Silver, bighead, and grass carps are all invasive fishes referred to as "Asian carp." They threaten to become established in Minnesota, where ~30 silver and bighead carp combined have been caught between 1996 and 2018. Asian carp pose threats to aquatic vegetation, food webs, commercial and recreational fishing, and silver carp pose additional threats to human health due to their propensity to leap out of the water when disturbed.
What they are
Grass carp were originally introduced to the United States in 1963 as a method for pond and lake maintenance (Mitchell and Kelly 2006) and are still stocked and sold in some states. Due to concerns about its invasiveness, grass carp are commonly produced as triploid, or sterile, fish. Illegal sale and transport of diploid fish can still result in unwanted fertile populations. The ecological risk associated with grass carp has been uncertain, however recent observations of wild grass carp in the Great Lakes basin have caused some environmental managers to call for the re-evaluation of the ecological risk and regulatory structure concerning this species (e.g., Conover et al. 2007).
Bighead and silver carp were imported to the U.S. in the 1970s and escaped from captivity shortly thereafter. Silver and bighead carps have become highly invasive in rivers such as the Mississippi and Illinois Rivers, where they can comprise up to 75% of the fish biomass (Kolar et al. 2005). Bighead and silver carp are considered a threat to native aquatic biodiversity and ecosystem health because of their wide environmental tolerances, large size, high fecundity, and voracious filter-feeding eating habits. Additionally, silver carp, with their tendency to jump up to 3 meters out of the water when disturbed (Kolar et al. 2005) can threaten human safety, recreation, and water-based economies.
The closest reproducing population of silver and bighead carps to date is thought to be in Iowa, and both species now threaten to become established in Minnesota, where approximately 30 silver and bighead carp combined have been caught between 1996 and the end of 2015. The location and abundance of bighead and silver carp in Minnesota has proven difficult to determine. Weaknesses that may be targeted for detection, prevention, and control include: a tendency for these fish to aggregate and shoal (Sorensen and Ghosal); their unique olfactory and eating habits; their swimming capabilities; and their exceptionally good sense of hearing as compared to many other native fish.
Bighead carp (Hypophthalmichthys nobilis) mature at 2-7 years old. Adults spawn in rivers, and their eggs drift into low-flow nursery areas. Each female can produce up to one million eggs per year. Both juveniles and adults eat zooplankton, the basis of the food chain.