The prolonged heat and drought together with a spate of pollution incidents are creating a “lethal” combination for Ireland’s rivers, Inland Fisheries Ireland (IFI) has said.
The agency is investigating six large fish kills that took place in the last six weeks, with two other pollution incidents also under investigation.
While discharges of polluting substances into the rivers is believed to be the primary cause of the deaths, IFI head of research, policy and risk Dr Fiona Kelly said high temperatures and low water levels were cutting chances of survival.
“Cold water fish, such as brown trout and salmon, need access to cold, clean water to remain healthy,” she said. “Summer 2026, featuring prolonged dry weather spells with heatwaves and little rainfall, is creating very challenging conditions. When pollutants enter watercourses that are already struggling, fish mortalities are far more likely to take place.”
RM Block
Thousands of fish have been killed in incidents since the beginning of July. In the worst case, at least 7,500 trout, eel, roach, lamprey and minnow died in an incident that affected a 2km stretch of the Cavan River near Cavan town.
On the Rye River in Co Kildare, more than 400 trout, lamprey, stone loach, minnow and gudgeon died over a 4km stretch of water. Endangered salmon were among the more than 120 fish killed in the Dunkellin River in Co Galway.
Several hundred juvenile bream were among the casualties at Curraghalickey Lake in Co Cork. At the start of August, more than 500 trout, eel and other fish died along a 2km stretch of the Tolka River in Dublin.
In the most recent case earlier this week, more than 500 trout were among the fish that died along a 9km stretch of the Moynalty River in Co Meath.
Two other pollution incidents were discovered at the Ross River in Co Cork, and the Deenagh River in Co Kerry in July, although no significant numbers of dead fish were recovered.
Kelly said fish kills were generally more prevalent between May and September, the warmest and driest months of the year. “Warm weather and low water levels exacerbate any underlying problems in a watercourse,” she said.
She said many peaks in fish kill numbers had coincided with heatwave and drought years, with 2018 and 2021 being recent examples.
Data published by the Environmental Protection Agency earlier this week show that in July, most of the country’s monitored rivers had below normal water flows, with some down to just over a quarter their normal flow.
Records maintained by the Office of Public Works, meanwhile, show that water temperatures in many rivers are higher than optimal for fish life.
A temperature gauge in the Cavan river showed the water was at 20 degrees around the time of the fish kill there, and had been up to 22 degrees in previous days.
The gauge on the Moynalty read 19.5 degrees around the time of the fish kill there while the Tolka gauge recorded 21 degrees.
IFI said it was very concerned about the number of kills and the potential for more.
As water levels drop, the remaining water heats up even faster. Temperatures peak in late afternoon which means they can stay high overnight, allowing no relief for fish.
“In combination, low flows and high temperatures increase the likelihood that low dissolved oxygen concentrations will also occur – especially in watercourses with additional pressures such as poor water quality,” Kelly said.
“In tandem, these conditions can lead to lethal conditions for fish.”
IFI said it was following “definite lines of inquiry” in relation to most of the recent kills.
