Exploring Ecosystem-Level Impacts of Alternative Salmonid Stocking Strategies in Lake Michigan

Lake Michigan supports popular salmonid fisheries that are primarily sustained by stocking.  Balancing angler desires for stocking and in-lake consumption demands of stocked piscivores is a great challenge to fisheries managers.  The balancing problem has been exacerbated in the last decade due to declining abundances of prey fishes that are thought to be driven by invasive species effects on energy flows through the food web.  Conservation objectives of rebuilding native lake trout populations via stocking are also vulnerable to disruptions in food web connectivity.  We used a mass-balance food web model to evaluate the potential outcomes of alternative stocking rates on stocked Chinook salmon biomass, expected angler catches, lake trout biomass, and prey fish populations.  We built our model using Ecopath with Ecosim and initiated the model using 1987 data, which was prior to significant recent invasions such as zebra and quagga mussels.  We fit the model to assessment and survey data available through 2008 and projected to the year 2030 under varying stocking regimes.  We evaluated several stocking scenarios including: 1) a baseline model with constant Chinook salmon and lake trout stockings at 2008 rates, 2) a conservation scenario that increased lake trout stocking and maintained Chinook stockings at the 2008 baseline rate, 3) an increased fishery scenario with increased Chinook salmon stocking and maintained lake trout stockings at the 2008 rate, 4) a prey fish sustainability scenario with both Chinook salmon and lake trout stockings reduced relative to 2008 baseline rates, and 5) increased Chinook salmon and lake trout stocking relative to 2008 rates.  Our simulation results provided expectations of alternative salmonid stocking strategies on an ecosystem scale for Lake Michigan with implications for fisheries management and conservation.