THE 2-STORY RAINBOW-TROUT FISHERY AND ITS EFFECT ON THE ZOOPLANKTON COMMUNITY IN A MINNESOTA LAKE

We studied the two-story rainbow trout fishery and its influence on zo oplankton communities in Big Watab Lake, Minnesota, in 1993. The main trout diet components were Chironomidae (larvae and pupae) and Daphnia pulex > 1.3 mm (53 % and 37% dry weight, respectively). Smaller trout were more dependent on D. pulex. Trout consumption of large D. pulex peaked in May (50% of diet) and decreased dramatically as the season p rogressed (3% of diet in August), being replaced by Chironomidae. This size-selective predation influenced the mean size of D. pulex in the water column, with 3% > 1.3 mm in May and 64% > 1.3 mm in September. T he change in trout diet was associated with changes in vertical distri bution of trout (detected by sonar surveys) as the lake stratified in mid-June. By August, trout were restricted to the thermocline and Daph nia found refuge from predation at greater depths. Results from a cree l survey showed Big Watab Lake is providing an excellent trout fishery . Trout stocked at 245+/-12 (2 SE) mm gained an average of 152 mm and 575 g after eight months in the lake. Angler return rates were 58% in number and 285% in weight for the first year. Estimated annual mortali ty was 32% for age 1 and 12% for age 2 and older trout. Estimated fish ing pressure was 337 h/ha and catch rates were 0.27 and 0.18 fish/h in open water and ice fishing seasons, respectively.