Chinese Grass Carp
Chinese grass carp are herbivorous fish that have been purposely introduced into many Florida lakes and ponds to control aquatic weed growth. For many, grass carp seem like a good solution for aquatic weeds: the fish are relatively inexpensive and they consume problem plants, especially hydrilla
Most people are surprised to learn that grass carp are members of the minnow family. Grass carp grow to an average of 15-20 pounds and 20-35 inches in length. In larger lakes with large numbers of plants, they can grow up to 50-60 pounds and exceed 50 inches in length. In Florida, the largest reported grass carp weighed 75 pounds.
Also known as white amur, Chinese grass carp sometimes appear silver in color, but often display an olive green or dark gray hue on top with light gold or pale yellow sides. Their belly is silvery or bluish-white and fins are light green or gray. Unusually large fish scales are another distinguishing characteristic.
Normally, grass carp require free-flowing river systems to spawn and produce eggs, so it is believed they don't pose a major threat to lake systems. However, grass carp have been known to escape from stocked lakes or ponds into neighboring waterbodies; this may explain the presence of numerous breeding populations in a number of states along the Mississippi River. In Florida, three adult fish, including one gravid female, have been captured in the Suwannee River.
When grass carp are able to produce viable eggs, they are said to be "diploid" – they have the right number of chromosomes to reproduce. Florida law dictates that grass carp used for weed control must be sterile, or "triploid."
"Triploid" Chinese grass carp are hatchery-raised fish that have been sterilized by “shocking” fertilized eggs with drastic changes in temperature or hydrostatic pressure. The result is an extra chromosome in the egg, leaving the fish unable to produce viable eggs. This technique is used to eliminate reproduction in Florida waters.
Sterilization techniques are not 100% effective. One study showed that milt from some triploid males was used to successfully fertilize eggs from normal diploid females. Other studies have shown that triploid grass carp are able to produce viable eggs and sperm, although in very low numbers.
Because of these anomalies, some scientists suggest that every single fish be genetically tested by blood or tissue analysis before stocking them into a lake. A number of states in the U.S. have taken a more drastic approach and have banned grass carp altogether — both diploid and triploid.
New Protocol Verifies Sterility of Newly Hatched Fish Standards for U.S. Fish & Wildlife Service /Grass Carp Inspection and Certification Program
Chinese grass carp were originally imported and stocked into Florida lakes in 1972 as part of an experimental effort to control hydrilla. When stocked in high enough numbers, the fish proved to be extremely effective.
However, when attempts were made to remove the fish, managers and biologists discovered the carp's uncanny ability to outsmart virtually every type of fishing technique. Nets, hooks-and-lines, electro-shocking, and poison baits were minimally successful, especially after the first attempts. It soon became apparent that once released, grass carp were nearly impossible to remove. As of 2010 there are still no easy ways to remove grass carp from a stocked lake.
Plants Preferred by the Grass Carp
Plants Not Preferred by the Grass Carp
Nymphaea odorata, N. mexicana
Aside from being hard to catch, grass carp have other characteristics to be considered:
- Chinese grass carp are not selective about the plants they eat. Once their preferred plants are consumed, they can eat every plant in a waterbody, including submersed, emersed, and floating plants. They’ve even been observed wriggling out of the water to eat grasses along the shoreline.
- At low stocking rates (two to five fish per acre), it can take from six to twelve months before plants noticeably decrease in abundance. And because grass carp generally consume plants starting with the roots instead of the leaves and flowers, their effectiveness is sometimes underestimated. When this happens, people have been known to stock more fish into the lake. In some instances, this has resulted in “over-stocking” and the unintended consumption of all plants, leaving the lake devoid of vegetation.
- If grass carp are stocked in high enough numbers, there is the potential for a lake to change from a clear-water, plant-dominated system to a murky, algae-dominated system. This happens when macrophytes (larger plants) are consumed by grass carp, and algae (phytoplankton) expand to become the dominant plants within the system. Within weeks or months, the water clarity (link to water clarity) can be significantly reduced as algae increase in number and are suspended in the water. This gives the water an opaque green color and can appear as a scum on the surface of the water. Aquatic plants also serve as stabilizers for bottom sediments. Once they are removed, there is increased potential for sediments to become disturbed and re-suspended in the water, further decreasing clarity. This dynamic is sometimes overlooked and, as a result, there have been disappointed lakefront homeowners over the years.
- Grass carp can live for 10 years or more. The older they are, the larger they become and the more food they will consume. If high numbers of fish are stocked, this can translate into the loss of a tremendous amount of plant biomass within a lake. For more information, view the following paper: The Water Quality and Fishery of Lake Baldwin, Florida: 4 Years after Macrophyte Removal by Grass Carp.
Ever since Chinese grass carp were introduced to Florida waters, efforts have been made to develop a formula to accurately predict the appropriate number of grass carp for stocking purposes, based on lake size and plant coverage. The ideal goal is to stock just enough fish so that aquatic weed consumption slightly exceeds aquatic weed growth.
According to the Florida Fish and Wildlife Conservation Commission (FWC), this can be achieved with approximately two to five fish per acre of submersed vegetation. However, according to another group of lake experts, this formula has produced mixed results. In fact, when using Florida LAKEWATCH data, one study shows a breaking point for the use of grass carp in aquatic plant control.* When stocked in high numbers (greater than 10-12 fish per acre of aquatic vegetation), grass carp were efficient at eating virtually every submersed plant in a lake. In some instances, they ignored a few less palatable plant species (water lilies, bacopa and water hyacinths), but the rest of the plants were effectively controlled. When stocked in lower numbers (less than 10-12 carp per acre of vegetation), the growth rate of the plants was often greater than the consumption rates of the fish and very little plant control was achieved.
According to Florida LAKEWATCH's Mark Hoyer, this "all or nothing" situation isn't always a bad thing. If the goal is to completely control the growth of nuisance plants in a lake, grass carp provide an economical long-term solution without the use of herbicides. Also, grass carp are particularly useful in small urban lakes, where submersed vegetation is not as imperative for fish habitat.
* Sandra G. Hanlon, et. al. 2000. Evaluation of macrophyte control in 38 Florida lakes using triploid grass carp. Journal of Aquatic Plant Management.38: 48-54.
When faced with hundreds of unwanted grass carp in Lake McMeekin in 1990, one individual tried to remove them using hook and line. He began by tossing soybean chum into the water from his dock. He then used molded dough balls made from uncooked biscuit mix. Using this technique, the angler and his neighbors reportedly caught more than 600 grass carp..
Other baits recommended include traditional bread balls, dog food, corn, celery, or live worms.
Some researchers investigated training grass carp to respond to underwater sound frequencies in order to herd them into smaller areas, capture and remove them once they were no longer needed. Two studies conducted at the University of Florida, IFAS Department of Fisheries and Aquatic Sciences suggest there was hope for removing grass carp from small lakes or ponds using this method. However, to date, nothing has proved successful.
In Florida, only triploid grass carp are allowed and a permit is always required, even when stocking privately owned waters. Permits may be obtained by contacting the Florida Fish and Wildlife Conservation Commission (FWC).
Last updated: 25 June 2014