The below is an abridged version of a report in The Conversation, by Prof. Alex Ford, Prof. Crispin Halsall and Prof. Mark Fitzsimons
In October 2021, thousands of dead and dying crabs and lobsters washed up along 70km of coastline in north-east England. This mass-mortality event coincided with the redevelopment of one of the UK’s largest ports at Teesside.
At the time, many local environment groups and fishers believed that dredging had released a toxic chemical known as pridine, an industrial substance thought to have settled in the water’s sediment, resulting in this mass-scale death of crustaceans.
But recent research outlines why this “pyridine hypothesis” is wrong. It was based on inaccurate and unpublished science, and the theory was propagated through mistrust in government, toxic local politics, and inaccurate media coverage.
An investigation by the government bodies the Environment Agency (EA) and the Centre of Environment, Fisheries and Aquaculture Science (Cefas) could not determine the cause of death, but speculated it could have been an algal bloom.
Pyridine had been widely used in industrial processes and was historically manufactured in the area. Results from EA tests showed that pyridine concentrations were high in some crabs near Teesside, but not significantly different from other locations in the UK.
A report – published by the Department for Environment, Food and Rural Affairs (Defra) in January 2023, written by independent scientists– deemed the pyridine hypothesis “very unlikely”. It also ruled out other potential causes of the crustacean die-off including algal blooms, deoxygenation, chemical spills and unusual weather events.
But the report was unable to rule out parasites or disease, as a comprehensive analysis of different potential pathogens had not taken place. While it noted that other global mortality events had reported unusual twitching and tremors in crabs dying with infections from viruses and bacteria, there wasn’t enough evidence to prove or disprove disease as a cause of these deaths.
Having taken part in this independent inquiry, we, along with other scientists, immediately encountered considerable scepticism from politicians and media. The Times reinforced the pyridine hypothesis and the inaccurate claim that “dead crabs contained 40 times more pyridine than control crabs”, stating that our independent panel “did not merit the name”.
Channel 4 News, a broadcaster that commissioned special reports about the pyridine hypothesis, also questioned whether the panel was independent.
Some journalists and politicians serving on the cross-party committee suggested we had deliberately made an alternative disease hypothesis to cover up for the government.
Even a nature charity, the RSPB, tweeted: “The results are clear, pyridine is extremely toxic to crabs.”
And it was not only scientists on the independent panel who were criticised. Those scientists who had earlier presented the pyridine hypothesis also received some unsavoury tweets when our panel’s report was released.
The idea that politically independent scientists from universities, research laboratories and consultancies had colluded on a fictitious report is, in our view, preposterous and unprofessional.
Photo by FlorianKunde/Shutterstock
Our recent peer-reviewed paper outlines why we reject the pyridine hypothesis for five key reasons.
1) Evidence that crabs contained high levels of pyridine is weak
Re-analysis of crab tissues by Cefas using an optimised methodology found low levels of pyridine and no significant difference between the affected areas in north-east England and other parts of the UK.
2) Pyridine is no more toxic to crustaceans than to other wildlife
Claims that pyridine is “exceptionally toxic” to crustaceans when exposed in milligram/litre concentrations are not accurate. There are many contaminants present in the environment which can kill crustaceans in nanogram/litre concentrations, which are 1 million-fold lower than the milligrams/litre concentrations of pyridine found to kill crustaceans.
We found that the non-peer-reviewed study based predictions of crab deaths on inaccurate toxicity calculations – probably because the number of crabs used in its experiments was too low.
3) Pyridine has never been recorded at concentrations likely to cause acute toxicity
The non-peer-reviewed study predicted no effect on crabs at pyridine concentrations below 20,000 micrograms/litre. The highest pyridine concentration ever recorded in the Tees estuary is 2.4 micrograms/litre in 2012, at a time when it was being actively manufactured and discharged under licence by the EA. This value is still approximately 400,000 times lower than the average values predicted to kill 50% of crustacean populations based on published literature.
4) Pyridine does not stick to sediments
Pyridine is a compound which breaks down very quickly – it doesn’t particularly adsorb or stick to sediments, so is unlikely to settle and accumulate on the seabed. Large reservoirs of pyridine in the sea sediment are therefore highly unlikely.
5) Pyridine wouldn’t persist long enough to cause mass deaths along a 45-mile stretch of coastline
Pyridine was not detected in the seawater at the time of the mass death, and concentrations in sediments were too low to be deemed toxic according to our research. Even modelling of a hypothetical spill of 20,000 litres of pyridine did not result in concentrations that would realistically cause acute toxicity. The claim that this compound could kill crustaceans along a 45-mile stretch of coastline without detection is unfounded.
Next steps
Unfortunately, neither the EA, Cefas nor the independent inquiry could determine a causal factor in the deaths of the crustaceans. Mistrust in the government led to a lack of trust in these public bodies. Broadcast and print media promoted unpublished research which hadn’t undergone rigorous scientific scrutiny with, we believe, an unusual degree of confidence.
We argue that this presents a problem for those serving on the cross-party committee and for wider society, who need politicians to make informed decisions based on the best available science. The committee’s decisions need to be based on rigorously tested scientific evidence, not politics.
Since 2021, there have been further dramatic shifts in the ecosystem of the waters around Teesside, such as major increases in the numbers of mussel beds and starfish. Climate change is also bringing new pressures on marine habitats, and exacerbating existing pressures from water quality, disease, coastal development and fishing.
We will probably never know the cause of the Teesside crustacean mortality event, unless something similar happens again. But better monitoring of our coastal environments is imperative. Only then can scientists understand what happens when potentially millions of keystone species across miles of coastline are disturbed or removed.
Read the full text of this report in The Conversation. A toxic chemical was blamed for killing thousands of Teesside crabs – but our study explains why pyridine wasn’t the culprit by Alex Ford, Professor of Biology, University of Portsmouth, Crispin Halsall, Professor in Environmental Chemistry, Lancaster University, Mark Fitzsimons, Professor of Environmental Chemistry, University of Plymouth.
The peer-reviewed journal article ‘Why there is no evidence that pyridine killed the English crabs’ by Alex Ford et al., in Environmental Science: Advances can be read here.