The Northern Ireland Environment Agency (NIEA) is currently investigating a major fish kill on the River Callan in Armagh City.
In a statement, the NIEA said they received a report via the Water Pollution Hotline on Tuesday evening (June 12), that there were dead and distressed fish in the river.
On hearing that the NIEA " deployed an inspector to the area to investigate and determine the environmental impact".
"From the immediate investigation, a small number of fish mortalities were confirmed and the river was noted to be running with black discolouration, read the statement. "Prior to poor light preventing further investigation, 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."
They added that a NIEA investigation team was on site early yesterday and "a definite line of enquiry is now being followed".
Around 1,000 brown trout and 100 salmon have been killed - and work is ongoing to assess the full impact.
"Statutory samples have been collected and evidence gathered with a view to initiating prosecution proceedings through PPS. The investigation, under the Water (NI) Order 1999 remains ongoing," added the statement.
"Following set procedures, the NIEA’s Emergency Pollution Officer notified the Department of Agriculture, Environment and Rural Affairs, (DAERA), Inland Fisheries, so that a joint investigation could be carried out. DAERA Inland Fisheries have confirmed that a Major Fishkill has occurred."