Seasonal and Spatial Distribution of Walleye Sex Ratios in a Large Nebraska Reservoir

The ratio of female to male Walleye Sander vitreus across the spatial scale of large reservoirs and during non-spawning times receives little attention, even though standardized sampling occurs during this time. This study evaluated whether the proportion of female Walleye collected from seasonal sampling at different spatial areas within a large reservoir fell within the 0.450-0.550 range, which would closely reflect a 1:1 female-to-male sex ratio. We used a Bayesian generalized linear mixed effects model with a binomial likelihood to assess the proportion of Walleye using season (spring and fall) and reservoir zone (riverine, transitional, and lacustrine) as fixed effects and year (2015 and 2016) as random effects. We collected a total of 2,163 Walleye using standardized Nebraska Game and Parks Commission gill netting methodology, and sex was determined on a random subsample of fish (n = 989) collected throughout each reservoir zone. There was no meaningful deviation from the 0.450-0.550 range in the mean posterior estimate of the proportion of female Walleye caught in the riverine and transitional zones during either spring or fall. The mean (SD) posterior estimate of the proportion of female Walleye in the lacustrine zone was 0.182 (0.024) in the spring and 0.621 (0.032) in the fall. These results are consistent with previous observations of increased male presence near spawning locations in the spring and demonstrate that increased fall female catch in the lacustrine zone could potentially bias sampling results. This study provides further insight into the distribution of sexes across the spatial gradient of a large Great Plains reservoir and demonstrates a need to sex Walleye during standardized fall surveys.