INFLUENCE OF WATER TEMPERATURE ON INTERACTIONS BETWEEN JUVENILE COLORADO RIVER CUTTHROAT TROUT AND BROOK TROUT IN A LABORATORY STREAM

We compared microhabitat selection by and behavioral interactions between juvenile Colorado River cutthroat trout Oncorhynchus clarki and brook trout Salvelinus fontinalis at two water temperatures in the laboratory. At 10-degrees-C, both species occupied similar depths but cutthroat trout used slightly faster current velocities. At 20-degrees-C, depth and current velocity use were similar for both species. The species were nearly equal competitors at 10-degrees-C, but brook trout showed a clear competitive dominance over cutthroat trout at 20-degrees-C. At the warmer temperature, brook trout were more aggressive, consumed more food, and occupied the lead position in a dominance hierarchy more often than cutthroat trout. Brook trout also maintained equilibrium longer during a thermal challenge test, suggesting they are more tolerant of heat stress than Colorado River cutthroat trout. Our results provide an example of condition-specific competition that may influence fish distribution patterns in streams.