CHEMICALS have been found in a Co Fermanagh river where almost 2,000 trout and more than 500 salmon died last week.
The "strange" kill happened on a 4km stretch of the Kesh River.
Stormont Agriculture Committee was told investigations are ongoing to identify the source of the pollution.
So far two chemicals `of interest' have been discovered, with more water samples being analysed.
Head of Department for Agriculture, Environment and Rural Affairs (Daera) Water Management Kerry Anderson said it was "really strange" that invertebrates on which fish feed were not affected.
There were 1,899 trout of all ages and 576 salmon up to two-years-old killed.
She said the investigation is focussed on a specific geographical area and dissolved oxygen levels were within the normal range with living fish were found in a 1km section of the river.
Assembly members expressed concerns dead fish remain uncollected in the river.
The official said nature will be allowed to take its course, with no plans to restock the river.
The trout population is expected to return pre-kill levels in three to four years.
