The relative abundance of brown trout in acidic softwater lakes in relation to water quality in tributary streams

The influence of the water quality of tributary streams on the relative abundance in benthic gillnet catches (catch per unit effort, cpue) of allopatric brown Salmo trutta was assessed in associated acidic, softwater lakes. The study was carried out over 6 years (1989-1994) in 15 lakes located al altitudes between 230-715 m a.s.l. in two Norwegian catchments. The water quality of the main inlets and outlets varied little, as indicated by their of pH range (4.93-5.51) and calcium concentrations (0.19-0.44 mg l(-1)), but varied more with respect to concentrations of inorganic, monomeric aluminium (7.0-41.0 mu g l(-1)). Most of the lakes were also fed by secondary streams with better water quality: a maximum pH of 6.56, calcium levels of up to 0.74 mg l(-1), and inorganic aluminium levels as low as 1.0 mu g l(-1). The cpue was inversely correlated with lake altitude (r(2)=0.50), and thus was adjusted to a mean altitude. The calcium concentration in the richest secondary stream to each lake, its richness judged on the basis of its acid-neutralizing capacity, had the highest predictive power of the variability in cpue (r(2)=0.49). The calcium content in the other secondary streams or in the main inlets and outlets did not correlate with cpue. Alkalinity in the main outlets correlated to some extent with cpue (r(2)=0.27). It is suggested that secondary streams with good water quality provide important refuges for the recruitment of brown trout in acidic softwater lakes. (C) 1998 The Fisheries Society of the British Isles.