A fish kill in the Castletown river is being investigated by the Eastern Regional Fisheries Board. The board found more than 200 dead fish at Toberona Bridge, in Dundalk, Co Louth, over the weekend, but has been unable to establish the full extent of the pollution because of high water levels. The board described the situation as an "environmental alert".
The pollution is thought to be the result of agricultural discharge in Newtownhamilton, Co Armagh, where agricultural waste was dumped into the White Water river early last week.
The source of the river is in the jurisdiction of the Northern Ireland's Environment and Heritage Service (EHS).
"The slurry has moved from its original source down the White Water river into the Castletown river, a distance of 20 kilometres, crossing the Border at Ballybinaby," the EHS reported. It believes the slurry was dumped into the river because bad weather prevented farmers accessing the land. The Castletown river rises in Co Armagh and enters Dundalk Bay just north of the town.