Do Nonnative Salmonines Exhibit Greater Density and Production than the Natives They Replace? A Comparison of Nonnative Brook Trout with Native Cutthroat Trout

The replacement of native species by nonnative species has been widely documented. In aquatic ecosystems, nonnative trout often replace native trout species to which they are closely related. For example, nonnative brook trout Salvelinus fontinalis have replaced native cutthroat trout Oncorhynchus clarkii throughout western North America. We hypothesized that when brook trout replace cutthroat trout, they exhibit greater density, biomass, and annual production than were achieved by the cutthroat trout before replacement. We tested this hypothesis in five paired streams in the Teton River drainage, Idaho, over a 4-year period. Streams were paired based on similar habitat characteristics and the presence of brook trout or cutthroat trout as the dominant species. Fish were surveyed annually by means of multipass depletion along representative stream reaches. Consistent with our hypothesis, brook trout exhibited 2.4 times greater density, 1.7 times greater biomass, and 2.5 times higher annual production than cutthroat trout. Because age-0 fish were difficult to capture during electrofishing, we also analyzed the data without this age-class. When age-0 fish were excluded, brook trout exhibited 1.3, 1.6, and 2.1 times greater density, biomass, and annual production, respectively, than cutthrout trout. These results are consistent with the hypothesis that nonnative salmonines exhibit greater density and production than the native fishes they replace, though the generality of this finding remains to be tested. The differences that we observed not only testify to the success of brook trout as an introduced species in western North America but also point to likely changes in ecosystem function that may accompany their replacement of native cutthroat trout.