Diet of walleye and sauger and morphological characteristics of their prey in Lake Winnipeg

Lake Winnipeg has the second largest commercial walleye (Sander vitreus) fishery in North America. The lake has experienced and will continue to undergo comprehensive changes such as eutrophication, invasion of non-indigenous aquatic species, and climate change. The North and South Basins of the lake differ in many ways, e.g. mean water depth and temperature, turbidity, and fish community. The study objectives were to establish a baseline for diet of walleye and sauger (Sander canadensis) in Lake Winnipeg, to determine whether the diets currently differ between basins and among seasons, and to investigate whether the non-indigenous rainbow smelt (Osmerus mordax) contributes substantially to their diet. Walleye and sauger diets in the South Basin had more diverse species composition, whereas in the North Basin, the diet consisted almost entirely of the introduced rainbow smelt (as well as troutperch (Percopsis omiscomaycus) for sauger). Some seasonal variation was also documented and found to be attributable to the emergence of young-of-year fish and mayfly larvae. The predominant incorporation of the abundant and large rainbow smelt in the diets of walleye and sauger in the North Basin may lead to increased growth and condition of fish in this basin. These data provide a baseline for comparison in future investigations of walleye and sauger diets as changes occur in the food web of the lake with invasion of zebra mussels (Dreissena polymorpha) and spiny water fleas (Bythotrephes longimanus).