Fish Kills in Ireland 1991-1992

Abstract

The numbers of fish kills were below the average for the ten years since the systematic recording of incidents began in 1983. Totals were 60 and 51 for 1991 and 1992 respectively. Both years therefore represent a continuation of the downward trend which began after the peak of 122 incidents in 1987. An important factor in the improved situation was the reduction in the number of silage discharges which had been the most serious problem for a number of years. Untreated sewage and industrial effluents in general have also shown downward trends. . Two serious problems remaining are 'enrichment' and run-off from farmyards, which includes spillage of slurry. Slurry and farm effluents in fact showed an increase in 1992 over 1991. Enrichment by excessive phosphorous, derived both from partially treated domestic sewage and from fertiliser, causes algal blooms leading to deoxygenation of the water and the death sometimes of very large numbers of fish - to say nothing of environmental conditions which are unacceptable to everybody who uses rivers or lakes. The effect of a discharge of effluent depends on many factors. In wet weather with high stream flows the damage will be minimised because the effluent is diluted quickly. A spillage beside a small river will led to a much more extensive fish kill than the same quantity of effluent will cause in a large one. These and other factors probably explain why, in spite of the small number of fish kills in 1992, nearly twice as much river was affected as in 1991.