Factors That Limit Growth and Abundance of Rainbow Trout across Ecologically Distinct Areas of Flaming Gorge Reservoir, Utah–Wyoming

Abstract

Rainbow trout Oncorhynchus mykiss stocked into Flaming Gorge Reservoir, Utah– Wyoming, are thought to be in poor condition and to rarely overwinter. In 2001–2002, we evaluated factors that affect the growth and abundance of rainbow trout, both temporally and spatially, based on a series of field measurements and growth experiments. Based on our field measurements, the growth and condition of rainbow trout in Flaming Gorge Reservoir actually parallel those of rainbow trout of various strains in other systems, and some rainbow trout overwinter as many as 3 years in the reservoir. Overall, rainbow trout appear to be spatially limited by physical factors (e.g., temperature) during only a few isolated times of the year and not in all areas of the reservoir. Although zooplankton were abundant throughout the reservoir, primary and secondary production vary spatially with concordant effects on fish growth and condition. Diet analysis indicated that rainbow trout consume primarily aquatic macroinvertebrates along with some zooplankton and demonstrate an extremely limited switch to piscivory at larger sizes. Rainbow trout reared in net-pens and limited to a diet of zooplankton lost weight in all three areas of the reservoir. In addition, the reservoir supports a large population of trophy lake trout Salvelinus namaycush, which probably affects the abundance of rainbow trout directly through consumption and the growth potential of rainbow trout indirectly through risk-averse behavior. The combination of spatial variation in reservoir productivity and habitat type, differing stocking strategies, and behavioral responses caused by predators appears to determine the varying success of stocked rainbow trout in the different areas of the reservoir. Our study suggests that stocking strategies may be most effective when specific characteristics of the prey and predator base, the suitability of different habitats, and the genetic strain are explicitly evaluated within the context of the targeted angler population.