Brown trout were among the fish killed
Almost 900 fish have been killed after a river in County Tyrone was polluted at the weekend.
It is believed farm effluent discharged into the Fardross river, a tributary of the Blackwater.
After an initial inspection about 870 fish - mainly brown trout and salmon parr (young salmon) - were found dead.
Samples have been taken with a view to prosecution, the Environmental Heritage Service has said.
Officers from the EHS and Fisheries Conservancy Board are investigating.