Effects of Water Temperature on Interspecific Competition between Juvenile Bull Trout and Brook Trout in an Artificial Stream

Abstract

The distribution and abundance of bull trout Salvelinus confluentus is influenced by a number of factors, including the presence of nonnative brook trout S. fontinalis, and populations are in decline across the native range. Where sympatric, bull trout are more likely to persist in higher-elevation, cooler streams. To test the hypothesis that competition between juveniles of these species is independent of water temperature, behavioral observations were made of each species in artificial streams. Two temperature (8°C and 15°C) and three density (4.1, 8.2, and 4.1 fish of each species/m²) treatments were used. The results support the contention that bull trout are competitively disadvantaged in warmer water. In warm water, bull trout significantly increased intraspecific agonism. Temperature and density did not influence intraspecific brook trout behavior; however, brook trout tended to increase interspecific agonism in warm water. In mixed-species treatments, brook trout agonism was positively associated with foraging rate at both temperatures, but the trend was weak for bull trout in the warmwater treatment and was reversed in the coldwater treatment. Although fish growth was variable and primarily negative, within temperature treatments the per capita bull trout weight loss consistently increased they were competing with brook trout and was greatest overall at 15°C. Intraspecific competition had the largest effect on brook trout growth in warm water; intraspecific and interspecific effects on brook trout growth were comparable at 8°C. If these condition-specific behavioral responses occur in natural streams, juvenile bull trout may be disadvantaged when competing with brook trout in warmer waters.