Bill Coulson was taking photos off Berry Head, Devon, when he captured something unexpected across a Risso’s dolphin’s back. A common octopus (Octopus vulgaris) was clinging to its dorsal fin like a passenger who forgot to get off.
The encounter came amid the largest surge in octopus numbers recorded off southwest England in at least 75 years, an event researchers call a bloom. Researchers are tracking what the population boom means for the food web, from crabs and lobsters eaten by octopuses to Risso’s dolphins, which feed on cephalopods, including octopuses, squid, and cuttlefish.
“These photographs offer a fascinating glimpse into the feeding ecology of Risso’s dolphins and the changing marine environment around our coasts. Risso’s dolphins are closely associated with cephalopods, and sightings like this help us understand how they are responding to changing prey availability,” Josh Symes, a University of Exeter researcher who studies the dolphins through photo ID, said in a Marine Biological Association press release.
How the Octopus Bloom Could Affect Risso’s Dolphins
(Image Courtesy of Bill Coulson)
Although native to British waters, the octopus became much easier to find in 2025. A Marine Biological Association report combined underwater video with responses from 120 divers and snorkelers. Of those surveyed, 96.5 percent saw the animals off southwest England, and 55.4 percent encountered them at depths of 33 feet or less.
Explore All OptionsThe recordings showed octopuses moving across reefs where shellfish crews set their gear. Fishing crews also reported finding the animals inside traps eating captured crabs and lobsters. During the same period, the amount of crab, lobster, and scallop brought ashore by local fleets declined, and some crews reported damaged catches and lost income.
Eggs and juveniles showed that the animals successfully bred in British waters. Researchers think the influx started south of Britain, with young octopuses drifting on currents from breeding grounds around the Channel Islands and northern France. Warmer seawater may have helped turn those arrivals into a bloom.
“We’re still seeing lots of reports of octopus across Southwest waters, and understanding what’s driving these changes is an important area of research,” said Bryce Stewart, a senior research fellow at the Marine Biological Association.
Read More: Rare Footage Reveals the Mysterious Seven-Arm Octopus Eating Its Prey
A Familiar Predator Meets Different Prey
Risso’s dolphins (Grampus griseus) often live in deep water near continental shelves and slopes, but those around the British Isles regularly enter coastal water less than about 65 feet deep.
Previous research found the curled octopus (Eledone cirrhosa) as the most important prey species for Risso’s dolphins in British waters.
A regional assessment called State of South-West Seas in 2025 also found that Risso’s dolphin sightings were increasing across southwest England. The trend remained even after its authors adjusted for the amount of time observers spent looking, and they said more cuttlefish and octopuses in coastal waters may help explain it.
DNA Connects Dolphins Around Britain
Researchers are also trying to determine whether dolphins seen off southwest England belong to a local group or mix with animals elsewhere around Britain. A study in the Journal of the Marine Biological Association of the United Kingdom looked at preserved samples from 72 dolphins stranded between 1992 and 2016. Their DNA placed the animals in one genetic group spanning the sampling locations.
Researchers found six versions of mitochondrial DNA unique to dolphins around the British Isles. When they mapped how those DNA sequences were related, the British versions grouped closest to those found around the Azores. Photo ID showed more localized movement patterns, with fewer than 3 percent of the dolphins photographed off western Scotland appearing in photographs from northern Scotland.
The Marine Biological Association and Risso’s Dolphin Project South West are asking the public to report octopus and dolphin sightings through Facebook or iNaturalist. Those records can help researchers track where and when the two species turn up. Coulson happened to be looking when they met at the surface.
Read More: Dolphins Create Invisible Vortices With Their Tails, Helping Them Power Ahead Underwater
Article Sources
Our writers at Discovermagazine.com use peer-reviewed studies and high-quality sources for our articles, and our editors review for scientific accuracy and editorial standards. Review the sources used below for this article:
- This article references information from the Marine Biological Association: Octopus bloom: We must “plan for a future where these changes may become more frequent” says leading marine scientist
- This article references information from the Marine Biological Association: Octopus bloom providing a feast for Devon’s Risso’s dolphins
- This article references information from South-West Marine Ecosystems: The State of South-West Seas in 2025
- This article references information from the Journal of the Marine Biological Association of the United Kingdom: Genetic data indicate a lack of population structure of Risso’s dolphins (Grampus griseus) around the British Isles
