Inland Fisheries Ireland says it's pursuing a definite line of inquiry into the cause of a major fish kill in Mayo last week.
The environmental agency is still analysing samples from the Glore River near Kiltimagh on Friday.
500 juvenile salmon and trout were found dead along a 2km stretch of the tributary of the River Moy.
It follows an overflow at Kiltimagh Water Treatment Plant that Mayo County Council and Irish Water told Midwest News they are investigating.
Greg Forde, head of operations at IFI is very concerned for the safety of one of the most important salmon rivers in Ireland.