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In October 2021, thousands of dead and dying crabs and lobsters along 45 miles (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 pyridine, an industrial substance thought to have settled in the water’s sediment, resulting in this mass-scale death of crustaceans.
But 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 It was based on inaccurate and unpublished science, and the theory was propagated through mistrust in government, toxic local politics, and inaccurate media coverage.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 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. Various interim reports from were published between December 2021 and May 2022. In funded by the fishing industry analysed the EA data from those interim reports, and proposed pyridine as a cause.
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.
The EA had used a technique to detect pyridine in crab tissue which was designed for water samples, and warned its results might not be accurate. It had also only looked at four crabs close to where deaths had occurred, and compared these with four crabs elsewhere in the UK.
This was a very low number to be statistically confident in the results. Yet, for those who believed the crab and lobster deaths were caused by dredging, this data was the “smoking gun”.
Other potential explanations from government agencies were not believed by local environmental campaigners and opposition parties, and some newspaper articles started to .
When unpublished data from Newcastle, Durham and York universities was presented in October 2022 to a , the researchers behind this data claimed that pyridine was “exceptionally toxic” to crabs, and that dying crabs exposed to pyridine display twitching behaviour similar to that observed on the beaches.
A by this team claimed that a simulation of dredged material drifting down the coast mirrored the mortality event. They believed that dredging had unearthed large reservoirs of pyridine from within the sediment, left there by past industrial processes, which then swept down the coastline killing crustaceans due to its exceptional toxicity.
Some MPs called for an into crab deaths and a pausing of the Teesside redevelopment.
However, the subsequent – published by the Department for Environment, Food and Rural Affairs (Defra) in January 2023, written by independent scientists including ourselves – 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. reinforced the pyridine hypothesis and “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 , also questioned whether the panel was . Some and serving on the cross-party committee suggested we had made an alternative disease hypothesis to cover up for the government.
Even a nature charity, the RSPB, : “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 had colluded on a fictitious report is, in our view, preposterous and unprofessional. Our recent 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 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 – 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 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 . Climate change is also bringing 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.
, Professor of Biology, ; , Professor in Environmental Chemistry, , and , Professor of Environmental Chemistry,
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