More than a thousand brown trout along with other species of fish have been killed following an effluent discharge into a river in Claremorris, Co Mayo.
The dead fish were found floating on the Ballygowan River on Wednesday evening in what is one of the prime spawning tributaries of the Robe River which falls into Lough Mask.
The fish kill was so substantial that it may take years for stocks in the river to recover because so many of the spawning fish have been killed.
Inland Fisheries head of operations Greg Forde said the pollution spill "wiped everything before it" and did a lot of long-term damage.
“Time is the only thing that will cure this. These are wild fish. We don’t believe in stocking where it is the native genetic stock. There is no substitute for nature,” he said.
The dead also included stone loachs and a protected species of crayfish.
The brown fish were found in an agitated state in the river on Tuesday evening. Inland Fisheries were alerted on Wednesday evening to the fish kill.
The fish were found lying dead on the surface of the water upside down.
There was no trace of the effluent left and fisheries officials believe that it had passed through the river at that stage.
Mr Forde said farmers have to exercise extreme care in spreading slurry because of the dry weather of late.
Water levels in some rivers are at an all time low, he explained, and a small amount of effluent can have “devastating results”.
He added: "Inland Fisheries Ireland is calling on all farmers and silage contractors, in particular, to be extremely careful in the current conditions to ensure that no effluent is released near drains, streams and rivers, and silage clamps are properly bunded.
“Extreme care should be taken when spreading slurry to avoid all water courses in order to protect our valuable natural rivers and streams.”
