MORTALITY OF BROOK TROUT, MOTTLED SCULPINS, AND SLIMY SCULPINS DURINGACIDIC EPISODES

Brook trout Salvelinus fontinalis, mottled sculpins Cottus bairdi, and slimy sculpins Cottus cognatus occur in many Pennsylvania streams tha t have depressed pH and elevated aluminum concentrations during episod es of high stream discharge (acidic episodes). We performed 20-d in si tu cage exposures with these species to determine their relative sensi tivities to field conditions. We also exposed fish in the laboratory t o synthetic soft water, without added Al, to elevate possible effects of Al on sodium flux rates and pH toxicity. Exposures were in five str eams: two with high pH (> 5.60) and low Al concentrations (< 80 mug/L) and three with low pH (usually between 5.0 and 5.5) and high Al level s (124-294 mug/L). Exposures were during two low-discharge fall period s, when pH tends to be seasonally higher and Al concentrations lower, and two relatively high-discharge spring seasons, when lower pH and hi gher Al concentrations are typical. Few fish died (generally < 10%) in the two streams that had higher pH and lower Al concentrations, where as mortalities typically exceeded 20% and were as high as 100% during spring exposures in the streams with lower pH and elevated Al concentr ations. All three species had higher mortality rates in spring, 20-100 %, than in fall, 0-29%. Mottled sculpins and slimy sculpins had simila r mortality rates and both had lower mortality rates than brook trout when exposed to similar conditions. We compared Na flux rates of mottl ed sculpins to those of brook trout in laboratory exposures at pH 4.0, 5.0, and 6.0 to determine if low pH alone could account for mortality rates in the field. Because Na flux rates at pH 5.0, without added Al , were similar to flux rates of the controls for both species, high Al levels were believed to have contributed to the increased mortality o bserved in streams.