2024 State of the Sea Report Highlights Major Changes in South-West Marine Life

Published On: 28 July 2025Last Updated: 28 July 2025By CornishStuff

The 2024 South-West Marine Ecosystems State of the Sea Report has been published, revealing major shifts in Cornwall’s marine life – from stinging jellyfish swarms to the first ever breeding eiders recorded in the county.

The report, now in its 11th year, reflects the combined efforts of professionals and hundreds of citizen scientists working under the South-West Marine Ecosystems (SWME) programme. The full report is available to read here, and topic-specific webinars can be watched on SWME’s YouTube channel.

Seals, Surprises and a Paint Tin Rescue

Two arctic seal species – Hooded and Ringed – were recorded in Cornwall, with the Ringed seal being a first for the county. Two harbour seals also made unusually long journeys, travelling over 250km from South Devon to the north Cornish coast.

One pup was photographed being fed by its mother alongside a rare observation of its birth sequence. Monitoring revealed a drop in feeding frequency around days 7 and 8, with weaning likely at day 14.

In one concerning case, a seal dubbed a ‘commuter’ had to be rescued by BDMLR after becoming entangled in an antifoul paint tin, marking what the report calls “an emerging issue of industrial entanglements”.

The Seal Research Trust highlighted a busy year, which included the Seal SW Exhibition at the Jackson Foundation in Cornwall (March to August), input at the Devon and Cornwall Police Wildlife Crime Conference, and drone research with Natural England and Cornish Lithium recommending a minimum flight height of 110m.

Jellyfish Blooms and Vanishing Plankton

Swimmers around the Scilly Isles faced a painful surprise in summer 2024, as unusually high numbers of mauve stinger jellyfish (Pelagia noctiluca) led to sting injuries during the Scilly Swim Challenge. Numbers of this species hadn’t been seen since 1966.

Other sightings included crystal jellyfish, By-the-Wind-Sailor (Velella velella), and large blooms of salps, with 2024 being the first time blooms of both salps and jellyfish were recorded in such high numbers and in so many locations.

Meanwhile, the copepod Calanus helgolandicus, a vital part of the food chain, hit its lowest recorded numbers. It had been stable for 25 years until 2015/16, when a sudden decline began. The reasons behind the drop remain unclear.

Mixed Fortunes for Fish and Seabed Life

A significant sardine die-off occurred on both Cornish coasts in March 2024, alongside common dolphin strandings. No cause has yet been identified.

Inshore mackerel catches were significantly reduced, while species like Bluefin Tuna and sharks saw an uptick. Divers and anglers also noted an abundance of conger eels and John Dory.

While common octopus numbers fell from their recent highs, rare sightings included rabbitfish, Saupe and two red scorpionfish.

The seabed told a different story. There were widespread concerns about reduced biodiversity, especially at familiar shorelines. Populations of spiny lobsters continued their recovery, while reefs of Honeycomb Worm (Sabellaria alveolata) expanded, potentially smothering other marine life.

Species new to science were also identified in the region: the worm Arabella ampulliformis and the sea slug Pleurobranchaea britannica.

Seabirds, Whales and the First Eider Nesting in Cornwall

Little terns at Chesil Beach reached their highest numbers in 20 years, while guillemots and razorbills continued to grow. In a first for Cornwall, a pair of eiders were confirmed breeding – a rare event for southern England.

Whales and dolphins showed shifting patterns. Minke whale sightings were at their highest in five years. Coastal bottlenose dolphins were no longer observed in western areas during the summer, while offshore types increased. Since around 2014–2015, these shifts have coincided with plankton changes likely linked to marine heatwaves and nutrient changes.

The report also confirmed that Cornwall continues to report the highest cetacean stranding rates in the UK, with bycatch a major concern.

Water Quality and Plastic Pollution

Combined Sewer Overflows (CSOs) continued to release sewage, regardless of storm activity. While South West Water reported a 13.8% reduction in leaks since 2017, pollution in rivers remains an issue. A major water quality incident in Brixham in May 2024 affected 17,000 homes.

Plastic pollution is still a major problem across the South-West. Over 22,500 kg of waste was removed by volunteers, worth over £435,000 in time. The most common items were ghost gear and single-use plastics. Surfers Against Sewage warned, “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.”

What Else Is Covered?

The report also includes updates on:

Marine planning: including evidence projects and developments in the South West Marine Plan.
Marine development concerns: around desalination proposals in St Austell Bay, seaweed farming in Port Quin, and Hinkley Point C’s impact on fish stocks.
Marine Protected Areas: which now cover over 30% of the region’s seas, with conservation advice completed for 98% of features.
Turtle sightings: numbers were average but included a higher proportion of hardshell turtles.
Fisheries management: with ongoing work to better understand the Celtic Sea ecosystem and shifts in predator species.