Effects of stream acidification and habitat on fish populations of a NorthAmerican river

Water quality, physical habitat, and fisheries at sixteen reaches in the Ne versink River Basin were studied during 1991-95 to identify the effects of acidic precipitation on stream-water chemistry and on selected fish-species populations, and to test the hypothesis that the degree of stream acidific ation affected the spatial distribution of each fish-species population. Mo st sites on the East Branch Neversink were strongly to severely acidified, whereas most sites on the West Branch were minimally to moderately acidifie d. Mean density of fish populations ranged from 0 to 2.15 fish/m(2); biomas s ranged from 0 to 17.5 g/m(2). Where brook trout were present, their popul ation density ranged from 0.04 to 1.09 fish/m(2), biomass ranged from 0.76 to 12.2 g/m(2), and condition (K) ranged from 0.94 to 1.07. Regression anal yses revealed strong relations (r(2) +/- 0.41 to 0.99; p less than or equal to 0.05) between characteristics of the two most common species (brook tro ut and slimy sculpin) populations and mean concentrations of inorganic mono meric aluminum (Al-im), pH, Si, K+, NO3, NH4+, DOG, Ca2+ and Na+; acid neut ralizing capacity (ANC); and water temperature. Stream acidification may ha ve adversely affected fish populations at most East Branch sites, but in ot her parts of the Neversink River Basin these effects were masked or mitigat ed by other physical habitat, geochemical, and biological factors.