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Population structure, density and reproductive potential of perch (Percafluviatilis L.) from 4 acidified lakes in a river system in Southern Norway were investigated. The upper 3 lakes were most affected by acidification and the number of perch caught per unit effort indicate three-fold increase in density from the upper Gjerstadvann to the lower Brsbsrvann. Low density of perch in the upper lakes is explained by; 1) Abnormal mortality occurring in episodes due to acidification, probably caused by aluminium toxicity at pH = 5.2-5.4.2). Acid water (pH 55.0) during spawning and development of eggs and larvae, resulting in recruitment failure.Juvenile mortality of perch may depend on the density of the parental stock. In the 3 upper lakes, the density of adult perch probably was too low to produce significant juvenile mortality, and in these lakes the recruitment probably depended more on the May water quality. Improved water quality increases egg hatching and survival and also benefits zooplankton production, the main food for perch during the first summer.Low population density has reduced competition for food and thus improved growth of perch in the 3 upper lakes compared to the lower lake.The perch in Brabsrvann mature at higher age and have lower individual fecundity than perch in Gjerstadvann, an effect of both slower growth and lower length specific fecundity in Brabsrvann. In Gjerstadvann, the perch therefore have a higher reproductive potential relative to the stock density, and may, when the water quality is good enough, give rise to strong year-classes and more frequent year-class fluctuations than perch in the less acid Brsbsrvann.

Smolts of Atlantic salmon (Salmo salar) were exposed for 70 h in the acidic River Vaaraana, southern Norway (Q = 1.3 m3/s, pH = 4.4–5.6, Ca = 0.6–1.5 mg/L, labile Al = 50–100 μg/L), to waters neutralized by addition of limestone slurry. Chemical and biological parameters were measured upstream of the liming site and 20, 100, 1000, 2000, and 3500 m downstream. Addition of limestone slurry effectively increased pH from 4.6 to 6.9, Ca from 0.6 to 2.6 mg/L, and reduced the labile Al concentration from 59 to 35 μg/L immediately downstream of the liming site (20 m, after 30 s). In the untreated upstream waters, salmon smolts lost plasma Cl rapidly, and all fish died within 36 h. In spite of the slight oversaturation of labile Al immediately downstream of the base addition, no mortality occurred in neutralized waters. Relatively high Ca concentrations may have mitigated potential toxic effects from Al oversaturation. Plasma Cl levels did not vary with distance downstream. Detoxification of running waters with highly toxic levels of pH and Al can successfully be performed by addition of limestone slurry.

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