Sulfur stable isotope analysis of liver tissue improves detection of stocked fish consumption

ABSTRACT Objective Fish stocking is used to manage recreational fisheries and conserve native species; however, stocking success can be reduced if predators consume the stocked fish at high rates. We evaluated how different stable isotopes and predator tissue types influenced estimates of predator reliance on stocked fish. Methods We measured δ13C, δ15N, and δ34S stable isotope values in Walleye Sander vitreus, stocked salmonids, and naturally reproduced prey fish before and after large salmonid stocking events in two Wyoming reservoirs. Shifts in the values for δ13C, δ15N, and δ34S stable isotopes across Walleye tissue types were assessed before, 1 month after, and 8 months after stocking using analyses of covariance, with length as a covariate. The contribution of stocked salmonids to Walleye diets was quantified using stable isotope mixing models in R (simmr) that were determined from Walleye muscle and liver tissues and combinations of stable isotopes. Results Sulfur stable isotopes differentiated stocked salmonids and natural prey sources for over 8 months poststocking. Liver tissues documented Walleye consumption of stocked salmonoids that was not detected by analyses of muscle tissue alone. Including δ34S values increased the precision of the mixing-model outputs and led to more consistent results across modeling approaches that were based on different combinations of stable isotopes. Conclusion Our results highlight the value of δ34S analyses and multiple tissues, with short- and long-term turnover rates, to identify the consumption of stocked fish by predators and describe the fates of stocked fish.