Dietary habits of an underperforming Walleye (Sander vitreus) population in a Northern Minnesota Reservoir.

Abstract

Walleye (Sander vitreus) are predatory fish that are highly desired by Minnesota anglers. Surveys conducted by the Minnesota Department of Natural Resources (MNDNR) on Island Lake Reservoir near Duluth, Minnesota, have found that Walleye length-at-age has declined over the last 30 years. The MNDNR speculates that prey limitation is one factor influencing the Walleye size structure. Notably, Island Lake Reservoir is infested with spiny water flea (Bythotrephes cederströmii) which have been associated with slower growth rates of young-of-year Walleye in Minnesota lakes. This study characterized the dietary habits of adult Island Lake Reservoir Walleye during summer 2021 and identified important prey items. This dietary assessment coincided with the implementation of new harvest regulations and Cisco (Coregonus artedi) stocking conducted by the MNDNR. Diet composition was assessed by direct microscopic observation of stomach contents. The Walleye diet results were compared to Walleye diet data from the literature as well as previous Walleye diet data from Island Lake Reservoir collected during 2006. This study found that adult Walleye in 2021 relied on a mixture of fish, insect, and zooplankton prey. Specifically, adult Walleye preyed heavily on atypical fish species, larval insects, and spiny water flea. It is likely that these dietary habits are contributing to the population’s poor size structure and body condition. This study found evidence to support a hypothesis of prey limitation in Island Lake Reservoir which should be useful information in the formulation of future fisheries management decisions.