Seasonal variation in condition, growth and food habits of walleye in a Great Plains reservoir and simulated effects of an altered thermal regime

Catch rates in gillnets and relative weight ( W _r) of walleye Stizostedion vitreum , in Glen Elder Reservoir, Kansas, were lowest during the summer (June–August) and highest during the autumn (September–November). Approximately 80% of their annual growth in length and mass was attained during late summer and autumn. Growth was minimal during winter (January–February) and spring (March–May). The number of walleye with empty stomachs was highest during the summer. Invertebrates (Cladocera, Chironomidae) were common in walleye stomachs during the summer and spring, but contributed little to the ingested biomass. Gizzard shad Dorosoma cepedianum dominated walleye diets (per cent by mass) throughout the year. A bioenergetics model predicted that the proportion of maximum consumption ( P _c) was highest during the autumn and was probably due to spatial overlap of walleye and gizzard shad once water temperatures were <22° C. The bioenergetics model predicted that walleye would lose up to 65% of their body mass during the summer if water temperature increased by 10% (as predicted by some global warming models). Growth during the autumn, winter and spring was enhanced up to 150% by increased temperatures. The results of this study indicate that lower condition, reduced consumption and slow growth are a generalized response of walleye to extreme temperatures. Elevated temperatures may have a net positive effect on walleye growth if they can survive the high thermal stress during summer.