“Asian carp” is the collective term that refers to bighead carp, silver carp, black carp, and grass carp. These invasive species were originally brought into the United States for aquaculture in the 1960s and 1970s, but through flooding, legal releases, and illegal releases, they have become established in the Mississippi and Illinois rivers and beyond. Impact and distribution vary among the four species.
Bighead carp
Bighead carp (Hypophthalmichthys nobilis) are native to southern and central China and were initially introduced to the United States for aquaculture and phytoplankton control in 1973.
Bighead carp—also known as noble fish, speckled amur, or lake fish—can reach up to five feet in length and weigh up to 90 pounds. They are deep-bodied, compressed fish, meaning they have very flat sides that allow them to swim quickly through the water. They exhibit a dark gray coloration on their backs that fades to white towards their belly. Bighead carp earn their name from their sizable scaleless head with a large mouth and protruding lower jaw with no teeth.
Bighead carp look similar to silver carp (Hypophthalmichthys molitrix) but can be distinguished by their larger head and dark blotches on their sides. The keel, a ridge on the fish’s belly that helps them form a compressed body shape, can be used to tell the two apart. Bighead carp usually have a smooth keel between their anal and pelvic fins that does not extend in front of the pelvic fins, while silver carp usually have a keel that extends forward past the pelvic fin base.
Bighead carp filter-feed a wide range of food, grow fast, and reproduce quickly. They lack a true stomach and must continuously filter-feed on plankton, consuming up to 40% of their body weight daily! Although the full impacts of this species are unknown, studies have suggested that bighead carp can deplete zooplankton populations. Most native larval fish, some native adult fish, and native mussels all rely on plankton for food, and this overlap may lead to declining native populations. Some of the fish that bighead carp compete with are paddlefish, walleye, yellow perch, lake whitefish, gizzard shad, and bigmouth buffalo. Additionally, declines in zooplankton populations can result in dense planktonic algae blooms.
Silver carp
Silver carp (Hypophthalmichthys molitrix) are native to several major Pacific Ocean drainages in eastern Asia. Their native range extends from the Amur River (Heilong Jiang) of far eastern Russia south through much of the eastern half of China to the Pearl River (Zhu Jiang), and possibly parts of Northern Vietnam. Silver carp were introduced to the United States in 1973 when they were intentionally imported for aquaculture, phytoplankton control, and human consumption. Silver carp are known colloquially as “flying fish,'' as they have been known to leap out of the water when startled by loud noises such as a boat motor.
Silver carp are generally smaller than bighead carp (Hypophthalmichthys nobilis) but still typically weigh around 60 pounds and can exceed three feet in length. This species is deep-bodied and laterally compressed. Silver carp are often bright silver and as they age their back darkens slightly into a greenish tint. They have a scaleless head and large mouth with pharyngeal teeth located in the throat, but no jaw teeth. As mentioned previously, silver carp are most similar to invasive bighead carp but can be distinguished by their smaller head, upturned mouth, keel which extends forward beyond the pelvic fin, and lack of dark blotches on their sides. When young, the bright coloration of silver carp can be confused with native gizzard and threadfin shad, however, gizzard and threadfin shad often have purple-blue shoulder spots near their gill covers.
Similar to bighead carp, silver carp filter feed on plankton and can compete with other planktivorous organisms such as some adult fish, many larval fish, and native mussels. Studies have shown that silver carp also compete with and impact gizzard shad, bigmouth buffalo, paddlefish, and emerald shiner populations. Competition can lead to declined body condition which can further lead to decreased fecundity, the ability to produce successful and abundant offspring throughout an individual's life.
Black carp
Black carp (Mylopharyngodon piceus)—also known as snail carp, black amur, and black Chinese roach—are native to most major Pacific Ocean drainages of eastern Asia. Their native range spans from the Pearl River (Zhu Jiang) basin in China north to the Amur River (Heilong Jiang) basin of China and far eastern Russia. They are also possibly native to the Honghe or Red rivers of northern Vietnam. The species was introduced to the United States in the early 1970s and 1980s as a means for biological control and human consumption. Black carp were used to control snails and other mollusks and yellow grub (Clinostomum marginatum) in lakes and ponds. A similar species, grass carp (Ctenopharyngodon idella) was also introduced during this time for aquaculture. More black carp were accidentally imported as a contaminant of those grass carp stocks. Like many of the invasive Asian carp species, flooding of aquaculture facilities has led to their escape into large river systems.
Black carp have an elongated, narrow body with a blunt head and a slightly down-turned mouth. Their coloration is brown to black on their back that fades to a gold sheen on their belly. Black carp most closely resemble grass carp in our waterways. Both species have similar body structures and fin placement, as well as very large circular scales. However, black carp are usually darker in coloration and their pharyngeal teeth, found in the throat, are large and look like human molars! Their large teeth help them crush the shells of the mollusks they consume. Some commercial fishermen have also noticed that generally, unlike grass carp, black carp have a somewhat pointed snout. Black carp can reach over five feet in length and weigh up to 150 pounds! The larger the fish, the bigger the appetite.
Although North America has the highest diversity of freshwater mussels worldwide, over half of the 78 known native mussels in the Midwest are classified as federally endangered, threatened, or gravely imperiled. River pollutants, habitat disturbances, and invasive mussels are already applying much pressure to native mussel populations. Invasive black carp, who consume up to 20% of their body weight daily, apply another great threat. Black carp also compete for food with other native species and are expected to considerably impact native fish, turtles, birds (including waterfowl) and vertebrates such as raccoons, otters, and muskrats. Additionally, black carp can host many parasites, flukes, bacteria, and viral diseases that may infect native fish species.
Grass carp
Grass carp (Ctenopharyngodon idella) are also known as white amur. This species is native to eastern Asia from the Amur River (Heilong Jiang) to eastern Russia and China south to the West River of southern China. They were originally introduced to the United States in the 1960s for aquaculture. Grass carp were imported as a form of biological control to manage aquatic weed and phytoplankton problems in lakes and ponds, and stocking of grass carp to manage nuisance aquatic plants continues today. Many factors have contributed to their spread in the U.S. including stocking by federal, state, and local government agencies; legal and illegal interstate transport via release by individuals and groups; and escape from ponds and aquaculture facilities. From there, natural dispersal has continued the species’ spread through our waterways.
Grass carp can grow over five feet long and weigh more than 80 pounds. Similar to black carp (Mylopharyngodon piceus) grass carp also have an elongated, narrow body with a terminal mouth, meaning their mouth is located in the middle of their head and points forward with no overbite or underbite. Grass carp have large, circular scales with a black spot at the base. They are usually lighter in color when compared to black carp and exhibit a gray to brassy-green coloration starting on their back and fading to whitish-yellow near their belly. Grass carp’s pharyngeal teeth are much more slender and narrower than black carp’s. Unlike black carp’s throat teeth that look like human molars, grass carp’s throat teeth look slender like teeth of a comb. In fact, the genus name Ctenopharyngodon means “comb-like” in Greek.
Grass carp's serrated pharyngeal teeth help them to shred and consume aquatic vegetation. Through this consumption, they can impact native populations both directly and indirectly. They are capable of impacting macrophyte, phytoplankton, and invertebrate communities through consumption, which in turn impacts the native species which rely on these as crucial food sources. As mentioned earlier, competition for food can lead to decreased body condition and decreased fecundity of native fish species. Grass carp’s consumption of large quantities of aquatic vegetation also impacts the native fish species that use vegetation as shelter and spawning substrates. Additionally, studies have shown that grass carp overeat. Grass carp can consume up to 99.2 pounds of plant material in a day but can only digest half that amount. In turn, they expel the rest into the water which enriches it and can promote algal blooms that reduce water quality and decrease oxygen levels. Grass carp can also infect native fish populations with parasites, bacteria, and viruses. It is thought that they introduced the Asian tapeworm (Bothriocephalus opsarichthydis) to native fish populations in the United States.
Additional reading material and references:
https://www.invasivespeciesinfo.gov/aquatic/fish-and-other-vertebrates/asian-carp
https://www.fws.gov/Midwest/News/documents/AsianCarp.pdf
https://secure.in.gov/dnr/fishwild/files/BIGHEAD_CARP.pdf
https://www.fws.gov/midwest/fisheries/library/fact-blackcarp.pdf
