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Triploid Grass Carp as a Biomass Control Technique
Goal:
Control of aquatic plant growth means arriving at, and maintaining, some percentage of cover that is acceptable. Generally 20 to 40 percent cover is considered optimal. Less than 20 percent cover can bring about ecosystem problems and a decline in water quality. In practice however, grass carp often fail to control plants or worse, all submersed plants are eliminated from the lake.
Rate of Success:
Success with grass carp is dependant on many factors and therefore highly variable. The metabolism of the carp, as well as the growth rate of the aquatic plants, will vary greatly due to environmental factors. The right number of fish for one season might be ineffective or cause complete elimination of all underwater plants for another. Bonar et. al. found that only 18 percent of 98 lakes stocked with grass carp achieved control to an intermediate level. In 39 percent of the lakes, all submersed plant species were eradicated. Leaving 43 percent of the lakes with no control observed. It has become the consensus among researchers and aquatic plant managers around the country that grass carp are an all or nothing control option. They should be stocked only in water bodies where complete elimination of all submersed plant species can be tolerated. As this is a common outcome some analysis should be done to forecast the effects of complete eradication of rooted plants.
It should be noted that grass carp stocked in the correct numbers can take up to five years to control nuisance plants. Survival rates of fish vary depending on factors like the presence of birds of prey or fish disease. The lake will probably need restocking about every ten years.
Grass Carp Diet Preferences:
Grass carp exhibit food preferences and some aquatic plant species will be consumed more readily than others. Pauley and Bonar performed experiments to evaluate the importance of 20 aquatic plant species as food items for grass carp. Grass carp did not remove plants in a preferred species-by-species sequence in multi-species plant communities. Instead they grazed simultaneously on palatable plants of similar preference before gradually switching to less preferred groups of plants. The relative preference of many plants was dependent upon what other plants were associated with them. The relative preference rank for the 20 aquatic plants tested was as follows: Potamogeton crispus (curly leaf pondweed) = P. pectinatus (sago pondweed) > P. zosteriformes (flat-stemmed pondweed) > Chara sp.(muskgrasses) = Elodea canadensis (American waterweed) = thin-leaved pondweeds Potamogeton spp. > Egeria densa (Brazilian elodea) (large fish only) > P. praelongus (white-stemmed pondweed) = Vallisneria americana (water celery) > Myriophyllum spicatum (Eurasian watermilfoil) > Ceratophyllum demersum (coontail) >Utricularia vulgaris (bladderwort) > Polygonium amphibium (water smartweed) > P. natans (floating leaved pondweed) > P. amplifolius (big leaf pondweed) > Brasenia schreberi (watershield) = Juncus sp.(rush) > Egeria densa (Brazilian elodea) (fingerling fish only) > Nyphaea sp. (fragrant waterlily) > Typha sp. (cattail) > Nuphar sp. (spatterdock).
Anecdotal evidence suggests triploid carp can have highly localized and variable preferences toward macrophyte species. A detailed understanding of macrophyte species distribution is crucial to the success of the triploid carp technique.
Grass carp facts:
1. Live for at least ten years and probably much longer in colder waters.
2. Will grow rapidly and reach at least ten pounds. They have been known to reach 40 pounds in the southern United States.
3. Although carp “fry” are omnivorous, adults are strictly herbivorous and will not eat fish eggs, young fish or invertebrates.
4. Have definite taste preferences. Plants like Eurasian Water Milfoil and Coontail are not preferred.
5. Generally carp do not consume emergent wetland vegetation or water lilies even when the lake is heavily stocked or over stocked.
6. Are dormant during the winter. Intensive feeding starts when water temperatures are above 68º F.
7. Are a river fish and have the desire to move from still waters into flowing waters.
Advantages
1. Grass carp are inexpensive compared to some other control methods and can offer long-term control, but fish need to be restocked at intervals.
2. Grass carp offer a biological alternative to aquatic plant control.
Disadvantages
1. Depending on plant densities and types, it may take several years to achieve plant control using grass carp.
2. The type of plants grass carp prefer may also be those most important for habitat and for waterfowl food.
3. If the lake is overstocked, all submersed aquatic plants may be eliminated. Removing excess fish is difficult and expensive.
4. Carp feed from the top of the plant down. If all readily available vegetation is eaten, hungry fish will disturb bottom sediments foraging in sediments for any remaining organic material causing increased nutrients in the water column and increased turbidity.
5. If not enough fish are stocked, less-appetizing plants, such as Eurasian milfoil, will take over the lake do to selective grazing.
6. Stocking grass carp will increase algal levels and may lead to blooms as the phosphorus formerly used by aquatic plants will be available for algal growth.
7. All inlets and outlets to the lake must be screened to prevent grass carp from escaping into streams, rivers, or other lakes. This can be an expensive undertaking and will depend on the arrangement and number of possible outlets (at all lake heights) and the classification of the streams above and below the lake.
8. They may not feed in swimming areas, docks, boating areas, or other sites where there is heavy human activity. In a recreational lake this can lead to a shift in plant distribution. Over time the densest vegetation will be concentrated in the most inconvenient areas.
9. In most cases control is never achieved and either no effect is observed or (more commonly) all submersed plants are eliminated.
References:
Bonar, S.A., Bolding, B., and Divens, M. 1996. Management of aquatic plants in Washington State using grass carp: effects on aquatic plants, water quality, and public satisfaction 1990-1995. Washington Department of Fish and Wildlife Research Report No. 1F96-05.
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