Color infrared photography for determining the efficacy of grass carp in aquatic weed control.

Lake Conroe is a 8100 ha (20000 acre) reservoir located 45 miles north of Houston, Texas. Hydrilla verticillata was first observed in the lake in 1975, and by 1978, had become a serious pest. In 1979, a five-year cooperative study involving the Lake Conroe Association and Texas Agricultural Experiment Station was begun to document the increase and the spread of submerged aquatic weeds and to introduce and evaluate the grass carp as a biological control agent. Aerial color infrared photography was used to monitor changes in vegetation. Submerged species increased from 2,350 ha in 1979 to 3,645 ha in July 1981, just prior to the first stocking. From September 1981 to September 1982, 270,000 grass carp were stocked into Lake Conroe. Significant decline in acreage of surfaced weed mats was evident throughout the lake during the year following stocking. By October 1983, all 9,000 acres of submerged weeds were gone from Lake Conroe. Feeding patterns typical of grass carp behavior, coupled with growth and abundance data of captured fish, indicated virtually all of the decline in vegetation was directly related to grass carp.