Effects of invasive trout on populations of benthic prey

Nonnative brook trout (BK) have replaced native cutthroat trout (CT) in streams throughout the western U.S., but may not be functionally identical. BK occur at higher densities and feed more from the benthos than CT, so we hypothesized that BK control benthic invertebrate prey more strongly than cutthroat trout. We conducted a field experiment, exposing benthic invertebrates to four treatments: (1) no fish; (2) CT at natural density; (3) BK at natural density (higher than CT); and (4) BK at the same density as CT. Trout diet, benthic invertebrate abundance, and drift propensity were assessed twice. Individual BK consumed more cased caddisflies than CT. At the population level, BK consumed greater numbers of most benthic prey. With few exceptions, differences in diet did not match differences in the abundance or drift propensity of benthic invertebrates among treatments. Differences in prey abundance could increase over time, where prey selected by BK (e.g., caddisflies) are more prevalent, or where variation in benthic abundance is less (e.g., owing to habitat heterogeneity). These results indicate that BK and CT are not functionally equivalent predators, and that BK may alter benthic prey communities when they replace native CT in mountain streams.