The two-story rainbow trout fishery and its effect on the zooplankton community in a minnesota lake

Abstract

We studied the two-story rainbow trout fishery and its influence on zooplankton 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. pules peaked in May (50% of diet) and decreased dramatically as the season progressed (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. The change in trout diet was associated with changes in vertical distribution of trout (detected by sonar surveys) as the lake stratified in mid-June. By August, trout were restricted to the thermocline and Daphnia found refuge from predation at greater depths. Results from a creel 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 mortality was 32% for age 1 and 12% for age 2 and older trout. Estimated fishing pressure was 337 h/ha and catch rates were 0.27 and 0.18 fish/h in open water and ice fishing seasons, respectively. © 1996, Copyright Taylor & Francis Group, LLC.