The effects of stocking and removal of a nonnative salmonid on the plankton of an alpine lake

Bighorn Lake, a fishless alpine lake, was stocked with nonnative brook trou t, Salvelinus fontinalis, in 1965 and 1966. The newly introduced trout rapi dly eliminated the large crustaceans Hesperodiaptomus arcticus and Daphnia middendorffiana from the plankton. In July 1997, we began to remove the fis h using gill nets. The population comprised 261 fish that averaged 214 g in wet weight and 273 mm in fork length. Thereafter, zooplankton abundance in creased within weeks. Early increases were caused by the maturation of Diac yclops bicuspidatus, few of which reached copepodid stages before the remov al of the fish because of fish predation. Daphnia middendorffiana, absent w hen fish were present, reappeared in 1998. Hesperodiaptomus arcticus, which had been eliminated by the stocked fish, did not return. The proportion of large zooplankton increased after fish removal, but their overall biomass did not change. Algal biomass was low and variable throughout the 1990s and correlated with water temperature but not with nutrient concentrations or grazer densities. Diatoms were the most abundant algal taxon in the lake, f ollowed by Dinophyceae. Chrysophyceans and cryptophyceans were eliminated a fter the fish were removed. Chlorophyll a concentrations were unaffected. G ill netting is a viable fish eradication technique for smaller (less than 1 0 ha), shallow (less than 10 m deep) lakes that lack habitable inflows and outflows or other sensitive species. Further work is required to define app ropriate removal methods for larger lakes and watersheds.