Responses of brown trout (Salmo truttaL.) to acidification and excess critical loads in lakes of western Norway with low ionic content

SummaryWe assessed the status of lentie brown trout (Salmo trutta) in relation to acidification in the County of Sogn og Fjordane in western Norway. Fish status was obtained by means of interviews, and characterised in terms of unaffected, damaged and lost populations, and related to various water chemistry parameters, ANC and excess critical loads of acidity. Water sampling was carried out in lake outlets in 1990, 1994 and 1995, and usually during the autumn. A total of 485 lakes were sampled, of which 472 lakes still contained, or previously had held, brown trout. We used an empirical steady-state (SSWC) model that incorporates ANC to estimate the critical load in individual lakes. In the model, we used a critical ANC (ANClimit) of 20 μeq/L, which means that no brown trout populations are regarded as damaged in lakes with higher values. The number of lakes >3 ha considered still to contain or previously to have contained fish was 3,116, while 250 lakes have never supported fish. The total numbers of lost and damaged brown trout populations are estimated to be 168 and 445, respectively. The study lakes were generally acid-sensitive, as 32% had no alkalinity, and 41% had pH below 5.5, while the mean calcium concentration was 0.77 mg/L. Generally, lakes with lost populations had much lower ANC and the critical load was highly exceeded compared to those with healthy populations, mean values being -8.57 vs. + 19.27 μeq/L and +25.70 vs. -35.91 μeq/m2/year, respectively. Lakes with lost and damaged populations of brown trout were more acidic, and had significantly lower concentrations of calcium and higher concentrations of toxic inorganic Al (Ali) than lakes with healthy populations. Analysis of variance and regression showed that Aliwas best correlated with brown trout status. Although the mean concentration of Aliin lakes with lost populations was low (mean values of 27.59 μg/L), its toxicity is probably related to low calcium content (mean value of 0.38 mg/L).