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 trou t (Salvelinus fontinalis). We compared movements of 10-15 brook trout in two streams with severe acidic episodes with movements of fish in t wo streams with less severe acidic episodes (reference streams). Study populations moved downstream hundreds of metres in the streams with m ore severe acidic episodes (pH <5.0 and >200 mu g Al/L). Median downst ream movement in spring was 250 and 900 m after 20 d for fish in the m ore 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 strea ms. Lower stream discharge in fall studies was associated with less se vere acidic episodes, less net movement, and no mortality. Water sampl es collected at individual fish locations indicated that few fish avoi ded adverse effects of acidic episodes by remaining in microhabitats w ith higher pH and lower Al concentration.