What is grass carp?
The grass carp (Ctenopharyngodon idella) is one of the largest members of the minnow family, commonly reaching weights in excess of 25 pounds. Native to the rivers of eastern China and the Soviet Union, it has been introduced into over 50 countries worldwide due to its uncanny ability to control a wide variety of aquatic plant species. Although they are related to both common carp and goldfish, distinct differences exist both in appearance and feeding habits. Grass carp lack the barbels and spiny dorsal and anal fin rays characteristic of goldfish and common carp, bearing a closer resemblance to a large creek chub or common shiner. Coloration of the fish ranges from dark gray to golden brown on the back, blending to white on the belly. Feeding is strictly done by grazing on aquatic vegetation and do not share the bottom feeding habits typical of common carp and goldfish. Goldfish, koi and common carp can cause cloudy water due to their bottom feeding technique.
Although fertile (diploid) grass carp have seen widespread use abroad. The majority of states in the US presently prohibit diploids. This is due to concerns over the potential impact fertile grass carp could have on sensitive aquatic habitats should uncontrolled reproducing populations of the fish become established. In New York State, use of diploid grass carp is prohibited!
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What is a Triploid Grass Carp?
Policy and Procedures Pertaining to the Use of Triploid Grass Carp in New York. In 1983, US producers began production of a sterile (triploid) form, mitigating the reproductive concerns associated with the diploid fish. Triploid grass carp were originally created through "shocking" the eggs immediately after fertilization with either hot or cold water. This temperature shock results in the retention of an extra chromosome set, rendering the fish incapable of producing viable young. Today, they are produced by exposing fertilized eggs to high pressure. With the exception of this extra chromosome set, triploid grass carp are identical to their diploid counterparts. The lack of any reproductive ability or drive to reproduce make grass carp eating machines. Triploid grass carp are the only form of grass carp legal in New York State.
Statement of NY DEC Policy
It will be the policy of the New York State Department of Environmental Conservation, Division of Fish, Wildlife & Marine Resources to approve and issue permits for stocking of up to 15 United States Fish and Wildlife Service certified triploid grass carp per surface acre for aquatic plant management purposes in ponds five (5) acres or less in size which lie wholly within the boundaries of lands privately owned or leased by the individual making or authorizing such treatments if:
- Aquatic plants targeted for control significantly impair the intended use(s) of the pond.
- The subject pond harbors no species of wildlife, fish, shellfish or crustacean identified by the Department as being endangered, threatened or special concern; or any species of plant identified as being endangered, threatened or rare.
- The subject pond is not contiguous to or part of a New York State regulated freshwater wetland.
- The subject pond is not an impoundment or natural pond on a permanent stream or a source of a permanent stream as designated by the most recent United States Geologic Survey (USGS) or New York State Department of Transportation (DOT) quadrangle covering the application site.
- At least two (2) years have elapsed since the last stocking of triploid grass carp, unless it can be demonstrated that a significant portion of the permitted fish were subject to mortality within the stocked pond.
Permit applications for waters other than those meeting these criteria, including waters greater than five (5) acres will not be acted upon until evaluated on a site-specific basis in accordance with the State Environmental Quality Review Act (SEQRA) and guidelines established by NYSDEC Division of Fish, Wildlife & Marine Resources.
Stocking Triploid Grass Carp to Control Aquatic Vegetation in New York Ponds and Lakes
Triploid grass carp are a biological aquatic vegetation control species of fish available to New York pond owners. However, as with any method of vegetation control, triploid grass carp must be used properly to achieve optimal results. The following stocking recommendations should assist you in getting the most out of the fish you purchase.
Triploid grass carp have distinct feeding preferences, preferring tender, succulent plant species over those that are tough and fibrous. Triploid grass carp will not control emergent species such as cattail or bulrush or floating leaved species such as water shield or water lily. Even among preferred submerged plant species, selectivity and consumption rate varies widely according to a vast array of factors including water temperature, dissolved oxygen and presence or absence of attached algae.
Triploid Grass Carp Stocking Rates
Triploid grass carp are extremely potent plant consumers. If overstocked, they are capable of eradicating all plants from a pond for periods exceeding 10 years. Besides the obvious impact such complete plant removal will have on vegetation-dependent fish and wildlife, total de-vegetation of a pond can also result in the development of severe algae blooms, foul smells and an overall decline in water clarity. To minimize or prevent such adverse impacts, plant populations should be maintained at approximately 20-30% of the pond's surface area.
Due to various factors that impact triploid grass carp feeding, it is impossible to precisely predict the exact number of fish to stock to achieve the 20-30% plant coverage target. The only way to prevent excessive plant control is through use of an incremental approach. This involves stocking at the rates suggested below, followed by a two-year waiting period. If needed, more fish are added in small increments at two-year intervals until plant populations are reduced to the 20-30% threshold. Remember, be patient. Plant control with triploid grass carp is a slow process. However, once control is achieved, it will last a number of years. If more rapid control is desired, other plant control methods such as mechanical harvesting or chemical applications can be integrated with triploid grass carp use.
Typical Recommended Initial Stocking Rates
Average Plant Density / Stocking Rate (# of fish per acre)
Low Plant Density / 5 fish per acre
Medium Plant Density / 10 fish per acre
High Plant Density / 15 fish per acre
When to Stock
Plant control, with triploid grass carp, is most successful when fish are stocked just as plants begin growing. Stocking of fish during the late summer often leads to unsatisfactory results for that season. The next season, after stocking, would show better results. High water temperature and low dissolved oxygen can result in a substantial loss of grass carp. This condition is typical in excessively weedy ponds. Aeration will help compensate for low oxygen levels and prevent winter kill.
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