Occurrence of salmonid parr and eel in relation to water quality in small streams on the west coast of Sweden

Electrofishing and water chemistry data, collected during a survey of 12 streams (88 stations) in the summer of 1983, showed that pH was above 6.0 at 89 per cent of the stations, and alkalinity was greater than 0.1 meq per litre at 75 per cent of stations. Occurrence or abundance of brown trout, Atlantic salmon and eel was correlated with alkalinity but not with summer pH. Occurrence of salmonid parr, and abundances of salmonid parr and eels were significantly higher (p less than 0.05) at alkalinities greater than 0.25 meq per litre. Sampling was continued throughout the winter of 1983-84 in 3 streams, and showed that alkalinity and pH decreased markedly during snowmelt, particularly at the higher altitude stations. Previously unpublished data, from electrofishing surveys of 7 similar streams in the region during 1956-84, showed yearly fluctuations in salmonid parr abundances. There was no overall trend in small/medium streams with summer alkalinities above 0.25 meq per litre but, in small streams with low alkalinity, parr abundances had decreased since the late 1950's. It was suggested that salmonid mortality was due to aluminium toxicity and, to a lesser extent, direct effects of low pH.