ABSTRACT: Pactola Reservoir in the Black Hills of South Dakota has historically been managed as a coldwater fishery for Rainbow Trout Oncorhynchus mykiss and Lake Trout Salvelinus namaycush. In 2003, Northern Pike Esox lucius were first sampled in fisheries surveys by South Dakota Game, Fish, and Parks (SD GFP) and have since become an established predator. Due to concerns of direct predation on stocked catchable (275 mm) Rainbow Trout, SD GFP partnered with South Dakota State University on a research project which evaluated Northern Pike diets and isotopes. The results of that study showed that fish smaller than 600 mm had δC13 isotope values similar to centrarchid prey species, whereas δC13 values for 600 mm and larger fish were similar to stocked Rainbow Trout. In response to this research and to reduce predation by Northern Pike, SD GFP increased the Rainbow Trout stocking size in Pactola Reservoir from 275 mm to 381 mm. The objective of this study was to evaluate Northern Pike isotope changes in response to the increased Rainbow Trout stocking size. Northern Pike (n=42) were captured in May 2021 with sizes ranging from 457 mm to 1,021 mm. δC13 values in Northern Pike smaller than 700 mm were similar to the centrarchid values seen in the previous study, suggesting that stocked Rainbow Trout had not become a major diet item for 700 mm and smaller individuals. This data was supported by an analysis of variance (ANOVA) among 100 mm size bins which revealed that significant differences in δC13 were detected between the 700 mm and 800 mm size bins, but not among 700 mm and smaller size bins. Furthermore, a piecewise linear regression technique in program R revealed a breakpoint value of 716 mm. These results indicate that the length at which Northern Pike prey upon stocked Rainbow Trout has increased by over 100 mm following the increase in trout stocking length.
