Mapping aquatic weeds with aerial color infrared photography and evaluating their control by grass carp

Abstract

Lake Conroe is an 8,100 ha reservoir located 75 km north of Houston, Texas. Hydrilla was first observed in the lake in 1975, and by 1978 had become a serious pest. In 1979, a study was begun to document the increase and spread of hydrilla and other submersed 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, prior to the first stocking. From September 1981 to 1982, 270,000 grass carp, mostly 20.3 cm or longer, were stocked into lake Conroe. Significant decline in area of surfaced weed mats was evident throughout the lake during the year following stocking and by October 1983, all 3,645 ha of submersed weeds were gone from the lake. Feeding patterns typical of grass carp behavior, coupled with growth, abundance, and distribution data of captured fish, indicated virtually all of the decline in vegetation was directly attributable to grass carp.