Stocking Strategies for Fingerling Walleyes: An Individual-Based Model Approach

The success of any program for stocking walleye (Stizostedion vitreum vitreum) fingerlings is strongly dependent on growth of the stocked fish during the summer and early fall months immediately following their release into lakes, reservoirs, or rivers. An individual-based model (IBM) was developed to describe growth of young-of-the-year (YOY) walleyes in Lake Mendota (Wisconsin, USA). The IBM was used to evaluate stocking strategies for walleye fingerlings. According to the rules of this simulation model, predation by a walleye would occur only if the walleye was sufficiently large relative to the prey individual. The length-frequency distribution of the YOY walleye population at the end of the 1989 growing season was predicted accurately by the model. During 1989, walleye fingerlings with a mean total length of 50 mm were stocked into Lake Mendota on 28 June. Simulations were performed to investigate the effects of the size of stocked fingerlings and the timing of stocking on subsequent YOY walleye growth. These simulations revealed that if walleye fingerlings were stocked on 28 June, at an average total length of 60 mm rather than 50 mm, then the proportion of large (total length of @>175 mm) fish in the YOY walleye population at the end of the growing season would have increased threefold over the observed proportion. Economic cost per large walleye was minimized when average total length at stocking was 62 mm. Stocking 50-mm walleye fingerlings on 14 June instead of 28 June resulted in a tripling of the percentage of large walleyes at the end of the growing season.