Temperatures in Irish lakes and rivers during last summer’s heatwave reached lethal levels for fish species, notably salmon and trout, according to Inland Fisheries Ireland (IFI).

The trend was most pronounced in the west of Ireland, according to pilot studies on the risk posed to fish communities by high temperatures which were submitted to the Department of Climate Action and Environment in December.

Water temperatures in 2018 exceeded the ideal temperature threshold, which is below 20 degrees for cold water fish species – such as Atlantic salmon, brown trout, sea trout and arctic char – across Ireland during the summer.

Prolonged periods of temperatures above 19 degrees are likely to increase fish mortalities and have a negative impact on species.

The state agency examined the temperatures in the context of climate change predictions. With climate models predicting a further increase of up to 1.6 degrees in air temperature in the next 30 years, “these weather conditions pose a significant risk to cold water fish in Ireland,” it concluded.

The highest temperatures during 2018 were in the Owenriff catchment in Co Galway where lethal water temperatures of more than 24.7 degrees were recorded over 13 days. The average temperature registered at 18.6 degrees for June and July, with a maximum temperature of 28 degrees.

If temperatures continue to increase, sensitive cold water fish species will be at risk

On the river Erriff in Co Mayo, regarded as a key indicator of the status of salmonid species (which includes trout, salmon and char), a maximum temperature of 25.6 degrees was reached and while cooler than Owenriff, “issues were reported with salmon survival due to high temperatures and low water levels”.

Temperatures recorded in the midlands, such as on the Clodiagh river in Co Offaly were less severe, with a high of 22 degrees and an average of 16.4 degrees.

The Dargle river in Co Wicklow had an average of 14.8 degrees and a maximum of 21.5 degrees, which while not lethal, was still above ideal temperature.

IFI head of research and development Dr Cathal Gallagher said: “The 2018 summer water temperatures need to be considered in the context of climate change predictions. If temperatures continue to increase, sensitive cold water fish species will be at risk.”

These temperatures could lead to localised extinction of native fish diversity in the future and economic losses as a result, he added. “We would reach a stage where the Owenriff catchment or similar catchments become inhospitable to brown trout and salmon over the summer period in the near future.”

IFI is looking at mitigation strategies to help safeguard against extreme warm weather events, such as planting trees to protect rivers as a starting point in a larger climate-proofing strategy. Research work will continue with a view to providing insight into which catchments need support most urgently.