Marine heatwaves, expanding warm-water species and changing marine habitats highlight a rapidly evolving marine ecosystem.
Partners in the world of marine research and observation, including the Marine Biological Association (MBA), have highlighted major ecological changes occurring across the region’s waters in the latest State of South-West Seas 2025 report.
The report, which brings together observations and evidence on marine species, habitats and environmental conditions across Southwest England and the wider Celtic Sea region, documents a range of significant shifts in marine species and habitats linked to sustained warming of sea temperatures, changing environmental conditions and broader ecosystem change. It provides one of the most comprehensive annual assessments of marine environmental conditions in the Southwest and draws on observations from researchers, conservation organisations, citizen scientists and marine industries.
Chapters were authored by experts from organisations including the Wildlife Trusts (Devon, Cornwall, Dorset and Alderney), Plymouth Marine Laboratory, the University of Plymouth, Exeter University, the Seal Research Trust, Natural England, the Marine Management Organisation, and the Cornwall Seal Group, as well as individual contributors.
Among the most notable findings is a dramatic increase in Common Octopus populations across Southwest waters. The phenomenon, described by contributors as the ‘Year of the Octopus’, has had widespread ecological and economic implications. Increased octopus abundance has affected shellfisheries targeting lobster, crab and scallops, while simultaneously providing a substantial food source for predators including grey seals, Risso’s dolphins, blue sharks and conger eels. The findings come during the third consecutive year in which spring and summer sea temperatures in the western English Channel reached marine heatwave conditions.
Clockwise from top left: Twin seals © Elizabeth Hall; Red Bream © Guilhem Duprat; Hair Curler Sea Slug © Britta Saffron Holme; Common octopus © Keith Hiscock MBE; Spiny lobster © Fran Hockley; Conger eels © Kirsty Andrews.
The report also highlights growing evidence of climate-related shifts in species distributions. Populations of Spiny Lobster, Black Bream, Two-Banded Sea Bream, Blackspot Bream and Comber continue to increase and extend their ranges, while increased abundance and extent of several warm-water species and of recent new records for Britain were documented during the year. These new and unusual records included the Hair Curler Sea Slug (Spurilla neapolitana), warmer water and potentially toxic dinoflagellates, and several rare fish records.
At the same time, some species appear to be under pressure. Basking Shark sightings remained at historically low levels, summer mackerel catches were again poor, and reduced numbers of certain starfish species persisted across the region.
Changes were also observed among marine mammals. Common Dolphins appear to be shifting northwards, Risso’s Dolphin sightings continue to increase, and coastal Bottlenose Dolphins are becoming less frequently recorded in some parts of the Southwest during summer.
The report identifies mixed fortunes for marine habitats. Seagrass meadows continue to expand and show encouraging signs of recovery, while prolonged periods of high temperatures caused bleaching in some intertidal algal communities. The first published UK record of the non-native sea squirt Didemnum pseudovexillum was also reported, highlighting ongoing biosecurity concerns.
Dr Keith Hiscock, MBA Senior Research Fellow and Editor of the report, said:
“These findings emphasise the value of sustained long-term marine observations. The changes recorded across Southwest seas illustrate how marine ecosystems are responding to a combination of warming temperatures, shifting environmental conditions and human pressures. Continued monitoring is essential if we are to understand and, where possible, react to changes effectively.”
Key findings
- Third consecutive year of marine heatwave conditions in Southwest waters.
- Major increase in Common Octopus abundance.
- Continued expansion of Spiny Lobster populations.
- Evidence of northward range shifts among several fish and marine mammal species.
- Expansion of seagrass habitats.
- Bleaching observed in some intertidal algal communities.
- Continued decline in Basking Shark sightings.
- First published UK record of the non-native sea squirt Didemnum pseudovexillum.
Read the full report here.
