Midwest Fish & Wildlife Conference 2018

AUTHORS. Benjamin Leonhardt, Purdue University; Harvey A. Bootsma; University of Wisconsin-Milwaukee; Benjamin A. Turschak; Michigan Department of Natural Resources; Serguisz Czezny, Illinois Natural History Survey; Austin Happel, University of Illinois; Matthew S. Kornis, U.S. Fish and Wildlife Service; Charles R. Bronte; U.S. Fish & Wildlife Service; Jacques Rinchard, SUNY-Brockport; Tomas O. Höök, Purdue University


ABSTRACT. Salmon and trout in Lake Michigan have relied primarily on pelagic Alewife as forage since the 1950’s. However, various ecosystem changes have led some salmon and trout to shift to other prey resources. Diminished pelagic production driven by reduced nutrient loading and filtering by dreissenid mussels, combined with the invasion of nearshore areas by a bottom-oriented forage fish, Round Goby, have collectively led to the increased importance of nearshore, benthic production for some species of salmon and trout. However, this shift does not appear to be ubiquitous: species differ in their relative use of various prey species, and individuals within each species display differential diet complexity and specialization. The most common method for quantifying the diets of fish is through stomach content analysis. Although stomach contents provide a relatively straightforward path to identifying prey items, this method is limited because the prey in a fish’s stomach may not reflect the long-term diet patterns. Instead, other trophic indicators, such as stable isotopes, better reflect long-term resource use. To understand the long-term trends of prey consumption by Lake Michigan salmonids, five salmonid species were collected in 2016 from April-November by the U.S. Fish and Wildlife Service, state departments of natural resources, and Little Traverse Bay Band of Odawa Indians from recreational anglers and from annual fishery-independent gill net surveys.  We quantified stomach contents as well as stable isotope ratios of dorsal muscle of Lake Michigan salmonids and used these data to evaluate diet variation by individual, species, region and season. We compare and contrast patterns of individual diet complexity and specialization derived from the two trophic indicators for each species, and discuss their implications in the context of sustainable fisheries.