Minister of State for Fisheries and the Marine Timmy Dooley this week chaired the fifth meeting of the inter-agency group investigating the recent mass fish mortality on the River Blackwater, Co Cork. Despite months of intensive testing and inspections, no conclusive cause has yet been identified.

Extensive laboratory analysis of 28 brown trout specimens was presented to the group by a technical subgroup. The results, combined with ongoing inter-agency investigations, showed no single factor could be firmly linked to the deaths of the large number of fish. The technical teams concluded that the most likely cause was an irritant present in the water, but said the source and specific compound could not be determined.

The laboratory results, including heavy metals testing and residue analysis for hundreds of potentially harmful substances, have been published on the Inland Fisheries Ireland (IFI) website, along with an explanatory note for the public.

Agencies involved in the investigation have carried out extensive on-the-ground work across the catchment since the incident was reported. The Environmental Protection Agency has inspected dozens of regulated sites, taken and analysed multiple samples, and completed a series of macroinvertebrate surveys in the Kanturk–Mallow area. Cork County Council has inspected licensed discharges, commercial activities and agricultural practices to check for pesticide use and compliance with Good Agricultural Practice rules. Inland Fisheries Ireland has tested over 900 compounds, conducted habitat surveys and used electrofishing to monitor fish health, while the Marine Institute has carried out diagnostic testing to rule out disease. Uisce Éireann has also reviewed drinking water monitoring results to confirm supplies remain safe.

Angling continues on the Blackwater and no water quality issues are currently detected. Monitoring from the Mallow water treatment plant shows the water is safe to drink. While fish populations are expected to recover, agencies acknowledge the incident has had a major impact on the local and angling communities.

The inter-agency group will now review all actions taken so far and prepare a comprehensive report setting out its findings and recommendations for future responses to fish kills, both in the Blackwater catchment and other river systems nationwide.

Minister Dooley described the outcome as “deeply regrettable” and said it underlined the need to improve coordination between agencies and step up preventative efforts. “These fish kills have a devastating impact on the local community, the angling community, and everyone in these areas,” he said. “It is essential that our agencies work with all stakeholders to ensure that our prevention, compliance and incident response actions are as effective as possible.”