New South West Marine Ecosystems report published

The South-West Marine Ecosystems – The State of South-West Seas in 2024 report has been prepared by a group of specialists working within the umbrella of the South-West Marine Ecosystems (SWME) programme. The SWME programme brings the marine and coastal communities together annually to consider the changes that are taking place in our local seas.

This is the eleventh report in this series. It includes detailed chapters on oceanography and storms, plankton, the seashore and seabed, fish and turtles, coastal birds, seals, whales, dolphins and porpoises. Management chapters include fisheries, marine planning, marine protected areas, water pollution and plastics pollution.

Webinars on these topics for 2024 can be viewed on this YouTube channel >>

The report reflects the work of both hundreds of volunteer citizen scientists and professionals working in the south-west. This summary below highlights findings that are covered in the chapters of the report, which was published on Monday 14th July 2025.

Report highlights

• Oceanography
  • The sea temperatures around the SW during 2024 continued to be well above the long-term average, with several instances of marine heatwaves during the year.
  • Contact: Prof. Tim Smyth, Plymouth Marine Laboratory (tjsm@pml.ac.uk)

• Storms
  • The patterns of storms in 2024 and in the winter of 2024 to 2025 was normal in the sense that there were no really extreme storms which prompted high profile responses in the marine ecosystems.
  • Contact: Bob Earll (bob@bobearll.co.uk)

• Plankton including jellyfish
  • Continued presence of late-summer salp blooms, especially along the south Cornish coast. These large, filter-feeding tunicates (unrelated to jellyfish) have become established here since about 2022. Large blooms of salps have occurred previously in SW England but never in such large numbers concurrently in so many places.  They also have not previously been reported as blooms in two successive years in the same region.
  • Exceptionally high abundances of ‘mauve stinger’ jellyfish (Pelagia noctiluca) in summer 2024, with juvenile individuals being reported into November 2024, although the species was not reported on the coast of North Devon. This species was recorded in numbers not seen since 1966. This jellyfish can cause a very painful sting which was reported by many swimmers including the Scilly Swim Challenge – about 120 swimmers encountered large swarms causing many sting injuries.
  • Lowest ever numbers of the major (biomass-dominant) copepod Calanus helgolandicus. Their populations had been stable for about 25 years until around 2015/2016 when they suddenly declined. This step-change coincides in timing with several other sharp changes observed in the larger marine fauna and its causes warrant further investigation.
  • Crystal jellyfish continued (since 2020) to be a common sight in south-west waters whilst the By-the-Wind-Sailor, Velella velella, has become more frequently reported during spring and summer than in winter.
  • Contact: For general plankton: Prof. Angus Atkinson, Plymouth Marine Laboratory (aat@pml.ac.uk). For observations of jellyfish: Jeanette Sanders (sea@seadreameducation.com) & Facebook: South Devon Jellyfish Survey

• Shore and seabed marine life
  • Seemingly more ‘lowlights’ than ‘highlights’ in 2024 with concern about reduced abundance and variety of seabed marine life in several locations. Effects of climate change may have been significant in terms of storminess and high rainfall early in the year but range extensions or increase in abundance of warmer water species seems minimal.
  • Recovery of populations of Spiny Lobsters Palinurus elephas continues after a ‘disappearance’ in some areas of the south-west for about 40 years until 2014.
  • After large numbers being present especially in 2022-23, Common Octopus Octopus vulgaris continued to be observed but in smaller numbers in 2024. Previous high numbers (looking back to 1900) were essentially one/two-year ‘wonders’.
  • Nudibranch sea slug species maintained the reputation of some species disappearing and re-appearing. There were notable increases in sightings of the Rainbow Sea Slug Babakina andoni, of Discodoris rosi and a sighting at Lundy of the nationally rare Trapania tartanella.
  • There were species new to science reported from the south-west in 2024: the worm Arabella ampulliformis and a pleurobranchid sea slug Pleurobranchaea britannica.
  • There has been continued work on seagrass survey, protection and restoration programmes in the south-west during 2024 including recording of significant expansion of intertidal seagrass beds (mainly Zostera nana) and some decreases but more increases in extent of subtidal Zostera marina beds in Plymouth Sound at least.
  • Many observers commented that seashore and seabed locations with which they are familiar were much less rich in species and/or with lower abundances of some species – sometimes continuing a trend from previous years. • Honeycomb Worm Sabellaria alveolata reefs were expanding in extent at several locations including North and South-east Devon alveolata has a smothering effect on other rock epibiota and seems not to attract a significant associated biota.
  • The poor condition of seabed marine life at Lundy continues to cause concern with several species are now difficult to find or of much lower abundance.
  • There were no additional non-native species that established themselves in the south-west in 2024 but some increases in abundance and extent of existing species. The finding of a crab native to the waters of Florida and living in the structure of a fishing marker washed-up at Chesil Beach in 2016 (published in 2024) demonstrates that human activities continue to bring non-native species to our shores.
  • Pacific Oysters, Magallana gigas, settled in large numbers, likely in 2023, at several locations in the south-west although seemingly individuals remained small during 2024.
  • Several experiments deploying artificial structures to make infrastructure more attractive to marine life have been undertaken.
  • Contact: Keith Hiscock (khis@mba.ac.uk)

• Fish
  • There was a major mortality event around Cornwall with the death of sardines on both coasts in March 2024, which coincided with common dolphin strandings. No obvious explanation has been determined.
  • Seasonal shifts in mackerel leading to very reduced catches inshore in the summer.
  • A shift from meso-predators (e.g. gadoids) to apex predators (Bluefin Tuna, sharks).
  • Conger eels and John Dory doing exceptionally well, confirmed by divers and anglers.
  • Sardine and anchovy are expanding in range and show overall increase in biomass. Sardine, sprat and anchovy show significant interannual biomass fluctuations with occasional ‘spike’ or drop.
  • Continued very low basking shark sightings to the new normal of very low or no sightings.
  • Range shifts in some southern species northward extensions e.g. Comber, and an explosion of juvenile Blackspot (Red) Bream.
  • Notable rare fish records including single records of rabbitfish, Saupe and two Red Scorpionfish.
  • Contacts: Douglas Herdson (Herdson@btinternet.com), mortality event: Matt Slater (matt.slater@cornwallwildlifetrust.org.uk) & sharks and rays: Simon Thomas, (bluedogfishing.simon@aol.co.uk).

• Turtles
  • The number of turtles reported in 2024 was about the average for the region. There was a higher than usual percentage of hardshell turtles, which was probably a ‘carry over’ on the conditions at the end of 2023.
  • Contact: Douglas Herdson (Herdson@btinternet.com)

• Marine and coastal birds
  • No significant outbreak of Highly Pathogenic Avian Influenza was evident in breeding colonies in 2024, though numbers of some tern species were down following impacts in 2022 and 2023.
  • Little terns at Chesil Beach reached their highest abundance in twenty years, and the third successive year of successful breeding productivity at rates required to maintain the colony.
  • Amongst the cliff nesters, recent trends held; general increases in guillemots and razorbills, and a mixed picture for kittiwakes.
  • Another good year for southern breeding seabird sightings, with influxes of species such as Cory’s and Great shearwaters in the thousands now the norm rather the exception.
  • A pair of eiders was confirmed breeding in Cornwall – the first ever in the county and a most unusual record for southern England more widely.
  • Contact: Alex Banks (alexnbanks@gmail.com)

• Seals
  • 2024 was not a normal year in the following respects: Two vagrant species (Ringed and Hooded). Grey seals pups down by 10%, pupping even earlier than previously and less on south coast.
  • 2024 was a normal year in the following respects: Range of seal movements, seal abundance which appears stable, seal demographics, entangled and hooked seals, substantial ongoing serious disturbance, and tagged seal recordings.
  • The range of migration: Seals link Cornwall to the Isle of Man (450km north), NW Wales, SW and S Wales, SE Ireland, the Isles of Scilly, NW France, N France, S France (800km south), N and S Devon, Dorset, Belgium and Holland (650km east).
  • Seal population: Undetermined as this is an open population across the entire Celtic Sea.
  • Grey seal demographics: 66% were adults and most adults were males (57.5%). White coated pups under three weeks old represented 1% of all seals. 3% were south coast pups.
  • Harbour seals: There were 72 harbour seal sightings with up to a max of three at 15 locations. Two young harbour seals made incredible journeys. After being released by the RSPCA West Hatch in South Devon, one (Kraken) swam 250km in less than 18 days and a second (Kelpie) swam 300km in 52 days up to the north coast of Cornwall (W Cornwall and Camel). No successfully weaned harbour seal pups were recorded.
  • Vagrant seal species: Two species: Cornwall’s first ever Ringed seal was recorded (a youngster that died) followed by a young Hooded seal in Falmouth harbour.
  • Entangled seals: 82 unique entangled seals were identified (3%). six hooked seals recorded. A new issue of industrial plastic ring entanglement was recorded after the rescue of adult male ‘Commuter’ by BDMLR from an anti-foul paint tin seal.
  • Disturbance: 160 serious disturbance incidents involved 930 seals (max 56 in a single disturbance incident). Looking at all levels of disturbance from level 1 (being woken up and alert) to level 3 (leaving the land to enter the sea, there were 322 seal disturbance incidents recorded at all levels on 8% of surveys impacting 2354 seals (6% of all seals recorded).
  • Rescued seals: 78 pups admitted to BDMLR Hospital.
  • Tagged seals: 164 different tagged seals (5%) Id’d.
  • Dead seals: 246 dead seals were recorded by Cornwall Wildlife Trust’s Marine Strandings Network. Almost half of which were white coated or moulted pups. Since 2016, the number of moulted pups has increased substantially. Four dead seals were identified by SRT died aged approximately 5, 10, 13 and 14 years old, less than the average life expectancy of 25-30 years.
  • Postmortems: 29 grey seals were examined: five adults, three juveniles, 17 moulters and four pre-moult pups. The primary conditions were: infectious disease (15), trauma (13) and Other (one).
  • Contact: Sue Sayer (sue@cornwallsealgroup.co.uk)

• Whales, Dolphins & Porpoises
  • The seas surrounding the South-West represent an important habitat for cetaceans.
  • The data suggest a significant coastal shift in the distribution of common dolphins, although recorded pod sizes appear to be decreasing.
  • Two of the datasets show a decline in effort-corrected sightings of harbour porpoises, while the third indicates a continued increase.
  • All three datasets show an upward trend in effort-corrected sightings of Risso’s dolphins, although this trend exhibits inter-annual fluctuations.
  • Coastal bottlenose dolphins are no longer recorded in the western part of the region during the summer months. However, observations of the offshore ecotype of bottlenose dolphins are increasing, although one dataset reported no sightings of this species in 2025.
  • Effort-corrected observations of rorqual whales, particularly minke whales, have increased over the past five years, with all three datasets reporting their highest sighting rates for this species in 2024.
  • Humpback whales continue to be recorded in the region, particularly during the winter months.
  • The notable increase in certain species within coastal waters has occurred since approximately 2014–2015. This represents a relatively short timescale for such marked ecological changes and warrants further investigation. The shift coincides with documented alterations in plankton community structure off the South-West coast, which are thought to be associated with changes in nutrient availability and the occurrence of extreme marine heatwaves currently affecting the region. These changes in plankton dynamics appear to be influencing the distribution of fish stocks, with increasing numbers of clupeid species, an important prey group for many cetaceans, being recorded.
  • Understanding the underlying drivers of these ecological changes is essential. The increasing coastal presence of odontocetes also raises concerns about heightened exposure to human impacts and anthropogenic disturbances. Cornwall continues to report the highest rates of cetacean strandings in the UK, with bycatch remaining a significant contributing factor.
  • Contacts: Duncan Jones (duncoliver@yahoo.co.uk) & Dan Jarvis (dan@bdmlr.org.uk).

• Marine Planning
  • The SWME report provides updates from the Marine Management Organisation’s Marine Planning team in the South West, including a summary of evidence projects, progress on implementing the South West Marine Plan, and regional highlights from other MMO teams such as the Strategic Renewables Unit and Marine & Wildlife Licensing.
  • Contact South West Coastal Planners; Mae van Loef (vanloef@marinemanagement.org.uk) & Ellie Hoad (ellie.hoad@marinemanagement.org.uk).

• Marine Developments
  • Desalination: Proposals for a desalination plant in St Austell Bay raised concerns about the ecological impacts of significant hypersaline discharge on the area’s extensive maerl and seagrass beds. Current data gaps, particularly for maerl, warrant a precautionary approach.
  • Seaweed farming: Seaweed aquaculture can provide ecosystem benefits, but poorly assessed proposals at Port Quin highlight the need for robust baseline data, clear mitigation plans, and sensitive siting. Key risks include benthic impacts from extensive moorings and potential entanglement of marine megafauna.
  • Nuclear power: The primary ecological risk associated with the Hinkley Point C development remains fish mortality through cooling intake entrainment and impingement. Effective mitigation depends on the combined function of low-velocity heads, fish return systems, and a functional acoustic deterrent.
  • Contact: Dan Barrios-O’Neill (barrios-oneill@cornwallwildlifetrusrt.org).

• Fisheries
  • Disentangling the respective effects of fishing and the environment is fundamental to improve our understanding of Celtic Sea ecosystem functioning and for providing insight to inform progress towards an ecosystem approach to fisheries management.
  • Southwest Marine Ecosystems plays an important role in bringing together transdisciplinary marine stakeholders whose collective knowledge is providing important insights which will prove fundamental in advancing our understanding of Celtic Sea ecosystem functioning.
  • Contact: Libby West (West@naturalengland.org.uk)

• Marine Protected Areas
  • The South-West (Devon, Cornwall, and the Isles of Scilly) has 53 Marine Protected Areas, covering over 30% of the region’s seas and forming a core part of England’s inshore MPA network.
  • Conservation advice for 98% of MPAs has been completed by Natural England, while condition assessments have been finalised for 23% of MPA features.
  • MMO fisheries management measures under Stage 1 and 2 are in force across 17 offshore MPAs, with good compliance from the fishing sector; Stage 3 and 4 measures are still in development.
  • The MMO has begun assessing the impact of recreational activities like anchoring, sailing, and diving at three South-West MPAs to better manage pressures on sensitive habitats.
  • Between 2019 and 2024, the ReMEDIES project targeted restoration of sensitive habitats across three South-West MPAs through active and passive techniques alongside public awareness campaigns.
  • The South-west seabed stores an estimated 35.8 million tonnes of organic carbon, with 32% of this blue carbon held within MPAs.
  • There are strategic changes ahead, including the MPA Network Review and Offshore Wind compensation plans.
  • Looking ahead, MPAs in the South-West must continue to adapt to dual priorities: conserving biodiversity and addressing climate change.

• Water Quality
  • Combined Sweage Outlets (CSOs) are now fully monitored and continued to release large volumes of sewage into the sea and freshwater, irrespective of storms.
  • South West Water have noted a 13.8% reduction in leaks compared to 2017.
  • Despite improvements, many rivers still face issues with chemical pollutants, such as arsenic, cadmium, and lead.
  • Approximately 17,000 households were affected by a water quality incident in Brixham, Devon, in May 2024. This incident underscored the importance of rigorous testing and monitoring to ensure water safety.
  • The Water (Special Measures) Bill was introduced in the House of Lords in September 2024. The bill aims to make changes to the regulation of private water companies and sewerage services in England and Wales.
  • Contact: Eleanor Ward (Ward@devon.gov.uk).

• Marine Plastic pollution
  • Plastic pollution is as prevalent as ever on the south-west coastline. A snapshot of evidence gathered by local coastal clean-ups shows at least 22,500 kg of pollution was removed by volunteers, whose time was worth over £435,000. The top plastic pollution items recovered were ghost gear and pollution stemming from single use plastic consumption.
  • The impact of this pollution is all permeating. Plastic has been found in the deepest part of the ocean and on the highest mountain peaks. It’s in water, soil and air. It’s in our bodies. Research claims humans eat a credit card’s worth of plastic a week. Marine wildlife is ingesting plastic, and entanglement is claiming lives. With plastic production set to soar in the next five years, the problem is only going to get worse.
  • But there are opportunities to influence and drive change.
  • An evidence-led approach, based in academic research, robust data collection and impact case studies, will tell the true story of plastic pollution in the south-west. From which we can work to help create the system change we need to see.
  • Contact: Rachel Yates Senior (rachel@sas.org.uk).