Brook trout (Salvelinus fontinalis) population dynamics and mottled sculpin (Cottus bairdi) occurrence in relation to acidic episodes in streams

We monitored water chemistry and populations of brook trout (Salvelinus fontinalis) and mottled sculpins (Cottus bairdi) in 5 streams and some tributaries to determine how the severity of acidic episodes (low pH and high Al) influenced density and distribution of these species. Among streams, Linn Run had the lowest pH (4.8) and highest concentrations of total dissolved Al (>400 μg‐l−1) during periods of high discharge. Densities of brook trout and mottled sculpins were low in Linn Run, but high in tributaries, where pH (6.5) and Al (30 μg‐l−1) remained well below lethal levels. Among the other streams, brook trout density was highest where the acidic episodes were least severe and the density of age‐0 brook trout was lowest when precipitation during the early months of the year was above normal. Immigration of brook trout from downstream areas maintained populations in the stream segments subjected to severe episodes. Mottled sculpins were not found in streams with severe episodes.