Using Stable C and N Isotopes to Assess Diets of Lake Michigan Salmonines: Implications for Ongoing Management

Lake Michigan currently hosts an economically valuable salmonine sportfishing industry which began in earnest with the return of planted Coho salmon to Platte River, Michigan in 1967. After Michigan DNR began stocking, Wisconsin, Indiana, and Illinois soon joined a multi species salmonid stocking effort which created fishing opportunities and helped control the invasive alewife prey population. Alewife numbers had reached nuisance levels during the 1950s-1960s in the absence of a top predator but salmon stocking efforts coupled with other lower food web changes have reduced alewife and other pelagic preyfish to record lows. As pelagic prey abundance has declined, the invading benthic round goby has reached high densities in nearshore areas. This has resulted in a major shift from a pelagic dominated preyfish community to one which is more benthic and nearshore. Understanding if and how introduced and native predators have responded to this transition is critical to preserving a diverse and healthy predatory sportfish community. We used stable C and N isotopes to assess the regional diets of predators in Lake Michigan in 2016. Bayesian mixing models, informed with gut content analyses, were used to reconstruct diet proportions, determine the proportion of assimilated energy coming from benthic versus pelagic sources, and determine niche overlap among predators. Regional usage of nearshore benthic prey was variable for some species such as lake trout and brown trout whereas other species such as Chinook and Coho salmon exhibited lower regional variance and narrower niche breadth. Although steelhead overlapped with Chinook and Coho salmon, their niche breadth was much broader indicating a more generalist feeding strategy. As Lake Michigan transitions from a pelagic to nearshore benthic preyfish production, maintaining predator-prey balance may require a more regionally-specific approach and greater focus on a diverse fishery including native lake trout and other generalist predators.