Comparative Behavior and Habitat Utilization of Brook Trout (Salvelinus fontinalis) and Cutthroat Trout (Salmo clarki) in Small Streams in Northern Idaho

Individual brook (Salvelinus fontinalis) and cutthroat (Salmo clarki) trout communicated with similar behavioral signals, both in laboratory stream-channels and in northern Idaho streams. Underyearling brook trout were less active socially than equal-sized cutthroat trout in laboratory observations. In study streams, brook trout maintained a 20-mm size advantage over cutthroat of the same age-groups throughout their lives, as they emerged from the gravel before cutthroat. Because of this size advantage, underyearling brook trout of sizes found in study streams in September consistently dominated in experiments the underyearling cutthroat with which they normally lived. But in study streams underyearlings of the two species utilized different microhabitats, particularly with respect to water depth, and so minimized chances for interaction.Yearling and older brook trout initiated 40% fewer aggressive encounters under laboratory conditions than did equal-sized cutthroat trout, and did not displace the cutthroat. In study streams with sympatric populations, cutthroat trout of these age-groups occupied territories with focal points of higher water velocities (averaging 10.2–10.3 cm/sec) than those occupied by brook trout (averaging 7.6–9.6 cm/sec). Considerable interspecific overlap in other habitat characteristics occurred for trout of age-groups I and II. The oldest members of the two species segregated more distinctly, as the brook trout lived closer to overhead cover.