Silver and bighead carp are filter-feeders which feed on plankton (drifting animal, plant, or bacteria organisms that inhabit the open waters of waterbodies), with an apparent preference for bluegreen algae). Asian carp can dominate native fisheries in both abundance and in biomass. Bighead carp can reach 110 pounds, although 30 to 40 pounds is considered average (silver carp are generally smaller). Bighead carp can live over 20 years, maturing at about 3 years. Asian carp can consume up to 50 percent of their body weight per day. As most native fish feed on plankton during their larval and juvenile life stages (and some native fish remain planktivorous for life), this high level of feeding on plankton by Asian carp can have serious impacts on the stability of the food web, with bighead carp potentially outcompeting native fish while eliminating the main source of food for larval fish and native planktivorous fish. Native fish considered most at risk include ciscos, bloaters, and yellow perch, which serve as prey to important predatory sportfish including lake trout and walleye.
The Great Lakes provide a wide range of habitat types which would serve as good spawning, recruitment, and maturation areas for Asian carp. Spawning habitat could be provided in the flowing waters of Great Lakes tributaries, while young Asian carp prefer warm, biologically productive, backwaters and wetlands. When not feeding on plankton, Asian carp have been known to feed on detritus and root in the bottom of protected embayments and wetlands. This disturbance could have significant impacts on Great Lakes wetlands and shoreline vegetation which provide spawning habitat for native fish and breeding areas for native waterfowl.
Black carp, being molluscivores, are not a threat to plankton. Should black carp reach the Great Lakes from the Mississippi Basin, however, they could become a threat to native Great Lakes native clam, snail, and mussel populations (particularly those that are rare or endangered), as well as to lake sturgeon (another molluscivore). Black carp can grow to more than 100 pounds and a length of up to seven feet.
In their native habitats, populations of Asian carp are held in check by natural predators. Unfortunately, there are no native Great Lakes fish species large enough to prey on adult Asian carp. White pelicans and eagles have been observed feeding on juvenile Asian carp in the Mississippi River Basin. The pelicans found in the western reaches of the Great Lakes and the eagles found throughout the Basin may be expected to do the same. Native predatory fish such as largemouth bass may feed on juvenile Asian carp. Given the growth rates of Asian carp, many juveniles can be expected to grow too large too quickly for fish predation to be a significant pressure to hold down carp populations.
Once populations of Asian carp become established with recruitment of young fish exceeding mortality, eradication will be difficult if not impossible. Populations might be minimized in some areas by denying access to spawning tributaries via construction of migration barriers, but this is an expensive proposition which may inadvertently result in negative impacts on native species. The best control of Asian carp is to promote commercial harvest.