Behavioral response of brook trout and brown trout to acidification and species interactions

Many factors have been associated with the decline of brook trout (Salvelinus fontinalis) throughout their native range including acidification and competition with brown trout (Salmo trutta), an introduced species. We observed the behavior of wild, adult brook trout and brown trout (alone and in combination) within study reaches that were manipulated to vary the level of acidity and CO₂. The aims of the study were to address 1) if acidification affects the segregation patterns of wild, adult brook trout and brown trout and 2) whether interactions among brook trout and brown trout influence the distribution of brook trout in acidified waters. After manipulating acidity levels, the majority of indwelling fish responded to acidification by moving to more neutral conditions (62 % of brook trout and 68 % of brown trout). Indwelling brook trout spent less time in acidic conditions during the acid treatment (41 ± 5 %) than during the control period (94 ± 2 %). Indwelling brown trout spent less time in the acid conditions during the acid treatment (44 ± 4 %) than during a control period (98 ± 1 %). The proportion of time spent in the acid water by indwelling trout decreased as negative interactions with other fish (such as chasing) increased. Presence of the opposite species did not influence the proportion of time spent in the acidic conditions. The results of this study do not support the hypothesis that acidification mediates the segregation of brook trout and brown trout in streams.