The inter-agency group set up last month to investigate the cause of the large fish kill on the River Blackwater near Mallow in Co Cork in early August has concluded its work, but says it has not been able to identify the cause of the fish kill.

It is estimated up to 32,000 salmon and brown trout died in a 37-kilometre stretch of the river when an unknown environmental irritant entered the water.

This would make it the largest single fish kill on record here.

However, in its final report published this evening on the Inland Fisheries Ireland website, the interagency group said it could find no evidence to support a link between the fish mortalities and a point source of pollution, or a specific environmental insult or waterborne irritant.

The report said that no change in water quality in the affected stretch of river has been detected between 2024 and 2025.

Diagnostic analyses by the Marine Institute on dead brown trout sampled from the river on 14 August had earlier found no evidence of systemic disease.

More recent analysis of brown trout for presence of chemicals, pesticides and heavy metals also did not identify any such possible irritant.

The report says the waterborne agent that caused the fish kill had a likely entered the river Blackwater around the 5 or 6 August and the first fish mortalities were observed 72 hours later, on 9 August 2025.

However, it says the pollutant dissipated quickly, rendering it undetectable in water samples and fish tissue samples.

There was no evidence of any mortalities of protected species such as otter or freshwater pearl mussels.
Neither were there any mortalities among bird species, nor indications that livestock health and the food chain from primary production was affected by the fish kill incident.

The report also details the outcome of 85 different investigations and inspections in relation to water quality issues along the affected stretch of river but could find no causal link to the fish mortalities.

It found no evidence of a chronic water quality problem, either before or after the fish mortalities.

This suggests, according to the report, that the cause of the mortalities was a short-term pollution event.

The EPA investigated regulated sites in the River Blackwater catchment, including industrial facilities, wastewater treatment plants and drinking water plants but it could establish no causal link between these activities and the fish mortalities in the River Blackwater.

20 complaints were received from members of the public in respect of the incident and were followed up by the EPA but again, no causal link with the fish kill could be established.

Cork County Council also inspected twenty light industrial and commercial sites within the catchment, comprising all businesses licensed under Section 4 of the Local Government Water Pollution Act as well as other commercial operators.

14 investigations of agricultural activities within the catchment were also carried out. But in all cases, a causal link to the incident could not be established.

In addition, Inland Fisheries Ireland itself carried out 198 habitat inspections, and 52 repeat habitat inspections, at 47 locations along the affected stretch of river.

It conducted macroinvertebrate sampling at 10 different sites. It also sent fish samples to Germany, via Eurofins Environmental Ltd, to be tested for a broad suite of 900 chemicals, pesticides and heavy metals.

Yet despite all these investigations, nothing was found.

Minister of State with responsibility for Fisheries and the Marine, Timmy Dooley, has welcomed the publication of the summary report from the inter-agency group, which he says provides a clear account of the response to, and the investigation into, the incident.

He 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.

“The investigation has now concluded. In this instance, a definitive cause could not be identified despite the extensive and significant investigation by members of the inter-agency group.

“The report does assert 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. It dissipated quickly, resulting in it being undetectable in subsequent water samples and fish tissue samples.

“The findings in the report 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.”