Silage discharged into a stream at two points caused it to be polluted by 64 times its normal level, a court was told.
The South Western Fisheries Board prosecuted Denis Collins from Killeens, Kilbrittain, Co Cork, on three charges brought under the Water Pollution Act, before Judge Brendan Wallace in Kinsale District Court.
Collins, who had taken measures to rectify the situation, was fined a total of £300 with £356 expenses and £100 costs.