Food Web Constraints on Chinook Salmon Recruitment in a Large Lake Michigan Tributary.

Abstract

Natural reproduction of Chinook salmon now supplies half of all recruits to the Lake Michigan sport fishery but may vary greatly due to environmental variability and biotic interactions in tributary nursery areas. From 2004 to 2007, I evaluated the relative effect of predation by stocked sport fish species on Chinook salmon recruitment variability in the Muskegon River, a large Lake Michigan tributary. Together, walleye and brown trout annually consumed from 17 to 47% of available Chinook salmon parr. Although brown trout consumed large quantities of Chinook parr, I found that hatchery trout dominated walleye diets. Walleye were size-selective for small hatchery trout but prey size was independent of predator size. In general, walleye showed neutral selection for prey species although they positively selected for rainbow trout and selected against Chinook parr in some years. Brown trout consumed smaller-than-average Chinook salmon parr although prey size was also independent of predator size. Chinook parr were positively selected as prey by brown trout but only in April when parr were < 40 mm in length. The presence of alternate prey significantly influenced walleye predation on Chinook salmon parr while brown trout appeared to be limited by Chinook size. I developed a functional response model using a Type-II curve for walleyes and a Type-I curve for brown trout to varying abundances of prey. Brown trout had the greatest impact on Chinook salmon recruitment based on their feeding behavior and consumption rates. Management efforts to reduce brown trout abundance via stocking could increase short-term survival of Chinook salmon parr by up to 31%. To assess long-term predation effects on Chinook recruitment from the Muskegon River, I used a stage-based matrix model (RAMAS Stage) parameterized with empirically-derived estimates of Chinook salmon growth, survival, abundance and fecundity. I simulated variable fry-to-smolt survival rates using 26 theoretical predator regimes and compared the results to baseline (i.e. observed) values. Only scenarios that involved removal of brown trout significantly increased long-term Chinook salmon recruitment. Based on the results of my dissertation research, I recommend elimination of brown trout stocking in all tributaries that produce Chinook salmon parr.