The vice-like grip with which waters around the UK have been subjected to a prolonged marine heatwave could be behind the early reported sightings of warmer water species, including barrel jellyfish, increased numbers of seabass, and pods of dolphins. It could also explain the recent spike in octopus currently wreaking havoc among fishers, scientists have suggested. 

Northwest European waters are currently experiencing an extreme marine heatwave, with sea surface temperatures reaching record highs for April and May since satellite monitoring began some forty years back, in 1982.

This event has now lasted over two months and is significant not only in its intensity but persistence, too.

Current sea surface temperatures are warmer by up to 4°C west of Ireland, and by 1.5 to 2.5°C around the UK coastline. These are, according to experts at the UK’s Met Office, “temperatures we would usually expect around mid-June.”

The heatwave itself is the result of several overlapping factors. Prolonged high-pressure systems brought a dry, sunny spring and weak winds and waves, creating “ideal conditions for warming,” the Met Office’s Dr Segolene Berthou, scientific manager of regional coupled modelling, explained.

“This means the sea started to warm mid-February, one month earlier than usual. Additionally, the waters around the UK were already warmer than usual coming out of winter, a trend that has been building over the past forty years, with an average increase of 0.3°C per decade.”

Marine heatwaves in UK waters usually last about two to four weeks. This year’s event began in early March and has continued into May, making it one of the longest on record for this time of year. The North Sea has already peaked in temperature, while areas of the west of Ireland and the Celtic Sea are currently experiencing their highest levels.

The waters of the southwest coast, meanwhile, have been experiencing a category I-III marine heatwave since the beginning of the year, with waters reaching 2 to 3°C warmer than usual for this time of year. And it’s been suggested that with these warmer waters, an increase of warmer water species has begun to populate UK coastal regions earlier than ever.

Dr Zoe Jacobs from the National Oceanography Centre in the UK, said: “This event may be behind reported early sightings of barrel jellyfish, increased numbers of seabass, and pods of dolphins spotted in shallow, inshore regions.

“It may also be behind the recent spike in octopus. We need to improve our monitoring of such species to collect as much evidence as we can to understand if the marine heatwave is indeed driving this sudden influx of warm-water species into coastal waters.”

Over the last few months, fishers along the southwest coast of England have reported a stark increase in the number and size of common octopus, describing the influx as an “explosion” that has gone “from catching nothing to catching 1,000 kilogrammes” of octopus a day.

While they are a valuable catch themselves, this increase of octopus has been making headlines across the south where numbers have begun to run riot of the local crab and lobster fishing industry, predating on catch by – oftentimes – sneaking into crab and lobster pots to devour the catch before sneaking back out again.

Alan Steer, a crab fisher based in Devon, told SkyNews this month that “since the octopus have turned up, we are seeing massive devastation to the crab and lobster and scallop stocks in the posts.” 

According to Steer, crab and lobster catches have dropped by “around 70%” since the arrival of octopus in their current numbers.

The Centre for Environment, Fisheries, and Aquaculture Science alongside Plymouth Marine Lab and the National Oceanography Centre are now all keeping a close eye on what the current marine heatwave means for our ecosystems and fisheries overall.

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