Invasive Species: Why Are Asian Carp a Problem?

Asian carp is the collective term used in the United States for several species of fish native to Asia. At least 10 species of Asian carp have been introduced outside their native range, and currently four species are considered invasive in the United States: grass, silver, bighead, and black carp.

Some carp were introduced intentionally in the 1970s, used in aquaculture facilities to clean algae and snails from the water, but accidental releases also occurred during high waters and flooding. Eventually, different species of carp established a presence in a number of waterways, with bighead and silver carp populations expanding exponentially in the middle Mississippi River.

The biggest problem with the spread of Asian carp is that some species are out-competing native fish for plankton and decreasing their populations. Some of these fish, like gizzard and threadfin shad, form the basis of food chains in the rivers that they occupy. Commercial fishermen in the Mississippi and Illinois rivers have also seen their nets increasingly full of Asian carp, meaning that each catch dramatically decreases in value because there isn't a significant commercial market for these species in the United States.

How Did Asian Carp Get to the U.S.?

Like most invasive species, the problem began with people. Pollution in the Mississippi River prior to the passage of the Clean Water Act in the early 1970s encouraged the development of an aquaculture industry in small towns in the Delta, with many raising catfish in large shallow ponds. Algae often develops in these types of ponds, and owners brought in plankton-loving silver carp to clean it. Bighead carp would eat detritus from pond bottoms, black carp would eat snails, and grass carp would eat aquatic plants.

Some aquaculture farmers believed that these carp were triploid species (incapable of producing young following genetic manipulation). That was proven at least partially false following repeated floods of the Mississippi River in the 1970s and 1980s, when silver and bighead carp escaped and eventually established a breeding population. (It takes more than one accidental release of a fish for them to establish an invasive population. Multiple fish have to be able to find each other and reproduce).

"They were originally released out of Arkansas aquaculture facilities into the Mississippi River. It was probably an accidental release; it may have been the flood of 1973. These fish escaped and went through an exponential growth period which hasn’t plateaued yet," said Dr. Jack Kilgore, Fish Ecology Team Leader with the U.S. Army Corps of Engineers in Vicksburg, Mississippi.

In addition to out-competing native species of fish and harming the fishing industry, silver carp also pose safety threats for boaters in lakes and rivers. "The first time you notice silver carp, they’re jumping out of the water and hitting you in the head or knocking someone out of their boat, or in my case, giving me a black eye," Kilgore said.

Types of Asian Carp

Asian carp belong to the Cyprinidae family, along with minnows and their relatives, a large and diverse group of 1,200 extant (still living) species of fish. Species of carp native to Asia have been introduced around the world and come from different scientific genera. In the United States, the term Asian carp refers to four of these fish, all considered invasive and native to Asia. Interestingly, recent research on hybridization of Asian carp has shown that two of these species — bighead and silver carp — are breeding and producing offspring that share characteristics of both species.

Bighead Carp (Hypophthalmichthys nobilis)

Bighead carp are native to large rivers and nearby floodplain lakes (often oxbow lakes, formed when the river's course changes) in eastern China. This fish has eyes set quite low, with mottled grey coloring, and can grow to be quite large, up to around 100 lbs.

While only 15% of anglers in one Missouri survey had eaten bighead carp, there is a small market for these fish in the United States, particularly in live or freshly killed markets in largely immigrant communities. Some states, including Illinois, have banned the sale of live bighead and silver carp to prevent further accidental releases.

All four species of Asian carp have pharyngeal teeth—"throat" teeth attached to the gill arches, or the bony supports that serve as attachment points for gill filaments and gill rakers. The pharyngeal teeth of bighead carp are long and rounded, and gill rakers are set very close together to facilitate water filtration.

Silver Carp (Hypophthalmichthys molitrix)

Imported to the United States around the same time as bighead carp in the early 1970s, silver carp are native to China and Siberia and have low-set eyes similar to those of their bighead cousins. One primary difference between these fish is that silver carp typically don't grow to be nearly as large as bighead carp in American waterways — usually reaching around 20 lbs., while bighead carp commonly reach around 40. 

The head of silver carp is scaleless, with an upturned mouth and thin gill rakers fused together into a sponge-like structure for filtering plankton, and their pharyngeal teeth have striated surfaces.  

Grass Carp (Ctenopharyngodon idella)

Compared to other Asian carp species, grass carp have very large scales and a more oblong body. Native to Russia and China, these fish can grow to reach almost 150 lbs. and have a broad head with long, serrated pharyngeal teeth, specifically designed for eating aquatic vegetation. 

Taiwanese fish mongers first sold grass carp to aquaculture farmers and fishermen in Alabama and Arkansas in the 1960s to control aquatic vegetation, and the fish have now been reported in 45 US states.

Black Carp (Mylopharyngodon piceus)

Also known as snail carp and black Chinese roaches, black carp resemble grass carp but are darker gray in color and have different pharyngeal teeth, which resemble human molars and are used to crush the shells of snails and mollusks.

Initially introduced unintentionally as part of a contaminated stock of imported fish, intentional introductions followed in the 1970s in an effort to control the spread of yellow grub, a fish parasite. In the early '80s, several dozen black carp, as well as over a thousand bighead carp, gained access to the Osage River (part of the Missouri River drainage system) when high flood waters entered hatchery ponds in the Ozarks. There is high potential that black carp could negatively impact native aquatic communities by reducing the populations and feeding on native mussels and snails, many of which are considered endangered or threatened.

The Problems

Asian Carp Compete With Native Species for Food

Bighead and silver carp have had a documented negative impact on native fish species in the Mississippi River and many of its connected waterways, particularly in the middle and lower parts of the river. While there is no system-wide research illustrating the overall impact of Asian carp, research in Missouri over the course of two decades of monitoring showed that as populations of silver carp increased, populations of bigmouth buffalo and gizzard shad decreased. Bighead carp out-competed native (and threatened) paddlefish for food resources in experimental pond studies, possibly due to dietary overlap. These fish consume large amounts of food each day, primarily plankton, and their overall impact on species that share their food sources is unknown.

They Threaten the Future of the Commercial Fishing Industry

Commercial fishermen earn money based on the value of the fish they catch. When compared to other native species in the Mississippi River basin, like catfish, Asian carp have little monetary value. The Asian immigrant communities where carp is most popular prefer to purchase fish live or recently killed, meaning Asian carp would have to be transported live from the river, which is typically prohibitively expensive and sometimes illegal, given their classification as an invasive species.

Some parts of the country, including the Great Lakes and the upper Mississippi River, are working to prevent the movement of Asian carp further upstream to protect their commercial fisheries and native species. Of particular concern is the Great Lakes, where commercial, recreational, and tribal fisheries are collectively valued at more than $7 billion annually and support more than 75,000 jobs. An electric fence barrier has been installed in the Chicago Sanitation and Shipping Canal, which connects to the Mississippi, to keep Asian carp from entering Lake Michigan.

Silver Carp Have Seriously Injured Boaters

Silver carp have a unique quality when compared to other Asian carp species — they propel themselves out of the water when startled by light and sound. Scientists suspect this could be a predator avoidance behavior, but no firm cause for their jumping activity is known, and the behavior isn't seen in their native Asia, likely because the fish are harvested and eaten at 1 lbs. to 3 lbs., and not allowed to reach 20 lbs. to 30 lbs., like many silver carp in American waterways. National Wildlife Federation officials fear someone may be killed by a silver carp sooner rather than later, as boaters have been knocked out of crafts unconscious during encounters with the fish.

Grass Carp Can Harm Water Quality

Grass carp are only capable of digesting around half of the grass that they consume each day, with the remaining material being expelled into the water, enriching it and promoting algal blooms, which can reduce water clarity and decrease oxygen levels. One analysis of the ecological effects of grass carp, found an overall negative impact on macrophytes (aquatic plants) as well as an alteration of water quality in stocked areas.

The Solutions

If You Can't Beat 'Em, Eat 'Em

In China, grass, silver, bighead, and black carp (among other species) are a common food source with cultural significance and have been cultivated for at least a thousand years.

Given their popularity around the world as a food source, one of the most commonly promoted solutions to the spread of Asian carp is to eat them. "Silver and bighead carp are the most intensively cultured fish in the world. You never see a 100-pound bighead carp in China because they eat them faster than they can grow. To me there’s only one solution, and that’s to eat them. If you can increase the price per pound of Asian carp to make it economically feasible for commercial fisherman, they will collect them," Kilgore said.

While it sounds great in theory, the problem is that efforts to establish a widespread market have yet to succeed. Chefs have prepared and promoted these fish, particularly silver carp, but there has yet to be any significant public change in their perception, though more than half of the participants in one study on human consumption of invasive species said they would try Asian carp if given the opportunity.

Make Fertilizers and Supplements

Some savvy entrepreneurs have decided that the best way to make Asian carp more palatable is to break it down and use it in other products. Illinois-based Schafer Fisheries sells flash-frozen Asian carp (mostly overseas) as well as a liquid fertilizer blend made with the fish. Because they are high in omega 3s and fatty acids, scientists have also suggested that Asian carp would be an ideal ingredient in vitamins and supplements for people and maybe even for pets.

Create Barriers to Stop Further Upstream Movement

In the late 1800s, officials in Chicago had a sewage problem. Their solution was to artificially reverse the flow of the Chicago River out of Lake Michigan and through the Chicago Shipping Canal, eventually reaching the Mississippi River. A century later, this man-made waterway has become a conduit for invasive species to reach the Great Lakes. In 2002, the Army Corps of Engineers created an electric fence barrier, formed of steel electrodes that are secured to the bottom of the shipping canal, to prevent fish from making their way into the Great Lakes.

Officials in Minnesota, where the Mississippi River originates, are also considering creating numerous barriers to keep the carp downstream. This part of the river sees significantly greater numbers of people recreating, and silver carp in particular would present a new danger. U.S. Fish and Wildlife Service staff are monitoring the impact of a barrier that uses sound to deter Asian carp at one lock and damn, with plans to expand both light and sound barriers if they successfully stop the fish.