William E. Kelso
For most freshwater fishes, spring is the time for spawning, a time when water temperatures warm, plankton populations grow, and conditions are perfect for survival of eggs and larvae. We often think of largemouth bass and bluegill nesting on shallow mudflats in Louisiana lakes and bayous when we think about the spring flush of fish reproduction. However, there are other spring-spawning fishes that you may be less familiar with, and while they have little commercial or sport value, they may be the most important species currently spawning in Louisiana’s rivers and streams.
The term “Asian carp” refers to four species of fishes that have become established in U.S. waters over the last four decades: silver carp (Hypopthalmichthys molitrix), bighead carp (H. nobilis), black carp (Mylopharyngodon piceus) and grass carp (Ctenopharyngodon Idella). Three of these species were imported by federal, state and private interests in the 1970s and ‘80s to support the aquaculture industry. The fishes were seen as potential food fish that could also control snails and improve water quality. Black carp eat molluscs, and reducing snail densities can help control trematode parasites in catfish. Bighead and silver carp can control algae and reduce off-flavor in channel catfish. The grass carp, which is herbivorous, was introduced by state fish and wildlife agencies in the Southeast to control nuisance aquatic plants. Bighead, silver and black carp escaped to the wild in the 1970s, and, together with intentionally released grass carp, have spread throughout the Mississippi River watershed and beyond.
Although imported for ostensibly beneficial reasons, Asian carp life history traits predisposed them to be successful invaders. They are tolerant of variations in temperature, dissolved oxygen and salinity. They grow quickly, escaping predatory mortality, and reaching large sizes of up to 100 pounds. They also have a long lifespan of over 10 years and are highly fecund, producing up to 2 million eggs per female. Originally, it was believed that the fishes would not spread quickly in U.S. waters because the species need large streams combined with specific sequences of photoperiod, increasing temperatures and increasing flow rates for successful reproduction. However, recent evidence from Illinois populations suggests they are able to spawn in a much broader range of river conditions than was previously thought, and their reproductive capacity and range expansion appear uncontrollable.
The most significant concern regarding successful spawning and colonization of U.S. waters by Asian carp is their ability to completely alter the structure of aquatic food webs. Adult silver carp are extremely efficient phytoplanktivores, consuming the microscopic plants that form the basis of many freshwater food webs. Adult bighead carp are also planktivorous, but they consume zooplankton, the tiny (0.5 to 2 millimeters) crustaceans that feed on phytoplankton and are in turn eaten by the larvae, juveniles and adults of many native fishes and invertebrates. Because of their size, feeding efficiencies and population growth rates, these two carps can shunt vast amounts of plankton away from native fishes and invertebrates and turn it into carp biomass. Black carp, because of their specialized feeding on molluscs, threaten native freshwater mussels, the most endangered group of animals in the U.S. today. Although grass carp are voracious herbivores, they are inefficient in digesting plant material, which means they must eat large amounts of aquatic plants to sustain themselves. Although initially stocked to control problem vegetation, many systems have seen the complete elimination of aquatic plants, which reduces habitat quality for native larvae and juveniles by thinning out hiding places.
There are few control options for reducing or eliminating Asian carps from U.S. waters. There is a small market for Asian carps in the U.S., but demand and prices are not sufficient to sustain a commercial industry. With no native piscivorous fishes big enough to eat even large juveniles, let alone adults, Asian carps are likely here to stay. The greatest threat these fishes pose is to the billion-dollar fisheries of the Great Lakes. When the Des Plaines and Calumet rivers were rerouted after 1900 to allow Mississippi River vessels to access the Great Lakes, it provided an avenue for Mississippi River fishes to access Lake Michigan. Despite three electric barriers that have been constructed on the Des Plaines River to stop up-running fish, a silver carp was captured well upstream of the barriers in the Calumet River in 2017. It seems just a matter of time before Asian carps will access Lake Michigan. Again, their spawning habits might limit successful reproduction to a few tributary rivers, but that would likely be sufficient to maintain populations that could ecologically and economically devastate Great Lakes food webs.
Spring is an incredibly productive time of year for Louisiana’s fishes, with huge numbers of eggs and larvae found throughout the state’s rivers, streams, bayous, lakes and ponds. Unfortunately, the last four decades have seen huge increases in spawning populations of Asian carps, which appear capable of dominating fish assemblages in our larger rivers and streams — and perhaps lakes as well — if tributary streams provide adequate spawning conditions. The legacy of Asian carps is a striking example of what can happen when alien fish species are released into rivers and lakes outside their native range, a legacy that will continue to impact aquatic systems in Louisiana and throughout the U.S. for a long time.
William E. Kelso is the associate director of the LSU AgCenter School of Renewable Natural Resources and the F.O. Bateman Professor of Renewable Natural Resources.
This article appears in the spring 2019 issue of Louisiana Agricuture.
Silver carp jump in a Louisiana waterway. These invasive species have spread across the United States. Photo provided by Louisiana Sea Grant
The silver carp is a species of Asian carp that has spread across the United States after being imported to the country in the 1970s and ’80s. Photo provided by Louisiana Sea Grant