Abstract
Kokanees Oncorhynchus nerka (lacustrine sockeye salmon) and Utah chub Gila atraria feed extensively on similar sizes and species of zooplankton in Flaming Gorge Reservoir. We measured the effects of Utah chub on the growth of kokanees in six 80-m3 enclosures containing ambient zooplankton populations. Three kokanees and 0, 3, 6, 12, or 24 Utah chub were weighed and placed in each enclosure. After 21 d, fish were removed and kokanee growth was compared among treatments. Zooplankton biomass, chlorophyll-a concentrations, and temperature profiles were monitored during each test. In May 1992, no competitive interactions were observed; in June 1992, however, zooplankton biomass and kokanee growth declined significantly as Utah chub densities increased. The best predictor of kokanee growth was Daphnia pulex density, which explained 76% of the variance in kokanee growth. Differences in temperature and initial zooplankton biomass affected the strength of observed competition. Our results suggest that monitoring forage resources rather than nongame fish densities may provide a better framework for assessing competition among pelagic fish species.
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