Use of Walleye Relative Weights (<i>W_r</i>) to Assess Prey Availability

Abstract

We investigated the effects of prey availability on relative weights (Wr; the ratio of fish weight to the weight of a standard fish of the same length) in populations of walleye Stizostedion vitreum from two reservoirs, Lake Ogallala and Lake McConaughy, in western Nebraska. Partitioned by season of capture and 50-mm length-classes, walleye Wr was compared with the abundance of prey fish by length-class. Walleye Wr changed seasonally and was heavily influenced by prey size availability. In Lake Ogallala, where smaller prey were abundant, there was no significant difference in Wr across length-classes by fall. In Lake McConaughy, where more than 90% of all prey fish were over 170 mm, there was a distinct trend by fall. Small walleyes (250–400 mm) exhibited the lowest Wr (92.0 ± 0.84); and large walleyes (550–700 mm) had the highest Wr (104.0 ± 1.57). Examining changes in walleye Wr among length-classes across seasons was more informative than using a mean population Wr because it was sensitive to changes in prey availability for individual walleye length-classes. We aggregated adjacent walleye length-classes with similar Wr values to form Wr-groups to determine the length-groups that were experiencing prey limitations. Identifying walleye length-groups with common Wr structure from standardized fall surveys may be a cost-effective method of detecting prey fish deficiencies in reservoirs.