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Controlling Aquatic Vegetation With Grass Carp

By Ken Williams and Glen Gebhart

EXCESS AQUATIC VEGETATION causes problems in both aquaculture ponds and in farm ponds used for either sport or food fish production. The main problems caused by rooted and filamentous aquatic vegetation in aquaculture ponds are: interference with fish harvest operations, use of nutrients that could be more efficiently utilized by phytoplankton for dissolved oxygen production, reduction of water circulation that increases stratification and lowers dissolved oxygen levels. Excess aquatic vegetation in farm ponds interferes with hook and line harvest and increases the possibility of overpopulated, stunted forage fish populations, and reduces the aesthetic value of the pond for swimming and recreation. Grass carp are used to great advantage in both situations.

GRASS CARP BIOLOGY

Grass carp, or white amur (Ctenopharyngodon idella) are originally from Asia and are a different species from the common carp. Grass carp are one of the largest members of the minnow family, (Cyprinidae). They may weigh up to 110 pounds but rarely exceed 35 pounds when stocked in ponds. Their life span is about 12 to 15 years but experience has shown that most ponds must be restocked after 7-8 years. Grass carp are distinguished from the common carp by its cylindrical body shape, terminal mouth (at the tip of its head), lack of barbels (whiskers), and lack of saw-toothed spines in its dorsal and anal fins.

Grass carp were first brought into the United States in 1963 by the U.S. Fish and Wildlife Service Fish Farming Experimental Station near Stuttgart Arkansas. The fish came from Malaysia and were used to document the potential of grass carp for weed control in vegetation choked ponds and lakes. Grass carp are legal to stock in Oklahoma without a permit, but some states require sterile triploid fish or outlaw grass carp completely.

Although, usually classified as an opportunistic feeder, grass carp aggressively feed on vegetation. Grass carp activity and growth is greatly reduced when water temperature drops below 57 degrees F. These fish prefer soft vegetation but consume tougher plant species as preferred food supplies diminish. Grass carp readily consume pelleted fish rations when available. Stocker fingerlings (8-10 inches) can reach a weight of 5-7 lb in a growing season, 15-20 lb after 3 years provided an adequate food supply exists.

In natural habitat, adult grass carp are found in back waters of large river systems in Asia. Aquatic vegetation is abundant and provides both food supply and cover from predators.

Spawning is determined primarily by photoperiod or day length and water temperature. The fish begin to spawn when flood waters swell rivers, water temperature reaches 70-75 degrees F. and photoperiod is increasing above eight hours of daylight per day. Mature adults form schools and swim upstream for many miles into the swift flowing waters found in upper reaches of the river. The grass carp spawn, releasing semi-buoyant, nonadhesive eggs which are fertilized as they float downstream. Swift currents(1-5 feet per second) are required to prevent eggs from lodging on the bottom, silting over and dying. Eggs moving downstream are oxygenated and hatch into fry after an incubation period of 24-28 hours. Grass carp will not successfully reproduce in ponds or lakes due to these restrictive habitat requirements.

The larvae receive nourishment from the yolk sac for 1-3 days before active feeding begins. Grass carp fry smaller than 1.25 inches feed on phytoplankton, zooplankton and invertebrates. As they grow larger, aquatic insects and algae become more important in the diet. Grass carp 6 inches long are large enough to consume rooted aquatic vegetation.

Grass carp reach sexual maturity in two-three years in Oklahoma. Reproductive organs reach an incomplete state of development and become dormant without the proper stimulant. As water temperature rises to above 80 degrees F. eggs and milt are absorbed into the fish.

STOCKING RATES

The stocking rate depends upon vegetation type, density and the desired control. In most aquaculture situations, complete vegetation eradication is the desired objective. Because these ponds are relatively shallow and nutrient rich, aquatic vegetation can quickly reach nuisance levels. Consequently, stocking rates are typically high to ensure quick and complete control. Grass carp fingerlings (8 to 12 inches) are typically stocked at 20 to 100 per surface acre. Actual numbers depend on the vegetation species present, plant density and the time available to eliminate the plants. Most plants can be controlled over the growing season with 20 grass carp per surface acre. Soft aquatic vegetation is quickly eradicated with 40-60 grass carp per surface acre while ponds with Chara spp. may require up to 100 grass carp per surface acre. Grass carp do not adequately control cattails, pond lilies and other emergent vegetation. However, as preferred vegetation is consumed the grass carp diet shifts to less palatable food items that include more coarse, woody stemmed plants such as smartweed and very young cattails.

Filamentous algae can be controlled with small, 2-4 inch grass carp, Stock 100-1000 fish per surface acre. This will effectively limit the food supply per individual fish and stunt them. This is desirable because grass carp feed more effectively on filamentous algae at a small size. Also, most of these fish will grow to the 8 to 12 inch size during one season and can then be sold to the sport pond stocking market where a slightly larger fish is needed.

Stocking rates for sportfish ponds are generally less than rates for aquaculture ponds because vegetation control objectives are different. Aquatic vegetation in sportfish ponds provides young fish with protective cover from predators. Complete eradication of these plants can ultimately reduce forage fish numbers and lead to stunted largemouth bass populations.

Small grass carp are highly vulnerable to predation and easily eliminated from ponds. Stock grass carp at least 8 to 12 inches long to insure their survival in ponds containing largemouth bass. In ponds containing numerous large bass, grass carp should be at least 12 inches long to avoid predation.

Many sportfish ponds are also used as habitat for ducks and geese. Complete control of aquatic vegetation will remove the soft plants that supply the necessary food to support waterfowl. Stock no more than 2 to 3 grass carp per acre in ponds used to attract waterfowl.

Grass carp can control vegetation in sportfish ponds when stocked at 5 to 10 fish per water surface acre or 15 fish per vegetated acre. This rate gives good vegetation control in about three years after stocking the fish. Little control will be noticed the first year, however, holes will begin to open in plant beds during the second year. It is usually best to remove vegetation slowly to prevent total aquatic plant eradication in sportfish ponds. If quicker vegetation control is required, increase stocking densities.

Grass carp can eventually eliminate weeds from the pond, lowering fishing quality and increasing turbidity. Remove the fish until the level of vegetation control desired is achieved. Grass carp can be caught on hook-and-line using celery, aquatic plants, dough balls or fish feed. Grass carp also can be removed by shooting them as they feed near shore. Grass carp control vegetation for about 7 or 8 years after stocking. After this time they must be replaced.

Grass carp, like many other species of fish are attracted to, and will follow a current of water. A spillway screen must be constructed to keep the fish in the pond while water is running across the spillway. Screens are about two feet high and reach across the width of the spillway. Screens can be made from one inch plastic or plastic coated wire mesh, supported by steel posts. Horizontal metal bars spaced one inch apart and welded into a frame also make good fish barriers, and are less likely to clog with leaves and other debris than mesh screens.

Grass carp provide excitement for the angler when caught on hook-and-line; and are an excellent fish for the table. Flesh is white, flaky and mild. Intermuscular bones are present but easily removed in large fish.

 

 

 

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