Food web constraints on chinook salmon recruitment in a large Lake Michigan tributary

Food web interactions that occur over short temporal and spatial scales may greatly affect population dynamics and management of important fisheries. In Lake Michigan, natural reproduction of Chinook salmon, a key target of the economically valuable sport fishery, may vary greatly due to abiotic factors and biotic interactions in tributary nursery areas. From 2004 to 2007, I evaluated the impact of predation by stocked sport fish on Chinook salmon recruitment in the Muskegon River, a large Lake Michigan tributary. I conducted electrofishing surveys to estimate growth, abundance and survival of Chinook parr, and estimated predator densities and sampled their diets. Together, walleyes and brown trout annually consumed from 18 to 49% of available Chinook parr. Although brown trout consumed large quantities of Chinook parr, walleyes consumed mainly hatchery trout. The presence of alternate prey significantly influenced walleye predation on Chinook parr while brown trout appeared to be limited by parr size. Walleyes positively selected for rainbow trout in some years, and brown trout positively selected for Chinook parr in April when parr were <40mm in length. Both walleyes and brown trout selected for prey that were smaller than the mean prey size in the environment, and prey size was independent of predator size. I developed functional response models to evaluate the potential for predator stocking reductions to affect survival of Chinook parr. Brown trout had the greatest impact on Chinook parr survival based on their feeding behavior and consumption rates. Management efforts to reduce stocking of brown trout could increase short-term survival of Chinook parr by up to 23.4%. I used a stage based matrix model (RAMAS), parameterized with empirically derived estimates of Chinook survival, abundance and fecundity, to simulate effects of predator or prey manipulation scenarios on recruitment and population stability of Chinook salmon. In comparison to a baseline scenario of mean predator and prey densities, only scenarios that involved removal of brown trout significantly increased long-term recruitment of Chinook salmon. Based on the results of my dissertation research, I recommend elimination of brown trout stocking in tributaries that produce Chinook salmon parr.