The Management of a Frustrating and Fascinating Invasive Fish
BY KJETIL HENDERSON, CARTERVILLE FWCO
Grass carp were introduced into the United States (U.S.) in 1963. The species is native to parts of China and Russia, and was introduced into the U.S. for limiting vegetation in aquaculture ponds. In most water bodies, an intermediate amount of vegetation is ideal. Too much or too little can be problematic, and fisheries professionals sometimes try to influence plant biomass towards a happy medium. This influence is exerted to reduce plant growth via drawdowns, mechanical removal of plants or spraying herbicides. Occasionally, another control method used to reduce plant growth which can have substantial negative ramifications is grass carp. This fish can grow large eating vegetation but this characteristic is as concerning as it is exceptional.
There are very few plant-eating fish in North America. These primary consumers (or plant eaters) include stonerollers and grass carp. Stonerollers consume algae and serve a vital role processing nutrients in stream ecosystems. Given their unique diets, both stonerollers and grass carp play substantial roles in the environments they thrive in. Unfortunately, grass carp often bring unwelcomed change when introduced. This species has caused reduced plant abundance and diversity, lowered water clarity, and substantially altered nutrient dynamics in some systems.
For these concerning reasons, the federal government passed 2008 legislation to avoid the spread of fertile grass carp. The Lacey Act makes it illegal to transport species deemed injurious into the country or across state lines without an exemption. Individuals can’t legally spread grass carp across state lines. The only way commercial fish farms can transport these fish across borders is with the documented approval of U.S. Fish and Wildlife Service (USFWS) personnel. This service is provided to states that do not allow diploid grass carp within their boundaries. With this program, only “triploid” grass carp can be transported between the states.
Triploid refers to the number of sets of chromosomes found in the nucleus of grass carp red blood cells. Diploid fish are fertile (2 sets of chromosomes in RBCs) and triploid carp are sterile. The inspection process starts several days prior to a shipment of grass carp from a fish farm. USFWS personnel are contacted by hatchery employees and an appointment is made. A large number of grass carp are selected for inspection from the pool of fish to be shipped. Blood is drawn from these fish and a Coulter Counter is used to measure chromosome diameter. If the test is passed, these grass carp can be shipped only with the proper documentation and after the appropriate states have been notified.
Triploid grass carp inspections represent one component of the role of USFWS in preventing the spread of this species in the United States. Most grass carp aquaculture is performed in the USFWS’s Southeast or Midwest Regions. This interesting species is valuable; over three million tons of grass carp is consumed worldwide each year. However, the potential for grass carp to cause environmental damage is very real, and has this been documented in many cases. Often, stocking grass carp is inappropriate or damaging to the pond or lake being managed. USFWS inspections help prevent the spread of this invasive fish.