An investigation is under way following the discovery of 850 dead wild Atlantic salmon in a Co Sligo river.
Inland Fisheries Ireland said the salmon were found in the lower Ballisodare River near Sligo town.
IFI said its staff is working with other officials in relation to the incident and is conducting a full environmental analysis" to investigate whether "water quality issues may be contributing to mortalities".
Its deputy chief executive Dr Cathal Gallagher said the organisation does not know if there are more fish dead elsewhere in the catchment.
Speaking on RTÉ's Today with Philip Boucher-Hayes, Dr Gallagher said it is a serious issue and water samples have been taken for analysis, with teams looking at the temperature of the water, oxygen levels and PH levels.
He said the first set of results should come early next week.
The river is now closed for angling for the immediate future.
"Everybody is putting their shoulder to wheel to try and understand what this issue is, in some cases with issues like this, we may never find out.
"It could be some naturally occurring phenomena that has passed. It's a difficult issue, we are hoping that we are at the peak of it and it doesn’t continue," Dr Gallagher said.
