Does short-term acclimation affect pH preference and avoidance of adult brook trout and brown trout

Competition with brown trout is one factor associated with the reduction of wild brook trout populations in some Pennsylvania streams.  Previous research showed a correlation between pH and the distribution of brook trout and brown trout in these streams.  Our goal was to determine if these species were capable of short term acclimation to low pH.  Groups of adult, hatchery brook trout and brown trout were held at various pH levels to address this question.  Avoidance was determined using a steep-gradient choice tank.  Preference was determined by allowing fish to choose from a gradient (pH 4.0 – 7.0).  Brook trout did not display avoidance until pH 4.0 and showed no preference in the pH gradient.  Brown trout held at pH 6.0 exhibited peak avoidance behavior at 0.5 pH units lower than those held at pH 8.3.  The pH preference of brown trout was 5.0 – 7.0.  Short term acclimation to low pH was not observed in brook trout, but a shift in avoidance pH was observed in brown trout.  This research indicates that wild fish used in further studies can be held at a different pH than the source stream for a short period time without altering their behavioral response