Effects of gizzard shad introductions on walleye and yellow perch populations in prairie glacial lakes

Fisheries managers often stock prey fish to increase abundance and growth of predatory sport fishes. One species commonly used in the U.S. Midwest is gizzard shad Dorosoma cepedianum; gizzard shad have been used to increase growth and condition of walleye Sander vitreus in many Midwestern systems. Additionally, with warming climates and stocking, gizzard shad are experiencing a natural range expansion. While this expansion may be beneficial for top predators like walleye, mixed results exist on the effects of gizzard shad on other recreationally important fishes in these systems. Our objectives were to determine if annual growth, relative abundance, and condition of yellow perch, Perca flavescens and walleye populations changed following the introduction of gizzard shad. Adult, pre-spawn gizzard shad were introduced in 2008 and 2009 at densities higher than those which resulted in self-sustaining populations in other South Dakota reservoirs. Yellow perch and walleye population dynamics were estimated during 2007 (pre-shad), 2008–2009 (shad present) and 2010 (post-shad) in two glacial lakes stocked with adult gizzard shad and a reference lake (not stocked with shad). Our results suggest that at the densities documented in this study and in the time frame assessed, gizzard shad did not negatively impact yellow perch as total length at age was similar, condition remained high and zooplankton resources were likely not limiting. Walleye did consume gizzard shad when available and shad appeared to have a neutral or positive effect on walleye growth, relative abundance and condition in these systems during this study. The addition of shad may be a viable option to improve walleye populations without negatively impacting sympatric yellow perch populations under the conditions tested in this study. However, the introduction of a non-indigenous species should be done with caution, especially a potential competitor like gizzard shad.