Diets and growth of age-0 walleye in a recently recovered population

Most fishes undergo ontogenetic diet shifts, progressing from small to larger prey as they grow. The availability of suitable prey throughout early ontogeny can influence growth, survival and ultimately, year-class strength. Simultaneously, due to their numeric abundance and high mass-specific consumption rates, young fish can serve as influential consumers and thereby affect abundance of various prey. The walleye Sander vitreus population in Saginaw Bay, Lake Huron, recently recovered and is now entirely supported by natural reproduction. Recovery coincided with a dramatic decline of alewife Alosa pseudoharengus, a preferred prey of walleye. Thus, we are uncertain what primary prey now support production of young life stages of this recovered walleye population. To this end, we collected young (larval and later-stage young of year) walleye in Saginaw Bay and characterized their growth, diets and cumulative consumption using bioenergetics models. Young walleye progressed from feeding entirely on zooplankton as larvae in April to feeding almost entirely on fish by September. Based on bioenergetics analyses, fish were the most important prey for young walleye cohorts. Shiners Notropis spp., along with invasive rainbow smelt Osmerus mordax and round goby Neogobius melanostomus, were the primary fish prey. In contrast, yellow perch Perca flavescens, an important prey for adult walleye in Saginaw Bay, were largely absent in young walleye diets. Young walleye growth rates were similar to rates observed in other systems, but lower than growth rates previously observed in Saginaw Bay when alewife were abundant and the density of walleye was low.