Managing for Coexistence of Kokanee and Trophy Lake Trout in a Montane Reservoir

Abstract

Kokanee Oncorhynchus nerka and Lake Trout Salvelinus namaycush are stocked for sportfishing in lakes and reservoirs throughout the western United States and Canada. However, where the two species co-occur, unsustainable levels of predation by Lake Trout can lead to declines in kokanee abundance and declines in Lake Trout growth and body condition. Such declines occurred in Blue Mesa Reservoir, Colorado. In 2009, managers began removing Lake Trout (<740 mm TL) in an attempt to sustain the hatchery-dependent kokanee population while still providing a trophy Lake Trout fishery. To evaluate this and other strategies for achieving the dual management goals, we developed age-structured kokanee and Lake Trout population models and linked them to a bioenergetics model of Lake Trout predation. We found that the existing level of Lake Trout removal (0.23; ages 4–9) is insufficient to prevent further decline and ultimately the extirpation of the kokanee population. If removal of age-4–9 Lake Trout is intensified to 0.38, the kokanee population would persist; however, removal would have to be increased to 0.63 to allow a return to historic kokanee abundance. Focusing removal on age-4 Lake Trout (0.78) would allow for persistence of kokanee and would leave more trophy Lake Trout for anglers, suggesting that the two goals are compatible under some circumstances. However, management costs of balancing kokanee with trophy Lake Trout are high and put both fisheries at risk unless Lake Trout abundance is controlled.

Received October 31, 2013; accepted May 5, 2014

ACKNOWLEDGMENTS

We thank Mark Jones and John Alves (CPW) for their financial support of the project, and we thank the many other CPW biologists and technicians for assistance in sample collection. We are also grateful to Ken Stahlnecker and Matt Malick (National Park Service) for funding the stable isotope research.