Research Document - 2000/050

Effects of low environmental pH on Atlantic salmon in Nova Scotia.

By G.J. Farmer

Abstract

Acidic rivers in Nova Scotia have low concentrations of Ca⁺⁺ (calcium) and high concentrations of dissolved organic matter and total dissolved aluminum. Dissolved organic matter binds to aluminum to form organic aluminum complexes which are not toxic to Atlantic salmon. Increased H⁺ ion concentrations (acidity) coupled with the low concentrations of Ca⁺⁺ have caused the mortality of salmon in Nova Scotia. When pH £ 5.0, the active uptake of Na⁺ (sodium) and C1^– (chloride) across the gill epithelium is reduced and the passive efflux is increased resulting in the net loss of both ions. The increased passive efflux of ions results from the displacement of Ca⁺⁺ from binding sites on the gill epithelium by H⁺. The loss of ions results in a shift of water from the extracellular fluid volume to the intracellular fluid volume causing a reduction in blood volume. The reduced blood volume and an increase in hematocrit causes an increase in blood viscosity which results in circulatory system failure. Sensitivity of the various salmon stages to low pH can be summarized in the following order of decreasing sensitivity:

fry > smolt > small parr > large parr > alevin > egg

Thus, significant mortality (19 - 71%) of fry occurs at a pH of about 5.0. Mortality of smolts also occurs at a pH of 5.0 but the rate is lower (1-5%). Mortality of parr and smolts is relatively great (72 – 100%) when pH declines to the 4.6 – 4.7 range. Mortality of eggs and alevins does not begin until pH declines below 4.8.

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