Relative Effects of Mortality versus Emigration on a Reservoir Walleye Population

Abstract

AbstractMechanisms influencing population fluctuations are an important component of fisheries ecology. Fishes can be lost from populations due to mortality or emigration, potentially resulting in population declines. Although emigration in large, open‐river systems is well recognized, emigration may also occur in reservoirs that are conventionally considered closed systems, potentially resulting in important population‐level effects. However, little is known regarding annual variation in reservoir emigration rates, the influence of reservoir discharge on emigration, or the influence of emigration compared to mortality in regulating populations. We used tagging data collected from 1990 to 2000 to evaluate the effects of various discharge patterns on reservoir Walleye Sander vitreus emigration and the importance of emigration versus mortality for regulating the population. Walleye annual mortality was variable, ranging from 0.05 to 0.46, but was similar between sexes. Walleye emigration ranged from 0.02 to 0.26 and was best explained by variation in mean April discharge and the number of days discharge exceeded 14 m3/s. Emigration rates were similar to mortality rates in 5 of 9 years but lower than mortality rates in the remaining 4 years, suggesting that both processes likely contribute to fluctuations in population abundance. Combined, our results indicate that wet years requiring prolonged reservoir discharge, even at relatively low rates, result in increased Walleye emigration, with up to 26% of the population emigrating. Larger, more severe precipitation events are predicted to occur in the future as a result of climate change, thus altering reservoir management plans and threatening the sustainability of reservoir fisheries.