Crab deaths on UK coast may be caused by unknown disease, finds report

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A scientific committee has ruled out chemical poisoning and algae as explanations for the deaths of crustaceans in north-east England, saying a new disease is the most likely cause

Environment 20 January 2023

By Jason Arunn Murugesu

The mass death of crabs seen in north-east England in 2021 and 2022 could have been caused by a never-before-seen disease, a scientific committee assembled by the UK’s Department for Environment, Food and Rural Affairs (Defra) has concluded. The committee found it was unlikely that algae or chemical poisoning were to blame for the deaths, as previous research has suggested.

In October 2021, tens of thousands of dead and dying crabs and lobsters started washing up along the Tees estuary on the North Yorkshire coast and then further south in the fishing town of Whitby. In May 2022, Defra’s investigation into the deaths pointed to a rapid natural increase in algae in the ocean, also known as an algal bloom, as a potential cause of the deaths. But the investigation also acknowledged that it had found no single causative factor behind the deaths.

In October, a group of researchers commissioned by a fishing collective published its own research into the mass deaths. The team argued that they were unlikely to have been caused by an algal bloom and instead suggested that the more likely cause of death was the release of pyridine in sediment that had been dredged up to make way for a new freeport on the Tees river.

The UK environment secretary, Thérèse Coffey, ordered Defra to set up an independent scientific committee to research the issue further. The committee has now reported that no single factor can be blamed for the mass deaths of crustaceans. Instead, it estimates that there is a 33 to 66 per cent chance that the mass die-off was caused by a novel disease that only affects crabs and lobsters.

Tammy Horton at the National Oceanography Centre in the UK, who was part of the committee that wrote the report, said at a press briefing that a new disease could explain why the crabs exhibited twitching behaviours as they died.

It would also explain why the mass deaths spanned such a long time and the fact that other marine life appeared unaffected, said Horton. However, no direct evidence of a new disease has been discovered so far, she added.

The report discounts Defra’s initial suggestion that an algal bloom was to blame for the deaths, saying this couldn’t explain the twitching seen in the crabs. “I don’t find fault with the earlier report,” the chief scientific adviser to Defra, Gideon Henderson, said at the briefing. “As is usual with science, our knowledge is deepening as time goes on.”

The committee also said that the length of time over which the mass deaths occurred ruled out pyridine poisoning. “It puts the pyridine story to bed,” says Crispin Halsall at Lancaster University, UK, another member of the committee. “You need an ongoing large source of pyridine to be causing that [crab deaths] and that’s clearly not the case.”

The last time the Tees was dredged before the mass die-off was December 2020 and there was no further dredging in the region until September 2022, the report says.

Henderson said if a disease is the main factor behind these crab deaths, it is hard to know if it is still causing more deaths in the region. “As more people are aware, people are reporting [crab deaths] more frequently,” he said. “It’s hard to tell if it’s unusual until we collate all the data.”

Horton said the disease is unlikely to be dangerous to humans as the it appears to only be affecting crabs. “Seafood will be safe to eat,” she said.

Rodney Forster at the University of Hull, UK, who took part in the second report on crab deaths that ultimately blamed dredging, says that this new report is good and thorough and that he largely agrees with its findings. He says the whole fiasco around the issue has highlighted the UK’s failing water-monitoring system. “We have a reactive system and not a proactive one,” he says.

Forster says that due to budget cuts to governmental bodies, water quality in the UK is only measured at the surface and not near the seabed, which is where crabs live. Levels of toxic algae also aren’t monitored in areas of the UK that aren’t used for oyster and mussel farming, he says. This lack of monitoring is a large reason why we still don’t know what exactly caused these crab deaths, he says.

“I think we underestimate the value of having healthy and safe rivers,” says Forster. “We have to measure certain parts of the marine system to understand it – we have to be ready for the changing climate.”