Fishery decline: mechanisms and predictions

The hypothesis that surface water acidification is responsible for fisheries loss depends on demonstration that fishery status is determined by acidity and related chemistry of lakes and rivers. The effect of acidity is shown to be moderated by the benefit of calcium and the adverse effect of aluminium: this is why no simple acid \`threshold' or \`acceptable pH' can be set for any species. The sensitivity to both acidity and aluminium toxicity varies with age in a complex way. Few data sets provide concurrent information on both water quality and fishery status, and a consistent picture does not emerge, possibly due to inadequate data, to natural differences between locations, but possibly because acidification alone may not be the cause at all locations. Critical pH-levels are not known for many species, and reputed thresholds for effects vary as much as 400-fold for H $^+$ toxicity. Some instances occur where fish are found in relatively acid waters, and where fish losses have occurred at levels of pH not judged toxic in the laboratory. Both laboratory data and field observations have shortcomings which limit the test of the hypothesis that acidification has led to loss of fisheries. The extent to which other environmental variables may affect fish populations in acid oligotrophic waters is quite uncertain. Unsuitable stream conditions associated with afforestation and unstable hydrological regimes may limit the benefit to be gained from remedial measures.