The investigation into last month's fish kill in the River Blackwater near Mallow has concluded with no definitive cause identified.

Inland Fisheries Ireland (IFI) estimates that up to 32,000 salmon and brown trout mortalities may have occurred.

A summary report on the fish mortalities was published on Thursday asserts that that a waterborne irritant likely entered the river up to 72 hours before the first mortalities were observed, at a point most likely upstream of the mortalities initially observed by IFI.

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It dissipated quickly, resulting in it being undetectable in subsequent water samples and fish tissue samples.

Minister of State at the Department of Climate, Energy and the Environment with responsibility for Fisheries and the Marine Timmy Dooley said: “The extensive fish mortalities that occurred on the River Blackwater represent a serious impact to local fish stocks and have had a deeply negative impact on surrounding communities.

Although the evidence points to this being a short-term pollution event, Inland Fisheries Ireland estimate that up to 32,000 salmon and brown trout mortalities may have occurred.

"When incidents of this severity occur, such as on the Blackwater, it is the duty of all State agencies with statutory responsibilities in this area to act decisively: to exhaust all avenues in identifying possible causes and contributing factors."

The findings in the report, he added, do give reassurance that this was a short-lived event, with no evidence of ongoing pollution risks.

"I want to emphasise that the investigation was exhaustive and involved extensive sampling, testing and monitoring – by multiple State agencies.

“This incident underpins the need to maintain a rigorous and transparent approach. Importantly, the learnings from this incident will now inform future inter-agency approaches to investigating significant incidents impacting water bodies across the country," he said.

Angling

Angling is continuing on the Blackwater, with no water quality issues currently detected. Uisce Éireann undertakes monitoring of drinking water supplies in accordance with the requirements of the Drinking Water Regulations. Monitoring results from the Mallow water treatment plant to date have shown that the water is safe to drink.

While fish populations are expected to recover, IFI and other agencies recognise the significant impact this incident has had on the area.

IFI collated and published the inter-agency report, drawing on investigative work undertaken by IFI, the Environmental Protection Agency (EPA) and Cork County Council, and supported by the Marine Institute, Uisce Éireann, the Department of Agriculture, Food and the Marine, the Health Service Executive, the National Parks and Wildlife Service (NPWS) and the Local Authority Waters Programme (LAWPRO).