There are a number distinct advantages in using grass carp for aquatic weed control. A major advantage is that long-term control (over a period of 5 to 10 years) may be attained with a single stocking, whereas treatment with herbicides is often needed annually or more frequently. Avoiding the use of herbicides also prevents the possibility that chemical residues may harm desirable aquatic vegetation, crops, fish, livestock, or humans. Oxygen depletion caused by the decay of vegetation after herbicide treatments and resultant fish kills are also avoided when grass carp are used.

Several disadvantages are associated with the use of grass carp. The fish are expensive to buy, and it may take two or more seasons to attain acceptable control. Grass carp are preferential feeders on submersed vegetation and hence may not be effective on many of the weeds of concern - especially floating, floating-leaved, and emergent species (Table 1). Part of the vegetation eaten is not completely digested. This partially digested material will decompose and release nutrients into the water, occasionally causing algal blooms. The fish also tend either to consume all of the vegetation present or not to control the weeds at all.

For a list of licensed grass carp distributors and additional information on grass carp or other means of aquatic weed management, contact your county Extension Service Center or a WRC district biologist in your area.

Table 1. Effectiveness of grass carp for control of aquatic weeds in ponds.

1. All of these species are submersed plants.
   
2. All of these species are floating, floating-leaved, or emergent plants, except Eurasian watermilfoil, stonewort, and filamentous algae. Most of these plants have rather tough, woody stems which can not be eaten.
   
3. The watermilfoils, particularly Eurasian watermilfoil, are less preferred than many of the other submersed plants and often are not readily eaten until more preferred species have been consumed first.
   
4. There is some confusion as to how effective grass carp are on these weeds. Small fish stocked in very high numbers (50 to 75 per acre) often are effective. The gill rakers of older fish are spaced too far apart to effectively strain out these plants from the water and thus the fish usually look for more suitable vegetation. Consequently, the grass carp normally stocked for weed control rarely are effective on these plants. Submersed aquatic macrophytes frequently are heavily encrusted with algae and often resemble mats filamentous algae. The fish readily consume these algae-covered plants. This probably has led to the conclusion that the fish are effective on filamentous algae.

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