A kill of more than 1,000 fish in the Glenavy River in Co Antrim at the weekend has prompted calls for stronger deterrents for polluters.
Members of the Glenavy Conservation and Angling Club raised the alarm with the Northern Ireland Environment Agency on Friday after seeing distressed fish gasping for air.
Further inspection led to the initial discovery of 510 dead fish but this number later increased to well over 1,000.
Club chairman Anthony McCormack said: "The source of the pollution was determined to be in an area close to the Gobranna Road, Glenavy.
"Above the site of the fish kill there was no sign of fish in distress and indeed fish were witnessed rising in a pool above the start of the upper limits of the kill.
"This in itself would appear to be a good indicator of where the incident stemmed from. The species of fish killed were brown trout, not stocked fish but wild trout.
"This does not take in to account of the numbers of other smaller fish killed as a result - species such as stone loach, an important fish that helps to maintain the cleanliness of the river bed by keeping unwanted harmful invertebrates in check.
"There were hundreds possibly thousands, of these little fish wiped out as well."