Julie Anderson, Michael D. Kaller and R. Glenn Thomas  

There is an invasive species in town that might not be welcome, but at least this one we would like to invite to dinner. Commercial fishers across Louisiana have been reporting increasing populations of both bighead carp and silver carp, known collectively as Asian carp, beginning in the early 1980s.

Subsequently, the invasive population of both species has been increasing in the Mississippi River Basin. Along the Gulf of Mexico, bighead carp have invaded states from Texas to Florida. While silver carp are only observed in Louisiana, Mississippi and Alabama, there is the potential for them to invade farther along the Gulf of Mexico coast similar to the bighead carp. The Asian carps can reach extremely high population densities with harmful effects on the environment, native species and safety. Silver carp often jump out of the water when disturbed, causing damage to property and injury to people in boats.

The bighead carp is native to east Asia, ranging from eastern China through Siberia to North Korea, but it has become an invasive species across Asia. It can tolerate a wide range of temperatures from 25 to 75 degrees mean annual air temperature. Silver carp have a similar native range and are endemic to the large rivers of southern Asia. Like the bighead carp, the silver carp’s territory has spread. It is documented in at least 88 countries.

These two species of Asian carp were originally imported to the United States for use in wastewater treatment lagoons as well as in aquaculture ponds in the 1970s to control nuisance algae and improve water quality. Bighead carp primarily eat zooplankton, but will also eat phytoplankton if available. Silver carp can consume smaller particles and consume primarily plankton. Consequently, Asian carp were purported to be biological controls of excess plankton preventing excessive, periodic plankton blooms and the resulting decline in water quality.

Eventually, the Asian carp escaped from holding facilities into nearby wetlands, streams, rivers and ponds. The first bighead carp in natural waters in the United States was found in 1981 in the Ohio River in Kentucky. Later, reproduction was documented in natural waters in 1989 with the discovery of a young bighead carp in Missouri, and postlarval fish were found in southern Illinois in 1992. The bighead carp population has been increasing ever since, ranking fourth in total commercial harvest on the Missouri portion of the Mississippi River Basin by 1998. Today, the bighead carp has been recorded in 23 states.

Silver carp have a similar history of invasion. The first specimens were found in Bayou Metro and in the White River in Arkansas between 1974 and 1975. By 1981, silver carp had reached the Mississippi River and began their spread throughout the Mississippi River Basin. Today, silver carp have been recorded in 16 states, including Louisiana where 1,600 larval Asian carp were collected from a backwater outlet of the Black River in 1996. Asian carp commercial landings increased from 4,700 pounds to 26,691 pounds annually between 2000 and 2005.

Both bighead and silver carp can tolerate a wide range of temperatures and salinity, making these unlikely factors to halt their invasive range within the United States. Bighead carp are known to actively spawn from 57 to 86 deg rees.

Additionally, fingerlings have survived 41 degrees, and adults are inactive only below 36 degrees water temperature. Silver carp also are active in the winter, and they may be more cold tolerant than bighead carp. Silver carp have been known to overwinter in Alberta, Canada, in ponds close to freezing from November to April. Because of their tolerance to salt water, Asian carp could compete with marine and estuarine fishes, not just freshwater species, further exacerbating an already precarious situation for coastal fishes.
 
Another trait that makes Asian carp a successful invasive species is their fecundity. With both bighead and silver carp, fertility increases with age and size. Newly mature females have an average of 280,000 eggs, and older spawners can have up to 549,000 eggs. Sexual maturity in their native range is three to four years with males maturing about one year before females. However, this varies by environment, and female maturity can be in as little as two years.

Spawning generally is initiated by rising water levels in the spring. Bighead carp migrate upstream and spawn in high-flow water that is turbid. Flow rates may be important for the eggs. Viable eggs are semibuoyant and hatch in 24 hours at 82 degrees. Turbulent water keeps the eggs buoyant until they hatch. However, in laboratory experiments, eggs resting on the bottom of still dishes also hatched. Therefore, flow may reduce predation or settling into mud, but may not be a requirement.

Rapid growth rates are another trait that increases the invasive threat of Asian carp. In waters above 55 degrees with plentiful food, bighead carp can reach 6 pounds in less than a year, and they can reach more than 80 pounds. Silver carp can grow to a size of 2.2 pounds in less than a year, and adults can reach more than 80 pounds over a 15-year lifespan.

Eradication of Asian carp in the Mississippi River basin is impractical, if not impossible. Attempts to limit their movement have been taken in Illinois to limit their access to the Great Lakes. However, even with electric and physical barriers, evidence of Asian carp has been found in Lake Michigan. A better alternative would be to utilize this resource and at the same time attempt to control their numbers and spread to minimize ecosystem disruption.
 
A regional commercial fishery has developed in Louisiana, but far more Asian carps are captured than are sold. Both species of carp are already captured and cultured as a human food product in Asia. Increased demand for the Asian carp, which are highly palatable, would improve profitability and would reduce the standing stock of these invasive species. While assumed to be a fish relatively low in potential contaminants due to its feeding behavior and low placement on the food chain, little data are currently available, although research is beginning.  

Julie Anderson, Assistant Professor and Fisheries Specialist; Michael D. Kaller, Assistant Professor, both with the School of Renewable Natural Resources, LSU AgCenter; and R. Glenn Thomas, Director of Marine Extension, Louisiana Sea Grant, Baton Rouge, La.

(This article was published in the fall 2010 issue of Louisiana Agriculture.)