The Central Fisheries Board informs me that there are no simple criteria which set down minimum standards with regard to water quality and fish. Different fish species respond differently to various pressures. Salmonids – salmon, trout, char – are cold water species, 4-150, they require very clean conditions with high oxygen levels and become stressed and more vulnerable to pollutants when temperature rises. Cyprinids or coarse fish – roach, rudd, bream, carp, eels – favour higher water temperatures,10-210, can survive lower oxygen levels and are more tolerant of pollution generally.
Reports on water quality in Ireland are published by the Environmental Protection Agency and the most recent review of the state of the inland waters covers the period 1995 to 1997. There are two main findings in relation to rivers and streams: a continuation of a trend of increasing slight and moderate pollution; a reversal of the trend of decreasing serious pollution which had been gradually falling off over the years.
The upward trend in the extent of slight and moderate pollution is attributed mainly to eutrophication by organic, animal manure, and artificial fertilisers and to a lesser extent by point source, i.e. sewage, discharges. The slight increase in the extent of seriously polluted channel is attributed mainly to suspected sewage discharges and to a lesser degree to suspected agricultural activities.
Of the 120 lakes examined in the review period, 81% have been assigned a trophic status, oligotrophic or mesotrophic, consistent with unenriched, satisfactory water quality conditions and a low probability of pollution. The water quality of the remaining lakes examined points to less than satisfactory conditions and the likelihood of significant impairment of beneficial use.