Competitive interactions between Arctic grayling and brook trout in the Big Hole River drainage, Montana

We investigated mechanisms through which nonnative brook trout Salvelinus fontinalis could negatively influence native Arctic grayling Thymallus arcticus. We assessed occupied microhabitats, growth, and condition of Arctic grayling and brook trout in sympatric and allopatric groups in stream enclosures. Microhabitat use by both species was also measured in a natural assemblage. In stream enclosures and the natural assemblage, Arctic grayling and brook trout exhibited similar preferences for pools and runs, but they spatially segregated in microhabitats. Arctic grayling occupied faster focal point and mean column velocities closer to the surface than brook trout. Brook trout occupied slower focal point velocities in proximity to structural cover. Specific growth rates indicated that intraspecific competition operated more strongly than interspecific competition in regulating growth of both species in stream enclosures. In sympatry, Arctic grayling grew faster at a low density with a high density of brook trout. In contrast, Arctic grayling at high densities in the control and a sympatric treatment exhibited slow or negative growth. Similarly, brook trout grew faster in a low-density enclosure in sympatry with a high density of Arctic grayling. We found little evidence that nonnative brook trout negatively affected microhabitat use or growth of native Arctic grayling.