Competition Between Brook Trout (<i>Salvelinus fontinalis</i>) and Brown Trout (<i>Salmo trutta</i>) for Positions in a Michigan Stream

Abstract

Competition between brook trout (Salvelinus fontinalis) and brown trout (Salmo truttta) was studied by measuring characteristics of daytime positions held by brook trout before and after removal of the brown trout from 1800 m of a stream. We used four criteria as indices of position quality: "water velocity difference" (the difference between velocity at the focal point and in the fastest current within 60 cm of the fish), water depth, distance to stream bed, and lighting. After brown trout removal, brook trout larger than 15 cm chose resting positions with more favorable water velocity characteristics and more often in shade. The position shift was greatest for the largest brook trout, those of 20–30 cm. Feeding positions of brook trout changed little upon brown trout removal according to our criteria. The shift in resting positions of brook trout after release from competition with brown trout indicates that brown trout excluded brook trout from preferred resting positions, a critical and scarce resource. The combined effects of such interspecific competition, differential susceptibility to angling, differential response to environmental factors, and predation of brown trout on juvenile brook trout may account for declines of brook trout populations while brown trout populations expand in many streams of the northeastern United States where the two species are sympatric.Key words: brook trout, brown trout, competition, ecological release, microhabitat use, resting positions, feeding positions, stream, Michigan