Stability of Microhabitat Use by Brook Trout and Juvenile Atlantic Salmon after Stream Acidification

Abstract

Microhabitat use by brook trout Salvelinus fontinalis and juvenile Atlantic salmon Salmo salar in a small acidic brook was examined in response to the acidification episodes typical of rivers of southwest Nova Scotia in the fall, and to a longitudinal pH gradient related to liming. Several microhabitat variables were measured for age-0 and age-1 and older fish of both species. Some of the measurements were made relative to the fishˈs focal point (height of the fish above the bottom) and included water depth and focal elevation, focal water velocity, focal water pH and temperature, and bed material grain size. The presence of a small, longitudinal pH gradient (<1 pH unit) along the brook did not alter microhabitat use by brook trout or juvenile Atlantic salmon, either in summer at pH greater than 5.5 or in the fall at pH less than 5.0. Similarly, a major acidification episode in the fall (>1 pH unit; final pH, <4.8) did not cause any major shift in microhabitat use by either species between summer and fall. The use of a particular microhabitat by fish generally reflected its availability, but some preferences in water depth and focal water velocity by age-0 fish ensured that Atlantic salmon and brook trout were effectively segregated at all times.