A large fish kill in the Blackwater river in Cork cannot be explained, the final report on the incident has concluded.
Some potent substance killed 32,000-42,000 fish – mostly prized salmon and trout – last month but multiple agencies involved in the investigation cannot say what it was.
Seven days may have passed between the incident that led to the kill and the first tests of possible sources.
The report by the agencies says the substance likely entered the river as early as August 5th and dead fish were spotted on August 9th but the Environmental Protection Agency (EPA) only became aware of the situation on August 12th. That date was the first day the EPA took samples from discharge flows from industries near the river and its tributaries.
RM Block
The subsequent investigation also involved Inland Fisheries Ireland, the National Parks and Wildlife Service, Cork County Council, Uisce Éireann, the Marine Institute, the Departments of Environment and Agriculture and the HSE.
Their joint report says dozens of industrial and commercial sites were inspected, multiple samples of water, fish and other river creatures were taken and 900 potentially damaging substances were tested for, but no conclusive results emerged.
“Despite the significant investigation by members of the inter-agency group, the pollutant or the source, that caused the fish mortalities has not been identified,” they said.
The most they could determine was an approximate time and location of the original incident.
“It may be concluded that a waterborne irritant likely entered the river Blackwater around 5/6 August, around 72 hours before the first mortalities were observed on 9 August 2025,” the report states.
This likely happened “at an unidentified point most likely upstream of the uppermost limit of Inland Fisheries Ireland observed mortalities (main channel between Gortmore and upstream of Roskeen Bridge 13 August)”.
However, it “dissipated quickly rendering it undetectable in water samples and fish tissue samples”.
Most of the dead fish were found around Mallow but dead and injured fish were found from Banteer, 22km upstream of Mallow, to Castletownroche, 17km downstream.
Anglers reported distressing scenes of dead and dying fish with multiple marks and lesions, swollen eyes and damage to their gills.
While the investigation identified no pollution source, the report says North Cork Creameries, the largest licensed facility near where the fish kill occurred, will continue to be closely monitored by the EPA after it found recent breaches of its licence.
[ Rules to protect Ireland’s fragile rivers are being repeatedly breachedOpens in new window ]
The report confirmed “non-compliances were detected in the wastewater treatment plant discharge from North Cork Creameries in the June to August period and were serious and entirely unacceptable”.
The licence breaches arose primarily due to a lack of organised management or control of wastewater treatment plant activities at the co-op, which discharges in the Allow, a tributary of the Blackwater.
The EPA also found “a lack of appropriate expertise to resolve significant operational issues, a failure to appropriately generate, manage, maintain and use critical data sets to inform corrective actions and a disregard for licence requirements and licence limits”.
“These compliance issues have not yet been fully resolved by the licensee, and the EPA is rigorously pursuing the enforcement of the licence breaches arising as a matter of priority and urgency, in line with its compliance and enforcement policy,” it added.
In April this year North Cork Creameries was convicted on eight counts for exceeding ammonia and nitrogen levels and fined €11,000 in a case brought by the EPA.
Minister of State with responsibility for fisheries Timmy Dooley said the investigation had been “exhaustive” and the findings provided reassurance that the incident was “a short-lived event, with no evidence of ongoing pollution risks”.
