretva River estuary
and relates these to their habitat, seasonality, and
population structure.
Study area
The present study was carried out in the estuary
of the Mala Neretva River situated on the southeastern Adriatic coast of Croatia, and the geographic
position of the site is: 40°52’N; 17°40’E (Fig. 1).
The mouth of the river is blocked with a dam preventing inflow of saltwater into the upper agricultural part of the estuary during dry months. The
sampling site was in front of this dam and on the
Adriatic Sea side where marine conditions dominate. However, significant outflow of freshwater
and pumped polluted freshwater from adjacent agricultural land is constantly added to the site, thus creating an environment of variable salinity. The water
mixing leads to daily and seasonal changes in temperature and salinity at the sampling site. Average
monthly temperatures varied between 25°C in
August and 9°C in February. Water temperature in
winter is affected by the inflow of freshwater, which
is colder (7.4°C) than the sea water (11.4°C). During
the winter period salinity varied between 4 and 38
psu, while during the summer it varied only between
30 and 38.
FIG. 1. – Map of the research area
stomach contents for abundance calculations was
based on the number of identifiable specimens and
on “remains”, the latter being composed of telsa,
jaws, legs, eggs, etc. Therefore, these categories
were assigned a value of 1 (Pais, 2002). The index
of vacuity was determined (Pais, 2002).
The samples were divided into three length fractions (Lt;), Lt I (<6 cm), Lt II (6-7.5 cm), and Lt III
(>7.5 cm), and three mass fractions (M), M I (<1 g),
M II (1-2 g), and M III (>2 g).
Analysis of Variance (ANOVA) and the SNK multiple range test were used to compare the differences
in length and body mass with the total numbers of
food categories (see Table 1).
The monthly prey composition of the stomach
contents of the samples was analysed with the
Principal Components Analysis method in order
to define similarities in the presence or absence
of certain prey items as well as similarities
between the compositions of stomach contents
between months. In addition, these similarities
were examined using a simple clustering technique based on the Bray-Curtis dissimilarity
index and the agglomeration profile was determined with the Thorndike criterion (Aldenderfer
and Blashfield, 1984; Ludwig and Reynolds,
1988).
MATERIALS AND METHODS
The fish examined (n = 1236) were collected
monthly from March 2001 to February 2002 at one
research station close to the dam at the mouth of
Mala Neretva River. The fish samples were collected using a small net (5 mm mesh size), used locally
for sand smelt fishery. It is practically one square
metre of net on a metal frame, which is connected by
ropes to a main rope and when the fish appear
above, the net is lifted out.
Temperature and salinity were measured on-site
using portable digital instruments (WTW). The
specimens were transported to the laboratory where
total (TL) length was measured to the nearest 0.1
mm using a digital calliper and body weight to the
nearest 0.01 g using an OHAUS digital balance.
After measurement of length and weight, the
stomachs were dissected and preserved in 96%
ethanol. Prey composition was determined under a
binocular microscope to the lowest possible systematic status.
The frequency of occurrence was determined for
each prey category, while the percentage number
was determined as the number of individuals,
expressed as a percentage of the total in all categories (Hynes, 1950; Pais, 2002). Quantification of
TABLE 1. – Monthly number of counted prey in the stomachs of 1235 (100 monthly) specimens of sand smelt from Mala Neretva River estuary.
Depth
(m)
0
1
0
1
0
1
0
1
Temperature (°C)
Salinity
pH
D.O. (mg L-1)
2001
March
11.9
12.3
24.3
35.9
6.9
7.2
10.8
11.2
Cladocera
0
Ostracoda
0
Copepoda
Calanoida
5
Cyclopoida
280
Poecilostomatoida
0
Harpacticoida
50
Decapoda Larvae
25
Decapoda
0
Mysidacea
20
Cumacea
0
Isopoda
0
Gammaridea
10
Insecta
5
Pisces
0
Gastropoda Larvae
0
Unidentified Eggs
mass
Reproduction Period
April
May
June
July
Aug.
26.9
26.6
26.6
27.1
7.2
7.4
7.8
8.8
Sep.
23.3
24.1
24.0
25.0
6.7
7.2
8.1
8.6
Oct.
15.4
14.5
28.3
33.1
7.7
7.5
9.6
9.6
21.4
20.8
24.1
31.8
6.7
5.8
10.0
10.1
25.2
24.7
29.6
32.8
6.9
6.9
9.7
9.6
27.0
27.2
29.9
31.3
8.3
8.3
8.1
8.0
16.8
17.2
20.0
24.0
7.3
7.5
8.8
8.7
5
0
5
0
0
0
0
0
135
0
5
0
1600
0
0
40
0
130
170
5
0
0
0
60
5
0
0
0
0
10
0
65
205
0
0
0
0
30
0
0
0
0
0
0
0
5
5
5
10
0
5
110
215
0
0
0
0
0
0
5
0
0
55
0
0
35
0
5
0
mass
0
15
5
35
0
0
60
5
0
5
0
5
0
0
0
0
0
0
5
0
5
0
0
15
0
0
0
0
15
30
10
45
0
0
0
0
0
10
5
0
0
0
•
•
•
•
Nov.
14.5
14.0
15.0
19.0
7.6
8.0
9.0
9.0
Dec.
2002
Jan.
Feb.
12.2
12.4
18.0
21.0
8.0
7.7
8.6
10.0
10.8
11.5
19.3
22.9
7.5
7.5
9.4
9.3
9.0
9.0
9.0
15.9
7.1
7.0
9.3
10.4
350
0
0
30
40
5
5
0
5
0
0
5
0
0
0
0
0
55
0
15
0
0
5
0
300
225
5
0
0
0
10
20
0
0
10
mass
40
10
600
625
5
25
0
5
0
50
0
0
10
0
1450
10
75
775
55
0
0
25
10
55
0
0
0
0
FOOD OF ATHERINA BOYERI IN THE ESTUARY 599
FIG . 2. – Frequency of occurrence (%) of different prey categories.
RESULTS
Sixteen different food categories were identified
(Table 1). The greatest diversity was recorded in
winter, especially in January, when 11 categories
were present. The minimum (4) was recorded in
September. Marine and estuarine species represented the bulk of the prey, while typically fresh-water
species (Daphnia spp. and Cyclops spp.) were found
only sporadically. Large aggregations of unidentified eggs—most likely of invertebrates—were
found in March, July and December.
Crustaceans were the most common prey and, of
these, copepods were the most common (45%), followed by gammarid amphipods (34%), cladocerans
(13%), decapod larvae (12%) and mysids (8%).
Insects represented 7% (Fig. 2). Five percent of the
FIG . 3. – Seasonal changes of the most common prey items (percent
abundance), with macrozooplankton as the sum of gammarids,
decapod larvae, and mysids.
600 V. BARTULOVIĆ et al.
material was unidentified. Among the copepods,
harpacticoids had the highest frequency of occurrence (70%), followed by poecilostomatoids (32%),
cyclopods (29%), and calanoids (27%).
The percentage number of prey showed high seasonal variations (Fig. 3). Copepods dominated in
autumn and winter, cyclopods—mostly Oithona
similis—in March, poecilostomatoids and harpacticoids in January, and calanoids (Paracalanus parvus
and Acartia clausi) in February. The maximum relative number of amphipods was recorded in June.
Throughout the year, in most cases we recognised
Perioculodes longimanus, followed by Guernea
coalita and Westwoodilla caecula. Decapod larvae
were dominant in April-May, the most common
species being Upogebia spp. Mysids were dominant
in summer (July and August), especially
Mesopodopsis slabberi and Paramysis helleri.
Insects (Coleoptera) were dominant in June. The
greatest fraction of prey in autumn (October) was
cladocera, Penilia avirostris (Table 1).
The mean index of vacuity was high: 36% ±
17%. The highest percentage values were in May
and July-September; these ranged from 40% in July
to 67% in September (mean 44% ± 11). In the colder season, the index of vacuity was lower, from 3%
in February to 29% in December and March (Fig. 4).
Fish length ranged from 4.5 to 11.6 cm (mean
6.80 ± 1.17 cm). Significantly higher numbers of
cladocerans and copepods were found in the stomachs of the smallest fish. Decapoda larvae, mysids,
and egg aggregations were prominently abundant in
the stomach of the largest fish (Table 2).
Comparison of body mass with the length of fish
with food in their stomachs showed a positive corre-
FIG . 4. – Percentage of empty stomachs (black) in monthly samples.
TABLE 2. – Results of Analysis of Variance (ANOVA) and SNK multiple range test between the three different length and body mass groups,
and number of prey categories found in all fishes (Differences were considered statistically significant at P<0.05; ns=not significant).
Length
LtI
Lt II
Lt III
Mass
MI
M II
M III
n=207
0.001
0.001
ns
ns
ns
n=334
ns
ns
ns
ns
ns
n=227
ns
ns
0.05
0.02
<0.01
n=251
0.05
<0.001
ns
ns
ns
n=221
ns
ns
ns
ns
ns
n=296
ns
ns
ns
0.05
0.05
Taxon
CLADOCERA
COPEPODA
DECAPODA LARVAE
MYSIDACEA
Unidentified eggs
lation: n = 755, r = 91, P < 0.001 (Pearson coefficient of correlation). Body mass ranged from 0.39 to
11.93 g (mean: 1.90 ± 1.33 g). There was a minor
significant difference between body mass size classes and the number of prey (Table 2).
Principal Components Analysis based on the
prey items compositions revealed significant eigenvalues for 4 axes (values >1). Prey composition was
different for June and December only, while the rest
of the months are clustered within the first axis,
which explains almost 50% of the total variability
(Table 3). Bray-Curtis similarity clustering based
also on the prey composition showed the highest
similarity between 3 of the copepod groups
(Harpacticoida, Poecilostomatida and Calanoida)
and between Gastropoda and Ostracoda. The rest of
the taxa identified cluster together at similarity levels of less than 50%, indicating that there are no particular patterns in the prey composition (Fig. 5b).
Clustering based on months showed that the
highest similarity occurred between April and May,
and between December and January (Fig. 5a). The
agglomeration profile, according to Thorndike’s criterion, showed two major clusters of months. Clustering based on prey items showed 2 clusters of
months (with varying inter-group similarities; Fig.
5a, b). The first group spans from February to
March; the second, from June to July. The data
(Table 1) show clearly that during the first period
(December-May) prey are more abundant and their
TABLE 3. – Principal Components Analysis results of sandsmelt,
Atherina boyeri from Mala Neretva River estuary.
Eigenvalues
PCA axes (value >1)
1
2
Value
5.7924
% of variability 48.27
Cumulative % 48.27
1.9396
16.16
64.43
3
1.3040
10.87
75.30
4
1.0350
8.62
83.93
Correlations between initial variables and principal factors :
factor 1
factor 2
factor 3
factor 4
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
0.6337*
0.6352*
0.7409*
0.7841*
0.9075*
0.4830
0.7112*
0.6326*
0.7703*
0.7553*
0.7365*
0.3930
-0.6557
-0.5235
0.2980
0.2004
0.0393
0.6702*
0.3742
0.0755
0.3513
-0.3925
-0.1285
-0.4649
-0.0225
-0.2552
-0.2608
-0.3238
-0.1913
-0.3484
0.2491
0.6641*
0.0874
0.1992
0.4271
-0.4181
(*) indicates characteristic factor (factor>0.5)
0.0130
0.0119
-0.2830
-0.3716
-0.2112
0.2158
0.4529
-0.0805
0.1890
-0.2840
0.1808
0.6039*
FIG . 5. – Bray-Curtis clustering scheme based on the similarities of:
a) monthly samples and b) the prey items and for the sand smelt,
Atherina boyeri in Mala Neretva River estuary.
FOOD OF ATHERINA BOYERI IN THE ESTUARY 601
diversity is higher. The second period is characterised by low numbers of prey (except in October,
when a very high number of cladocerans appeared)
and low prey diversity.
DISCUSSION
Crustaceans were the most common prey of sand
smelt in the Mala Neretva River estuary. Copepods
were the most common crustaceans (45%). Of these,
harpacticoids were found throughout the year, with
especially high numbers in winter and spring.
Cladocerans were the most numerous in autumn.
Macrozooplanktonic were very frequent prey. Gammarid amphipods were the second most common
prey and were present throughout the year. Mysids
and cumaceans were also important. Along with
decapoda larvae, they were the most important prey
in spring and summer. These three groups are
numerous in the nocturnal zooplankton of Mali Ston
Bay (Lučić, 1986), and the mysid Mesopodopsis
slabberi was extremely abundant (10-15 ind. m-3)
during the day at the mouth of the Neretva from
May to July (Lučić, unpublished data). Of other
prey, insects and as yet unidentified eggs were
prominent. Typical elements of the benthos, such as
adult decapods, were rare.
According to optimal foraging theory, the probability of prey capture is a function of a prey’s density, size, total visibility, and motion (Lazzaro, 1987).
Although the prey variability found in sand smelt
stomachs in the study area is in accordance with previous studies (Kiener and Spillman, 1969; Trabelsi
et al., 1994), its seasonal composition is different.
For example, sand smelt from Suez Channel fed on
copepods mostly in summer (Fouda, 1995), which
contrasts with our results. The same discrepancy
was reported by Vizzini and Mazzola (2002), but in
our case mysids and isopods dominated in winter
and spring. Our results point to opportunism as the
dominant feeding strategy, as well as a clear preference for zooplankton, as found by others (Castel et
al., 1977; Gon and Ben-Tuvia, 1983). When zooplankton is scarce, however, sand smelt shift exclusively to benthic prey (Trabelsi et al., 1994), and in
our case when benthic prey was available the sand
smelt used it.
The diversity of prey in the stomachs of sand
smelt varied seasonally, with greater diversity in
winter. This is consistent with the local composition
of zooplankton, which was also more diverse in
602 V. BARTULOVIĆ et al.
winter (Lučić, 1986; Lučić and Onofri, 1990; Lučić
and Kršinić, 1998).
The relative abundance of prey does not necessarily correspond to the distribution of their biomass. Copepods, for example, have a higher specific weight than cladocerans (Fonda-Umani and Specchi, 1979), and that of macrozooplankton is higher
still. Further, the high nutritional value of copepods
(Stottrup, 2000) and mysids (Mauchline, 1980) is
well established. Lučić (1985) estimated that
mysids, decapoda larvae and amphipods contributed
significantly to the total production of net zooplankton in the Mali Ston Bay, and it was this group that
dominated the gut contents of sand smelt during and
after the spawning period.
Significantly higher numbers of cladocerans and
copepods were found in the guts of the smallest fish,
while decapoda larvae, mysids, and egg aggregations predominated in the stomachs of the largest.
The mid-length group of fish preyed on all of these
items. These results reflect the importance of sizeselection in sand smelt nutrition (Table 2). This also
points to the importance of all age classes of sand
smelt as major consumers of a wide spectrum of
planktonic prey, including prey diversity and dimensions, in the estuaries. The well-structured sand
smelt population preyed on virtually all kinds of
suitable planktonic food in the estuaries, providing
higher members of food webs, mainly different fish
species, with an abundant and rich food source.
The great number of empty stomachs was found
during spring spawning and in summer, especially in
September. On the other hand, the smallest number
of empty stomachs was found during winter. A high
percentage of empty stomachs in summer was also
found in sand smelt from Bardawil Lagoon, Israel
(Gon and Ben-Tuvia, 1983). The overall vacuity
index is also very low (36%), while the percentage
of empty stomachs is rather high. This suggests several important conclusions. It seems that the food
composition of the plankton in the study area may
not be fully appropriate, either in terms of quality
and quantity, at least during the summer season.
Thus, the species probably appears and stays in the
area for reasons other than food. The study area is
part of its natural course of migration between
marine and freshwater, and this course is interrupted
by the dam, which only periodically opens the gates
(Bartulović, 2003). Hence, we may have dealt with
part of population waiting for the free road to freshwater, at least during some parts of the year. This
indicates that the study area may be a transition-
waiting zone between marine and freshwater
ecosystems. The other explanation for low stomach
fullness throughout the year could be constant and
unpredictable abrupt changes in salinity and temperature that affect fish behaviour and feeding (Bartulović et al., 2004). However, these changes were
extreme in winter when we observed the lowest percentage of empty stomachs. Hence, even if this has
some influence, it is not a major reason. Finally, it is
difficult to name one reason for the high level of
empty stomachs, but the instability of the ecosystem
in general may lead to lower prey availability and
lower feeding frequency of sand smelt in the Mala
Neretva River estuary.
The recent propagation of sand smelt in the
Neretva River estuary associated with riverbanks
public works has created new food webs and new
potential fishery resources in the area. This event
provides us with a potentially useful case study in
promoting new habitat development as a benefit,
and not only an ecological disaster. The description
of biological and economic values of new food webs
in the estuary will be investigated in future, with the
results of this paper forming the basis.
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Scient. ed.: G. Pequeño
FOOD OF ATHERINA BOYERI IN THE ESTUARY 603
Jakov Dulčić