Evaluating the Effects of Rainbow Smelt on Larval Walleye Densities and Bioenergetics in Wisconsin Inland Lakes

Since the introduction of rainbow smelt (Osmerus mordax) to the Laurentian Great Lakes region in 1912, the species have invaded numerous inland lake systems.  Rainbow smelt are known to cause declines of several species of native fish, including walleye (Sander vitreus).  Young of year walleye densities have been observed to decline in many inland lakes after rainbow smelt invade, but little evidence exists to demonstrate how recruitment declines.  We tested alternative hypotheses that walleye recruitment declines because adult smelt consume larval walleye or because smelt monopolize zooplankton resources critical to larval walleye development.  We studied four lakes in Vilas County, Wisconsin with contrasting rainbow smelt populations to identify potential constraints on walleye recruitment in lakes that contain smelt.  We evaluated the larval fish community, rainbow smelt diets and zooplankton communities of each lake.  Lakes that contain rainbow smelt had much lower densities of larval walleye throughout spring when compared to lakes that had no rainbow smelt.  Additionally, zooplankton abundances and sizes were reduced in lakes that contained rainbow smelt. Rainbow smelt diet analysis indicated that larval walleye were not consumed.  Alternatively, our results suggest that a lack of zooplankton resources may lead to a larval bottleneck that prohibits strong walleye year class development in lakes containing rainbow smelt.  Previous studies in the region indicate that these changes in the zooplankton community are a direct result of rainbow smelt zooplanktivory.  An individual based model was developed to assess how larval walleye bioenergetics differ between lakes that contain rainbow smelt and those that do not.  The model suggests that the altered zooplankton community in lakes with rainbow smelt, as a result of the zooplanktivory of that species, inhibits the natural dietary ontogeny of larval walleye.  This may ultimately lead to recruitment declines of this important recreational species.