An investigation has been launched after a major fish kill on the River Callan in Armagh City.
The Northern Ireland Environment Agency (NIEA) received a report on Tuesday evening, indicating there were dead and distressed fish in the river.
The agency deployed an inspector to the area to investigate and determine the environmental impact.
The immediate investigation found a small number of fish mortalities and the river was noted to be running with black discoluration.
The inspector traced the discoloured pollutant and identified that the impact was coming from a specific tributary to the River Callan upstream of Armagh City.
An NIEA investigation team was on site early on Wednesday and a definite line of enquiry is now being followed.
The Department of Agriculture, Environment and Rural Affairs has confirmed a major fish kill has occurred.
So far around 1,000 brown trout and 100 salmon have been counted and work is ongoing to assess the full impact.
Water samples have been collected and evidence gathered with a view to initiating prosecutions proceedings.
The investigation remains ongoing.