While the relationship of SO2 and NOx emissions to ambient air quality and deposition, and to surface water quality, are still matters for debate and clarification, it is generally recognised that fisheries do not thrive in acid waters. An international advisory commission (EIFAC) has reviewed much of the literature and concluded that below pH 5 progressively more severe effects on fish can be expected. Records of acid lakes in Scandinavia and America show that about 50% of lakes below pH 5 are fishless, compared with only one in seven of similar oligotrophic lakes with pH greater than 5. However, it is clear that the status of a fishery is not a simple function of the reported pH of a lake or river. Both field and laboratory studies provide evidence that other water quality factors (such as the levels of aluminium and calcium), biological factors (such as food supply, genetically determined tolerance, adaptation), and ecological factors (such as lake morphology, patterns of hydrological events) are also important. How else can we explain the continued presence of fish in some waters well below pH 5? The paper will present a review of current information from both field and laboratory investigations concerning the responses of fish and fish populations to natural acid conditions in Scandinavian and N. American lakes and rivers, identifying the factors which determine survival and mortality. The conclusions have some significance for remedial actions that may be proposed to alleviate acid conditions in Scandinavian and N. American waters.

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