Invasive species is reproducing, with bad results
The black carp is one of four species included in the group of Asian carp that includes grass carp, bighead carp and silver carp. These fish grow to more than 6 feet and 150 pounds in their native rivers of eastern China and Russia.
Black carp eat primarily mollusks (snails and mussels) when available. They were initially stocked into aquaculture ponds to eat the snails that are the intermediate host of flukes that parasitize cultured fish like channel catfish.
Although black carp stocked at fish farms were genetically manipulated to be triploid fish (three sets of chromosomes) and, thus, genetically sterile, the fish appear to be reproducing in the wild, so some escapees were diploid (two sets of chromosomes) and capable of reproducing.
Introduced, non-native plants and animals usually negatively affect native plants and animals and their ecosystems. In the case of black carp, their selective consumption of snails and mussels — the most imperiled group of animals in the world — poses a significant threat to aquatic ecosystems.
While many management agencies decry the effect of introduced animals based solely on their known or expected behaviors — including what they eat — a study by graduate student Nate Hodgins at Mississippi State took a deeper look at potential effects of black carp by addressing how much they eat.
Small (3- to 5-ounce) and large (1- to 2-pound) juvenile black carp were fed high-protein feed pellets or live snails in controlled laboratory conditions.
The average daily consumption rate —the amount of food consumed daily as a percentage of the fish’s body weight — of small black carp fed pellets ranged from 1.4% to 1.7% at water temperatures ranging from 68° to 86°F. The average daily consumption of the large black carp fed pellets was 1.3% to 2.1% at the same range of water temperatures.
On the other hand, the average daily consumption rate of small black carp fed live snails was 5.9% to 6.3%.
The pellets have no water and, therefore, an ounce of pellets has much more energy than an ounce of live snails that are mostly water and inorganic shell. When daily consumption was calculated as energy, small black carp fed pellets consumed twice as much energy per day as those fed live snails. Possibly, energy consumption of black carp fed snails was mechanically limited. The snails were small, and the black carp ate 296 to 674 snails per day.
The energy consumption rates of small and large juvenile black carp were used to estimate the impact of black carp on natural snail and mussel populations. Across 84 studies, average mussel biomass ranges from about 0.03 to 3.0 ounces per square yard. Converting that biomass to energy and using the ranges of average daily energy consumption rates from the laboratory studies, a standing stock of 2.3 to 9.5 pounds per acre would consume the entire snail and mussel biomass in an average stream during a 180-day growing season when the water temperature is 68° to 86° F, the temperatures at which consumption was measured.
Estimates of black carp biomass measured as pounds per acre are not available. It is unlikely that standing stocks exceeding 2.3 pounds per acre are present at this early stage of black carp colonization and population expansion, but when individual fish quickly grow to sizes larger than 50 pounds, only a few black carp in a couple miles of stream can achieve the biomass necessary to extirpate the mollusks.
Small mussels impacted more
I think there is another — and more realistic — way to use the result from Hodgins’ study to forecast the potential threat of black carp to native mussels. Much of the biomass of mussels in our streams and rivers results from a few large, old, thick-shelled individuals. Although black carp attain large size, these large mussels will largely escape consumption. Rather, consider the thousands of small, young mussels and snails that will be consumed annually by even a few pounds per acre of relatively small black carp. As noted, small black carp in the laboratory trials consumed 296 to 674 live snails per day! Thus, the data supports decimation of an imperiled mollusk fauna by even a low biomass of black carp that can eliminate the smaller, younger mollusks and prevent recruitment to adult stages.