Muskellunge (Esox masquinongy) Predation on Walleye (Sander vitreus) Affects Population Size Structure and Harvest: A Modeling Approach

Densities of Walleye (Sander vitreus) have declined in some lakes in Wisconsin, USA, over the last two decades. Increased predation on Walleye due to shifts in densities and size structure of other piscivores is one hypothesized driver. We estimated predation by an apex predator, Muskellunge (Esox masquinongy), on Walleye and its potential effects on the fishery. We combined an empirically derived individual-based Muskellunge bioenergetics model with an age-structured Walleye population model. The Walleye age classes consumed by Muskellunge include those most critical to Walleye reproduction. The largest individual Muskellunge in our analysis (1250mm) consumed as many as 20 age 2-3 walleye year^-1. At typical densities and under the mean predation rate, a population of Muskellunge consumes 2-12 age 2-3 walleye ha^-1year^-1. High predation on Walleye results in Muskellunge consuming 7-51 age 2-3 walleye ha^-1year^-1.  Either scenario has serious implications for the Walleye fishery. The magnitude of predation on Walleye and the subsequent effect on the fishery is highly sensitive to small changes in Muskellunge foraging behavior, density, and size structure. While Muskellunge predation is only one potential driver of Walleye decline, our findings highlight the potential for unanticipated species interactions when managing for multiple predators.