Modeling the implications of multiple hatching sites for larval dynamics in the resurgent Saginaw Bay walleye population

The early life environment experienced by most larval fish is largely dependent on a combination of hatch site and water currents. Until larvae are able to swim fast enough to overcome currents, they are largely passively transported and have limited control over ambient environmental conditions, including temperature and prey availability. These factors strongly influence growth and survival of larvae, with direct consequences for subsequent recruitment. Early life survival of Saginaw Bay walleye was formerly limited by alewife predation on larvae; but following the collapse of Lake Huron alewives, the walleye population has rebounded and recruitment success may now be influenced by other factors including spawning habitat. We sought to assess the implications of successful hatching at multiple locations in Saginaw Bay, using a hydrodynamics model, particle transport model, and an individual-based bioenergetics model in series. Model results were compared to locations of young larvae collected in Saginaw Bay during 2009-2010. Results suggest that larval growth is strongly influenced by hatch date, driven by seasonal variation in temperature between sites. Larvae hatched at any location could be transported extensively within inner Saginaw Bay before reaching a sufficient size to swim independently of currents, and retention within the productive inner bay varied among years and sites. Our results indicate multiple larval walleye origins in the field, augmenting the continued production from the Saginaw River system. Successful hatching at more locations would serve to buffer walleye recruitment variation through portfolio effects, supporting arguments for more emphasis on diverse spawning habitat management and restoration.