Growth and Survival of Larval Walleyes in Response to Prey Availability

Although larval fish can be highly susceptible to changes in prey availability, the effects on growth and survival are difficult to examine in the field. In addition, previous studies often have examined these relationships only with zooplankton communities common to northern oligotrophic lakes. We used mesocosm and pond experiments to better understand the relationship between communities of larval walleye Sander vitreus (formerly Stizostedion vitreum) and zooplankton common to eutrophic midwestern USA reservoirs. Treatments of low, medium, and high crustacean zooplankton density (1-50 individuals/L) were created in mesocosms by filtering pond water. Treatments in ponds were created by adding copper sulfate to create low-density zooplankton ponds and adding liquid fertilizer to create high-density zooplankton ponds. Walleye growth rate (0.7-1.5 mm/d) increased with crustacean zooplankton density in both mesocosms and ponds. Densities necessary to maintain good growth were higher than previously observed at northern latitudes, probably due to lack of large-sized zooplankton. Survival increased with zooplankton density in the mesocosm experiments (11-37%) but not in the ponds. Walleye consumption of zooplankton increased with zooplankton density up to 20-30 individuals/L. At low zooplankton densities, larval walleyes fed more heavily on chironomid larvae. Our results demonstrate the importance of zooplankton abundance and size composition for survival and growth of larval walleyes.