Niche partitioning and resource use of stocked salmonid species across Wyoming reservoirs

Abstract Objective Stocking of salmonid species creates and maintains recreational fisheries in western U.S. reservoirs, yet success varies widely among systems and their resource use and potential for competition are not well understood. We investigated isotopic niche space and overlap among stocked salmonids in Wyoming reservoirs to quantify niche partitioning and ontogenetic shifts in resource use to inform ecologically compatible stocking strategies. Methods Stable isotope ratios (δ13C, δ15N, δ34S) were analyzed for Brown Trout Salmo trutta, Bear River Cutthroat Trout Oncorhynchus virginalis utah, kokanee O. nerka, Rainbow Trout O. mykiss, and Snake River Cutthroat Trout O. v. bouvieri across four Wyoming reservoirs. Recently stocked individuals were identified and excluded from further analysis using k-medoids clustering. Isotope values were standardized across reservoirs using two-source mixing models based on benthic reliance and trophic position. Isotopic niche region size and overlap were estimated using nicheROVER, and Bayesian regression models evaluated relationships between fish length and resource use. Results Mean benthic reliance values for salmonid populations ranged from −0.28 to 0.67, and trophic position ranged from 2.74 to 3.30. Bear River Cutthroat Trout and Brown Trout occupied the largest isotopic niche regions (0.97 and 0.88), whereas Snake River Cutthroat Trout occupied the smallest (0.24). Most reservoirs supported broader niche areas and high interspecific overlap (66–71%), except for New Fork Lakes (0.8%). Brown Trout and Bear River Cutthroat Trout shifted from pelagic to benthic energy pathways with length, whereas kokanee and Rainbow Trout showed minimal change. Conclusions Salmonid species showed consistent, species-specific patterns of resource use across reservoirs, with Brown Trout and Bear River Cutthroat Trout acting as generalists, kokanee functioning as pelagic specialists, Rainbow Trout serving as lower-trophic level generalists, and Snake River Cutthroat Trout occupying a narrow niche that remains largely pelagic. Although species relied on similar broad energy pathways, most populations occupied fundamentally different isotopic niche space, and notable overlap occurred primarily among the more flexible generalist species. Integrating species-specific foraging strategies and niche separation into stocking decisions could therefore improve poststocking performance in reservoir systems.