Persistent marine heatwave raises concerns for salmon farmers in UK and Irish waters.
A sustained marine heatwave in UK and Irish coastal waters is raising concerns about potential biological impacts on salmon farming operations, particularly in Western Scotland and Northern Ireland.
According to the Met Office and National Oceanography Centre, sea surface temperatures off the west coast of Ireland, as well as parts of Cornwall, Devon, and Western Scotland, are currently between 2.5°C and 4°C above the seasonal average. This follows what scientists are calling one of the warmest springs on record, with April and early May sea temperatures the highest observed in over 45 years of monitoring.
The elevated temperatures are part of a broader marine heatwave event that has persisted since late winter — an unusually early onset for this type of phenomenon in the region. The current average sea surface temperature reached 12.69°C on 19 May, well above the UK marine heatwave threshold for May of 11.3°C.
While temperatures have not yet surpassed levels known to cause acute mortality in farmed fish, prolonged exposure to elevated sea temperatures can lead to sub-lethal stress, increased susceptibility to disease, and disruptions to feeding behaviour.
“There are signs this could disrupt breeding patterns and favour harmful algal blooms,” Dr Zoe Jacobs of the National Oceanography Centre told the BBC. Jellyfish blooms, which are often correlated with warmer waters, have also been reported in similar heat events in recent years, posing potential risk to open-net sites.
In 2018 and 2023, similar heat events contributed to increased jellyfish activity and widespread blooms of algae, causing operational challenges. Scientists have also warned of the potential for shifts in fish migration patterns, with species such as bluefin tuna already appearing more frequently in UK waters.
For the salmon sector, which is particularly sensitive to fluctuations in water quality and temperature, prolonged marine heatwaves present growing operational and planning risks.
The longer-term outlook suggests that marine heatwaves in northern European waters may become more frequent and prolonged due to broader ocean warming trends. The North Atlantic has warmed by approximately 0.3°C per decade over the past 40 years, and scientists warn that ocean temperature anomalies of the current scale may no longer be exceptional.
While a short-term cooling is expected later this week due to changes in atmospheric conditions, researchers caution that this may represent only a brief interruption in a longer warm phase extending into summer.