Temperature-Dependent Interactions between Juvenile Steelhead and Sacramento Pikeminnow in Laboratory Streams

Abstract

We examined the temperature dependence of interactions between juvenile steelhead Oncorhynchus mykiss and juvenile Sacramento pikeminnow Ptychocheilus grandis in laboratory streams. Growth of dominant steelhead in water 20-23°C was reduced by more than 50% in trials with Sacramento pikeminnow compared with trials with steelhead alone. Comparison of the two-species treatment with one with an equal density of fish comprised of steelhead alone indicated that the per capita effect of Sacramento pikeminnow was similar to the intraspecific effect of steelhead in water 20-23°C. In contrast, the growth of dominant steelhead in water 15-18°C was unaffected by Sacramento pikeminnow. The temperature-dependence of the effect of Sacramento pikeminnow on dominant steelhead was also reflected in the size of the area defended by steelhead and in the behavior of Sacramento pikeminnow under the two thermal regimes. Compared with all other treatment combinations, dominant steelhead defended the smallest areas when accompanied by Sacramento pikeminnow in warm water. Behavioral interactions with steelhead initiated by Sacramento pikeminnow were about 50 times more frequent in warm water. For the growth of subdominant steelhead, the effect of intraspecific competition exceeded the interspecific effect of Sacramento pikeminnow regardless of water temperature. While thermal regime affects the distributions of these two species, this experiment suggests that it also affects the outcome of competition between them.