Potential Factors Limiting Recruitment of Walleye and Yellow Perch in Saginaw Bay, Lake Huron: A Modeling Exercise

Walleye and yellow perch are important ecological and economic species throughout the Great Lakes.  Historically, these fishes supported large commercial and recreational fisheries in Saginaw Bay, Lake Huron, but overfishing and degradation of water quality led to extirpation of walleye from the bay.  Stocking, improvement in water quality since the 1970s, and the extirpation of the invasive alewife has led to the reestablishment of walleye.  However, these increases in walleye abundance have been accompanied by declines in yellow perch abundance.  To determine what factors might limit recruitment of walleye and yellow perch in Saginaw Bay, we coupled a biophysical model with a spatially-explicit individual-based model.  The biophysical model (SAGEM) uses a fine-scale spatial structure and estimates water current, temperature, light, particulate carbon, zooplankton biomass, and dreissenid biomass for each cell on a 6 second time step.  Using output from the SAGEM model, the individual-based model tracks the growth and numbers of walleye and yellow perch throughout the first year of growth using a 6 minute time step.  Individuals forage using a multispecies functional response model that includes encounter rate, capture probability and selectivity indices.  Metabolic costs are determined using previously established bioenergetics parameters.  Individuals also experience starvation and size-based predation mortality.  Movement among horizontal cells is dependent on fish size, water currents, food availability and mortality rates.  We simulated conditions within Saginaw Bay using the SAGEM model to determine what potential bottlenecks exist for yellow perch and walleye recruitment.  Potential bottlenecks include changes in thermal regimes and light levels, changes in larval predation rates, and changes in prey abundance.  Preliminary results reveal that yellow perch are more sensitive to changes in zooplankton and benthic prey densities than walleye, largely due to the early transition to piscivory by walleye.  Results from this study can inform managers and direct future research on factors limiting recruitment of yellow perch and walleye in Saginaw Bay.