Experimental Effects of Nonnative Brown Trout on the Individual- and Population-Level Performance of Native Bonneville Cutthroat Trout

Abstract

In western North America, nonnative trout invasions threaten the persistence of native cutthroat trout Oncorhynchus clarkii subspp. through competition, predation, and hybridization. While small-scale individual-level studies on nonnative–native fish interactions have helped elucidate mechanisms of impact, very few investigations have occurred at a scale consistent with that of species replacement. Thus, large-scale population-level studies are needed to understand the effects of nonnative fish as well as to more effectively conserve native fishes in the presence of nonnative fish. For this reason, we conducted an evaluation of the effects of nonnative brown trout Salmo trutta on the individual- and population-level performance of native cutthroat trout using a field-experimental approach. We studied local populations of cutthroat trout in the presence and absence of brown trout in long stream reaches and subsequently compared growth, condition, stable-isotope-based measures of dietary habits, movement, and survival between treatment groups. We found clear evidence for reduced cutthroat trout growth owing to the presence of brown trout. Additionally, we observed differences in cutthroat trout dietary habits based on δ¹³C signatures resulting from our experimental manipulation of brown trout abundance. Finally, brown trout suppressed cutthroat trout movement but had no apparent effect on their survival. Considering previous studies, we concluded that the individual-level effects of brown trout on cutthroat trout are scale invariant. Higher-level impacts (i.e., on movement and survival), in contrast, appear to be more sensitive to the scale at which the investigation is conducted. Overall, our results indicate that brown trout can have strong negative effects on cutthroat trout performance and should therefore be more explicitly considered in native fish conservation plans.