Natal Contributions of Kokanee Salmon to Flaming Gorge Reservoir, Wyoming–Utah: An Evaluation Using Otolith Microchemistry

Abstract

In a system that uses supplemental stocking to enhance a fishery that serves a dual purpose, an understanding of the contributions from natural and hatchery-produced fish is important so that hatchery resources can be appropriately allocated. Kokanee Oncorhynchus nerka were first stocked in Flaming Gorge Reservoir (FGR), Wyoming-Utah, in 1963 and serve a dual purpose as a prey resource and sport fish. Although natural recruitment occurs in the reservoir, a supplemental stocking program was initiated in 1991. The goal of this research was to identify the natal origin (i.e., natural, hatchery) of kokanee in FGR using otolith microchemistry. Return to the creel, composition of spawning aggregates, and growth of kokanee in FGR were evaluated with a focus on differences associated with natal origin. Kokanee otoliths collected from hatcheries (n = 60) and FGR (n = 1,003) were analyzed for the strontium isotope ratio, 87Sr/86Sr, using laser ablation and a multi-collector inductively coupled plasma mass spectrometer. Kruskal-Wallis tests were conducted to compare the Sr isotope ratios from the otolith edge of kokanee sampled from hatcheries and FGR. Based on 87Sr/86Sr ratios, natural-origin kokanee could be distinguished from eleven out of the twelve hatcheries (P < 0.01); however, the Wigwam Hatchery was not significantly different from FGR (P = 0.84). Model-based discriminant function analysis was used to assign natal origins for kokanee caught in FGR. Hatchery contribution to the population at large varied from 21% to 50% among year classes from 2014 to 2018. The percentage of hatchery origin kokanee in the creel (18-50%) was similar to what was observed in the population. Hatchery-produced kokanee contributed a higher proportion to tributary-spawning aggregates (40-90%) than shoreline-spawning aggregates (19-58%) by sample year. Growth of natural and hatchery kokanee was similar, suggesting similar performance in the system. Results from this study identify that hatchery supplementation contributes to the population and recreational harvest of kokanee in FGR. This research also provides insight on the ecology of kokanee that is useful for better understanding kokanee population dynamics in reservoir systems.