Foraging Patterns of Walleye and Sauger as Revealed by Stable Isotope Analysis

Sauger Sander canadensis populations have declined throughout the Midwest over the past 50 years. One hypothesis to explain this phenomenon relates to competition with walleye S. vitreus, a highly sought after sport fish that is frequently stocked. Examining isotopic overlap and variability provides quantitative insight of energy transfer and diet breadth in sympatric walleye and sauger populations. In this study, we compared relative overlap and diet variability using stable isotopes (δ¹⁵N and δ¹³C) of walleye and sauger collected from three, Missouri River impoundments. To account for variable sample size, we used re-sampling statistics to compare mean isotope values and measures of diet variation between walleye and sauger. We found high isotopic overlap between walleye and sauger in all three reservoirs, suggesting high diet overlap. However, isotope variability was significantly greater for sauger compared with walleye. Walleye isotope variability was consistent among reservoirs, whereas isotopic variability among sauger decreased from upstream to downstream reservoirs. Moreover, patterns of diet variability for sauger were congruent with reported hybridization rates; that is, as diet variability decreased, hybridization rate increased for sauger. Thus, differences in isotopic variability could be reflective of reduced niche breadth and(or) displacement by sympatric walleye.