In North Carolina, stocking of grass carp is not allowed in rivers or in large lakes from which fish are likely to escape. This restriction results partly from concern that the grass carp will not stay in the targeted areas and feed on the weeds but instead may feed on desirable vegetation (such as muskgrass) and eliminate habitat that sustains commercial or sport fisheries.

Grass carp generally either eat all of the edible vegetation available in a pond or are entirely ineffective. No scientific basis has been found for determining precisely how many grass carp are required to provide effective control in a given body of water without total loss of vegetation. However, many desirable plants are not usually consumed by grass carp, minimizing their adverse impact on fisheries and wildlife habitats. Once grass carp have eliminated the edible vegetation, they may begin to starve. In these cases, the fish can be fed grass clippings or commercial fish food, or may be harvested.

Though very little is known about the effects of large-scale grass carp stocking, what we do know suggests that the actual adverse effects may be substantially less than has been predicted. Studies at Lake Baldwin, Florida, and Lake Conroe, Texas, revealed only minimal impacts on sport fisheries, even though essentially all of the vegetation was removed by grass carp stocked for hydrilla control. Current studies in Texas and at Lake Marion, South Carolina, may provide the information needed to evaluate whether using grass carp in large lake systems is an environmentally sound practice.

There has also been some concern that escaped fish migrating downstream might damage desirable vegetation, particularly the seagrass beds in the estuaries, and thereby have an adverse impact on shellfish and marine fisheries. To date, there is little evidence to support this hypothesis. Fertile (diploid) grass carp were released in the Arkansas River and have been recovered from the Mississippi River system and its tributaries. Similarly, fertile grass carp released in Lake Conroe also have been found downstream. No adverse impacts have been seen in these locations.

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