The UK's marine environments are undergoing a significant transformation, driven by persistent marine heatwaves that are altering the delicate balance of underwater ecosystems. Experts are warning that while some species, such as octopuses, are thriving in the warmer conditions, others crucial to the local fishing industry are suffering, posing a serious threat to traditional livelihoods.
These heatwaves have been intensified by the 'heat domes' that caused record-breaking air temperatures across the UK in May and June, exacerbating long-term ocean warming trends linked to climate change. Earlier in July, sea temperatures off Guernsey reached 17.7C, a notable 2.7C higher than the average recorded for the same week between 1980 and 2019, according to Sea Fisheries.
Nicky Harris, who leads the marine biology section at La Société Guernesiaise, highlighted the urgent need for proactive strategies to enhance the resilience of the seas against these heatwaves. Harris noted that while certain species are benefiting, vital crustacean species like chancre crabs, which prefer colder waters and are crucial for the fishing industry, are being forced into deeper, cooler habitats. There are also concerns about a 'downward shift' in phytoplankton and plankton, which form the base of the marine food web, potentially impacting all species.
Observations from divers like John Fallaize in Guernsey corroborate these changes, with sightings of comber fish, typically found in the Mediterranean, now common in island waters. Fallaize has also noted an increase in jellyfish, including mauve stingers, which are more common in warmer European seas, and has observed dolphins consuming jellyfish, possibly due to a scarcity of mackerel. Octopus populations, he added, have been 'more plentiful and of a slightly smaller size' over the last two years, a trend Harris attributes to milder winters reducing typical die-off events.
The economic impact of these shifts is already being felt. A 2025 report by the UK Marine Biological Association (MBA) revealed that a boom in common octopus populations in South West waters led to a reduction of over £500,000 in the value of local shellfish landings. The number of octopuses landed by commercial fisheries in the region was almost 65 times higher than recent annual averages, demonstrating the significant disruption. Deputy Marc Leadbeater of Guernsey has called for government support for the fishing industry, advocating for access to bluefin tuna quotas from the Department for Environment, Food & Rural Affairs (Defra) to help keep the island's 'decimated' fleet afloat, noting that Jersey has already secured such quotas.