An octopus bloom off southwest England is eating the catch inside the shellfish fleet's traps

Ninety-six and a half percent of 120 respondents works out to 115.8 people, and 55.4 percent to 66.5, so neither figure was computed on the full response set ^[1]. That is a reporting detail rather than a flaw, but it is a reminder of what a diver survey is: a measure of detection by people who chose to answer, not a measure of how many animals are down there. The depth result has the same shape. Most reported encounters were at 10 metres or shallower ^[5][2], which is also where recreational divers spend their time.

The parts of the case that do not depend on volunteers are the stronger ones. Eggs and juveniles showed the animals bred successfully in British waters ^[9], video recorded octopuses moving across reefs where shellfish crews set their gear ^[6], and crews reported opening traps to find octopuses eating the captured crabs and lobsters ^[7]. That last observation supplies a mechanism, and a plausible one, since a pot concentrates prey and an octopus can walk in.

What it does not supply is a fraction. Landings respond to effort, weather, market and stock as well as predation, and the coincidence in time between the bloom and the drop ^[8] leaves open how much of the missing crab and lobster the octopuses ate. Bryce Stewart of the Marine Biological Association described the drivers themselves as an open research question ^[11]. Researchers suspect the influx began south of Britain, with young octopuses drifting on currents from breeding grounds around the Channel Islands and northern France, and warmer seawater helping the arrivals establish ^[10].

The predator side carries a wrinkle that the photographs cannot resolve. The bloom species is the common octopus, Octopus vulgaris ^[1], and Risso's dolphins do eat octopus, squid and cuttlefish ^[13]. But earlier work identified a different animal, the curled octopus Eledone cirrhosa, as the most important prey species for Risso's in British waters ^[15]. Josh Symes of the University of Exeter, who tracks the dolphins by photo ID, called the images a glimpse into their feeding ecology and their response to changing prey availability ^[12]; a single frame records contact between animals, and contact alone does not confirm a meal.

The regional assessment did the harder statistical thing by adjusting for how long observers looked, and its authors offered more cuttlefish and octopus in coastal water as a candidate explanation for rising sightings ^[16]. But correcting for effort only removes the influence of how long someone watched; it does not tell you whether the animals being seen are more numerous or simply closer to shore. Risso's around the British Isles already come into water shallower than roughly 20 metres ^[14][3], so animals moving inshore can lift an encounter rate without any change in how many exist.

Whether the southwest animals are a distinct local group is being approached with two methods that answer different questions. DNA from 72 dolphins stranded between 1992 and 2016 put them in a single genetic group across sampling locations, with six mitochondrial versions unique to the British Isles and closest to those around the Azores ^[17][18]. That is roughly three samples a year ^[4], and mitochondrial lineages integrate over generations. Photo ID works on the scale of survey seasons, and there fewer than 3 percent of dolphins photographed off western Scotland reappeared in northern Scotland images ^[19]. One gene pool over many generations is entirely compatible with individuals that rarely commute.

At least 75 years of records counted back from 2025 reaches to about 1950 ^[5], so the claim is about the length of the record, not about the species' history in these waters ^[2].