Estimating Trophic Position of Lake Oahe Walleye Using Stable Isotope Analysis

Variable rainbow smelt Osmerus mordax recruitment has important implications for walleye Sander vitreus growth and condition in Lake Oahe, South Dakota. In recent years, the rainbow smelt population has decreased appreciably in Lake Oahe. This decline, combined with strong (successive) year classes of walleye, has resulted in reduced walleye growth. Although traditional diet studies are useful for determining changes in prey use, they can be time and labor intensive. Furthermore, they offer only a snapshot of recent feeding patterns when populations are sampled infrequently. I used stable isotope analysis to estimate walleye trophic position and then compared these values to trophic position estimates from diet content analysis for fish sampled from May 2001 to May 2002. Trophic position estimates generated by isotopes were not correlated to those estimated from diet composition. However, because isotope signatures reflect an integrated feeding history, when corrected for seasonal time lags, isotope signatures strongly corresponded to cumulative diet composition. Furthermore, walleye isotope signatures quantified both annual changes in prey use and spatial differences across reservoir zones (riverine, transition, and lacustrine). Spatial differences in prey consumption strongly corresponded to spatial differences in isotope estimates. Using biplots of trophic versus ᵟ13C, I show how seasonal and spatial variation in energy flow can be used for monitoring changes in prey us