Case Studies in Fisheries Conservation and Management: Applied Critical Thinking and Problem Solving
Case 28: Managing Prey Resources in Colorado Reservoirs
Many large western reservoirs are managed with non-native fish assemblages. In higher elevation reservoirs (cold water systems) a popular sport fish assemblage created by managers was lake trout (top level predator) and kokanee salmon (lacustrine sockeye salmon) and/or rainbow trout (mid-level planktivores and insectivores). This simple fish assemblage is easy to manage (fish culture and stocking techniques are well known) and is very popular with anglers. After the establishment of many reservoir fisheries with this combination, managers sought to increase kokanee and rainbow trout growth rates by introducing an additional invertebrate prey, opossum shrimp. These freshwater shrimp are native to the great lakes region and parts of Canada, and were first introduced into western U.S. systems in 1949 (Kootenay Lake, British Columbia; Spencer et al. 1991). This initial introduction resulted in marked increase in growth of kokanee, prompting managers to introduce opossum shrimp into many other systems, including several Colorado reservoirs. However, most of these introductions had unintended consequences…
The focus of this case study will be Lake Granby, a high elevation reservoir in northern Colorado. Opossum shrimp were first introduced into Lake Granby in 1971, and several years of research and monitoring are available to determine the effectiveness of this stocking. Your assignment is to predict possible food web effects of introducing opossum shrimp into Lake Granby.