River Blackwater disaster

The Munster Blackwater Fish Kill – What Really Happened?

In mid-August 2025, the Munster Blackwater River suffered one of the worst ecological disasters in recent memory. Over the course of just a few days, an estimated 10,000–50,000 wild fish – mainly brown trout, but also salmon, eels, and other species – died along a 30 km stretch near Mallow, Co. Cork. The sight of lifeless fish piled along riverbanks shocked local communities, anglers, and environmentalists. But what exactly caused this mass die-off?




1. The Evidence From the Water

The Environmental Protection Agency (EPA) collected water samples from wastewater treatment plants (WWTPs) and industrial sites around the Blackwater catchment in the immediate aftermath of the event. The results paint a clear picture:

  • North Cork Creameries (Kanturk)

    • Ammonia: 26 mg/L (limit: 0.5 mg/L) – 52× over limit

    • Phosphorus: 3.6 mg/L (limit: 0.5 mg/L)

    • Total Nitrogen: 34 mg/L (limit: 15 mg/L)

    • BOD: 13 mg/L (limit: 6 mg/L)

    • COD: 120 mg/L (limit: 75 mg/L)

    • Effluent temperature: 26 °C (limit: 25 °C)

    • EPA inspectors reported blue-grey cloudy effluent and confirmed the company failed to notify them of the incident.

  • Lombardstown WWTP

    • Ammonia: 54 mg/L

    • CBOD: >270 mg/L

    • COD: 662 mg/L

    • Suspended solids: 117 mg/L

    • These values show extremely poor treatment, with untreated sewage-like discharge.

  • Dromohane WWTP

    • Ammonia: 0.5–0.8 mg/L (low compared to others)

    • But BOD up to 110 mg/L, COD up to 533 mg/L, Suspended solids up to 450 mg/L

    • Suggests treatment failure, contributing to oxygen depletion in the river.

2. The Evidence From the Fish

The Marine Institute performed autopsies on affected brown trout:

  • Gill necrosis and erosion

  • Epithelial lifting

  • Eye haemorrhaging and swelling

  • No bacterial or viral pathogens detected

  • Pathology concluded: exposure to an environmental insult / waterborne irritant

This rules out natural disease and strongly points to toxic chemical exposure plus oxygen stress.

3. The Impact Chain

Technical Version

  • Pollution Sources: North Cork Creameries & failing WWTPs

  • Pollutants Released: Ammonia, phosphorus, BOD/COD, solids, warm effluent

  • River Chemistry Impact: toxic irritants, oxygen depletion, eutrophication, thermal stress

  • Fish Pathology: gill necrosis, eye haemorrhage, severe stress

  • Outcome: Mass fish kill across 30 km stretch

Simplified Story

  • Pollution sources → River damage → Fish kill

4. Likely Culprits

  • Primary: North Cork Creameries – massive exceedances, visible effluent, and failure to notify the EPA.

  • Contributory: Lombardstown WWTP & Dromohane WWTP – high organic loads, oxygen crash, and suspended solids.

Together, these discharges created a perfect storm of toxic shock and oxygen starvation for fish.

5. What Happens Next?

The EPA and Inland Fisheries Ireland have launched investigations, but pressure is growing for:

  • Full transparency about the sources and scale of the pollution.

  • Legal accountability for non-compliant discharges.

  • A long-term recovery plan for the river’s ecology.

  • Stronger monitoring of wastewater and industrial sites to prevent recurrence.

The Munster Blackwater is one of Ireland’s greatest salmon and trout rivers. Its future now depends on how seriously regulators, industry, and communities respond to this.

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