Understanding Seasonal Food Habits and Growth of Age-0 Walleyes with the Use of Stable Isotope Analysis

In this study we will examine the intrapopulation variability in growth of age-0 walleye in Harlan County Reservoir.  Growth can be an important indicator of recruitment as the length of age-0 fish after that first growing season may influence over-winter survival. Similar to other irrigation reservoirs in Nebraska, Harlan County Reservoir has a low density marcoinvertebrate community, possibly confounding the normal ontogenetic diet shifts of age-0 walleye from zooplankton to macroinvertebrates and then to piscivory.  We hypothesized that the ontogenetic diet shifts of age-0 walleye would differ among the observed range of lengths; with the larger age-0 walleyes becoming piscivorous within the first growing season while the smaller age-0 walleyes may remain feeding on zooplankton and/or macroinvertebrates.  In September, age-0 walleyes ranged in length from 113 mm to 159 mm during 2009 and 131 mm to 239 mm during 2010 indicating the 2010 year class displayed more intrapopulation variability in growth and achieved greater lengths within the first growing season.  The average weight of stomach contents from age-0 walleye captured in August was 0.13 g (0.055) during 2009 and 0.48 g (0.178) during 2010, with fish comprising 33.5% of the diet by weight in 2009 and 99.9% of the diet by weight in 2010. Stable isotope analysis and stomach samples from the faster growing 2010 year class have shown that age-0 walleye of all lengths transitioned directly from zooplanktivory to piscivory (largely age-0 gizzard shad Dorosoma cepedianum) during the first growing season.  Because age-0 walleye of all lengths switched to piscivory, additional factors must be affecting intrapopulation variability in growth.