Industrial toxins were the "most likely" cause of a mass die-off of marine life along the North East coast, university researchers have said.
The research was carried out following the deaths of thousands of dead crabs and lobsters, which washed up on beaches across Teesside in October 2021.
Government scientists concluded that algal bloom, a harmful but naturally occurring algae, was "the most likely cause".
Video report by Rachel Bullock & Kris Jepson.
That finding prompted months of fury from fishermen across Teesside and North Yorkshire who believe dredging on the River Tees had disturbed pyridine, a chemical once used in steelmaking.
They say fishing businesses from Hartlepool to Scarborough have been severely impacted this year.
DEFRA rejected their claims, reporting that while pyridine was found in the dead crabs, it was also found in healthy crabs tested in Cornwall. The government department also found no trace of pyridine in water samples taken from the Tees.
The North East Fishing Collective (NEFC) commissioned scientists from Newcastle, Durham, York and Hull universities to investigate the sediment dredged from the River Tees, as well as the toxic effects of pyridine on crabs.
The findings, released on Thursday 29 September, are based on research which has been taking place since July.
The key findings, which are in a report which they are submitting to DEFRA, are:
Pyridine was discovered in sediment samples taken from the River Tees.
When exposed to less than 25% of the pyridine found in crabs found dead in Saltburn, healthy crabs gathered from waters around Croquet Island, Northumberland, died within six hours.
Crab deaths along the North East coast are more consistent with industrial toxins than by natural algal toxins.
Joe Redfern, chairman of the Whitby Commercial Fishing Association, said: "We are not activists; we are fishermen and all we want is the truth. I am very glad the university findings support our initial worries that dredging was the cause of the mass mortalities of shellfish.
"I hope in light of these findings the authorities will act on them and thoroughly investigate the events of the past year and will strive to prevent this catastrophe happening again.
"Our fishing businesses and livelihoods from Hartlepool down to Scarborough have been severely impacted this year. The amount of crab on our traditional fishing grounds has been decimated.
"The loss of earnings has resulted in fishermen being put out of business indefinitely with multiple boats being put up for sale in the past months. Over the past year our fishing fleets have been forced to work further afield, spend more time at sea, and use more fishing gear, just to make ends meet."
The association is calling on Government agencies to "act quickly and transparently" and wants dredging to be halted.
A spokesperson for DEFRA said: "A comprehensive investigation last year included extensive testing for chemicals and other pollutants such as pyridine.
"It concluded a naturally occurring algal bloom was the most likely cause. We recognise the concerns in regards to dredging, but we found no evidence to suggest this was a likely cause.
"There have been no materials licensed for disposal at sea in the area which would fail to meet international standards. This is a complex scientific issue, which is why we took a thorough, evidence based approach.
"We welcome research carried out by universities and will continue to work with them, including studying this report carefully. We are aware there have been some localised reductions in catch rates and we are continuing to monitor shellfish populations in the area."
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