DOWNSTREAM MOVEMENT AND MORTALITY OF BROOK TROUT (SALVELINUS-FONTINALIS) EXPOSED TO ACIDIC EPISODES IN STREAMS

Episodes of low pH and high Al concentration were associated with net downstream movement and increased mortality of ratio-tagged brook trout (Salvelinus fontinalis). We compared movements of 10-15 brook trout in two streams with severe acidic episodes with movements of fish in two streams with less severe acidic episodes (reference streams). Study populations moved downstream hundreds of metres in the streams with more severe acidic episodes (pH <5.0 and >200 mu g Al/L). Median downstream movement in spring was 250 and 900 m after 20 d for fish in the more acidic streams; one third of the fish were found dead during this time. We found no net movement and no dead fish in the reference streams. Lower stream discharge in fall studies was associated with less severe acidic episodes, less net movement, and no mortality. Water samples collected at individual fish locations indicated that few fish avoided adverse effects of acidic episodes by remaining in microhabitats with higher pH and lower Al concentration.