Silver carp (Hypophthalmichthys molitrix) are one of four non-native fish species belonging to a group commonly referred to as invasive carp. Native to eastern Asia, silver carp were introduced to the United States during the 1970s and 1980s to private fish farms and wastewater treatment facilities as a biological control agent to reduce algae growth and improve water-quality conditions of the ponds. By the 1990s, they had escaped into the Mississippi River during high-water flooding events and spread rapidly throughout the Mississippi River drainage.
These plankton-eating fish have narrow, deep bodies that are laterally compressed and large, toothless mouths with upturned lower jaws that lack barbels. Their bodies are covered in very small silver scales, except for their head, which is scaleless. Their eyes sit far forward on the head, below the mouth, and project downwards. They have keels that extend along the belly from the anal fin to the base of the gills.
Silver carp can grow upwards of 3 feet long and commonly reach 20 pounds, with the largest individuals reaching upwards of 80 pounds. They are popularly known for acrobatic jumping out of the water when startled by the noise of boat motors, becoming potential hazards for recreational boaters.
In the United States, silver carp are found in the Mississippi, Missouri, Ohio, Tennessee and Cumberland rivers, as well as many of their tributaries. They have been reported in states such as Alabama, Arkansas, Illinois, Indiana, Iowa, Kansas, Kentucky, Louisiana, Minnesota, Mississippi, Missouri, Nebraska, Nevada, North Dakota, Ohio, Oklahoma, South Dakota, Tennessee, Texas and Wisconsin.
Since 2007, silver carp have been listed as injurious wildlife under the Lacey Act. All forms of live silver carp (Hypophthalmichthys molitrix), gametes, viable eggs and hybrids may not be imported into the United States or transported between the continental United States, the District of Columbia, Hawaii, the Commonwealth of Puerto Rico or any possession of the United States without a permit. The best available information indicates this action is necessary to protect the interests of human beings, as well as wildlife and wildlife resources, from the purposeful or accidental introduction, and subsequent establishment, of silver carp and large-scale silver carp populations in ecosystems of the United States. Live silver carp and large-scale silver carp, gametes, viable eggs and hybrids can be imported only by permit for scientific, medical, educational or zoological purposes, or without a permit by federal agencies solely for their own use.
Silver carp spawn from early spring through fall and have the ability to spawn multiple times during a reproductive season. The reproductive potential of silver carp is high and increases with body size. Estimates range from 145,000 to 5,400,000 eggs for fish 3.18 to 12.1 kilograms. Silver carp mature anywhere from 3 to 8 years and male silver carp usually mature one year earlier than females. Maturation rate of silver carp, as in bighead carp, has been found to be related to water temperature, requiring 1,000 days at 15°C and 500 days at 30°C. When silver carp are ready to spawn, ripples have been seen on the water surface from spawners chasing each other. Males and females ascended close to the water surface, chasing each other and shedding eggs and sperm. Spawning usually occurs in association with a sharp rise in water level because this decreases the possibility of egg mortality and helps larvae to enter floodwaters rich in the food they need. They produce semi-buoyant eggs that are carried by currents through the hatching stage and deposit larvae to slow-flowing backwaters, creeks, reservoirs or other flooded areas that become nursery areas. Their eggs range in diameter from 4.9 to 5.6 millimeters, similar to eggs of grass carp but smaller than those of bighead carp. Silver carp eggs need water hardness ranging from 300 to 500 milligrams per liter calcium carbonate to develop and hatch properly otherwise they can burst prematurely in soft water conditions. Freshly deposited eggs that aren’t water hardened yet are clear and can be distinguished from grass carp eggs which have a yellow tinge.
In its native range, silver carp mature by age 3 and live for as many as 8 years, whereas individuals in introduced areas, like the United States, appear to be reproducing by age 2 and only reach 5 years of age. Adults breed in rivers or tributaries over shallow rapids with gravel or sand bottom, in upper water layer or even at surface during floods when the water level rises. Juveniles and adults form large schools during spawning season. Mature individuals undertake long distance upriver migration at start of a rapid flood and water-level increase. After spawning, adults migrate to foraging habitats and in autumn, adults move to deeper pools of the main river. Larvae drift downstream and settle in floodplain lakes, shallow shores and backwaters with little or no current where the develop before migrating as adults to habitats such as main and side channel borders.
Commonly reach 3 to 5 years of age, but can live upwards of 15 to 20 years.
?Silver carp eat plankton and filter feed on both phytoplankton and zooplankton.
?Silver carp are a schooling species and can be found grouped in large numbers. They are popularly known for acrobatic jumping out of the water up to 9 feet (3 meters) when startled by boat motors.
Silver carp occur in a variety of freshwater habitats, including large rivers and warm-water ponds, lakes and backwaters that receive flooding or are otherwise connected to large rivers. They also have been introduced widely to ponds, lakes, reservoirs and canals, where they have been reported to thrive but probably can’t reproduce without access to an appropriate riverine habitat.
Silver carp prefer open areas and eutrophic zones of standing or slow-flowing waters and occupy the upper and middle layers of the water column. In the United States, they prefer contiguous backwaters, tributary streams, main-channel borders, side-channel borders, behind wing dikes and slow-flowing portions of large rivers, where water may pool. They avoid main-channel habitat.
Silver carp are quite tolerant of broad water temperature ranges, from 4 °C to 40 °C. Silver carp are known to feed at water temperatures of 10 to 19 °C; in the Missouri River, silver carp sometimes had full guts at temperatures lower than 4 °C. Although they’re a freshwater species, they can also live in slightly brackish water and tolerate salinities ranging from 1.5 to 12%.
Bighead carp (Hypophthalmichthys nobilis) is often confused with silver carp. They’re both from eastern Asia and overlap in both their natural and introduced ranges inhabiting many of the same rivers and lakes. There are a few distinguishing characteristics that separate the two species, however. They have similar shaped bodies, but bighead carp are typically dark gray above and cream-colored below with dark gray to black irregular blotches on the back and sides. Their head and mouth are disproportionately large in comparison to their body and other fish species. They have a keel like silver carp, however their keel only extends between the anal and pelvic fins. The length of their pectoral fins overlaps their pelvic fins unlike silver carp. The shape of their gill rakers is another useful characteristic in distinguishing the two species. Bighead carp have long, thin gill rakers that are not fused whereas silver carp have gill rakers that are long and thin, but are fused to form a sponge-like apparatus. The eyes of both carp species are situated low on the head, but the eyes of bighead carp differ from those of silver carp by facing at a sharper downward angle and more forward on the head. This is especially noticeable when looking at the head from underneath. Bighead carp are also typically grow larger than silver carp reaching lengths upwards of 5 feet in length, weighing more than 100 pounds. They, however, do not acrobatically jump out of the water at the sound of motor boats unlike silver carp.
Young black carp feed primarily on zooplankton and later on insect larvae and detritus. Adult black carp feed primarily on mollusks, such as mussels and snails, using their pharyngeal (throat) teeth to crush the shells. They also eat freshwater shrimp, crawfish, and insects
The silver carp is a narrow, deep-bodied fish that is laterally compressed that as a large, toothless mouth with upturned lower jaw and lacks barbels. Their body is covered in very small silver scales except for their head which is scaleless. Their eyes are situated far forward on the head, sit below the mouth and project downwards. They can be distinguished from bighead carp due to their keel which extends along the belly from their anal fin to the base of their gills whereas bighead carp have keels that only extend from their anal fin to pelvic fin, and their pectoral fins only extend to the base of their pelvic fin as opposed to beyond the pelvic fin in bigheads. They can grow upwards of 3 feet in length.
Individuals commonly reach 20 pounds with the largest individuals reaching upwards of 80 to 90 pounds.
Their bodies are very silvery in color, especially when young, and later develop a greenish hue on their back to silver on the belly as they age. Their scales are small, and trout-like while their head and opercles are scaleless, as noted by C.S. Kolar and others in 2005.