The dead crustaceans have been washing ashore since October last year, and local fishermen claim their livelihoods are at risk because they are struggling to catch enough to cover their costs.
An investigation, led by the Department for Environment, Food and Rural Affairs (Defra), concluded that a naturally-occuring algal bloom is “the most likely cause”.
Fishermen in the area are adamant the crustaceans died after dredging in the River Tees has released harmful chemicals into the water, but the investigation found it “has been ruled out as a likely cause”.
Sir Robert Goodwill, the new chairman of the Environment, Food and Rural Affairs Committee, raised the issue in the House of Commons and said it “still has not gone away”.
Sir Robert, who is also the MP for Scarborough and Whitby, added: “Catches of lobster are 50 per cent down despite vessels venturing further out to sea. I know Defra have attributed this to an algal bloom, but other theories are circulating.
“Could the minister publish all the toxicology data available for sediment, seawater, and dead crustaceans for independent scrutiny?”
Environment minister Victoria Prentis said she is “very concerned by what happened last year”.
She added: “As he said, we are not entirely sure what the cause of the mortality was, but algal bloom seems the most likely outcome.
"I have made it very clear that we need to publish every single piece of information that we have available, and academics need to work together on this.”
The investigation, which involved a number of government agencies, also ruled out chemical pollution, sewage leaks and disease as likely causes.
A group of fishermen paid for marine pollution consultant Tim Deere-Jones to conduct his own investigation earlier this year and review Environment Agency data.
He disputed the findings of the government investigation and found the water temperature has been too low to sustain large quantities of that type of algae in recent months.
He also said the data, obtained via the Freedom of Information Act, shows that “very high concentrations” of the chemical pyridine were found in samples taken from dead crabs.
During Defra’s investigation, “varying amounts” of pyridine was found in some of the crabs, but it did not cause the deaths, and it has been found in crabs in other parts of the country.
The investigation report said pyridine can “form naturally” in the crabs after death and the chemical was not found in the water samples around the River Tees.
It added: "Dredging has been ruled out as a likely cause. Samples of dredge material must meet the highest international standards protecting marine life before it is permitted to be disposed of at sea.
"If samples analysed for contaminants do not meet the standards, the disposal to sea of that material will not be licensed.
"Nothing in the testing of sediment prior to disposal or evidence from Environment Agency sampling suggests a chemical contaminant is a cause."