Dead and dying marine life has been washing up on beaches around Teesside for almost a year after a "mass-mortality event" of crabs and lobsters there in the autumn of 2021, according to official documents and eyewitness reports seen by Sky News.
Scientists and local campaigners said the reports are evidence of a continued ecological disaster affecting more than 30 miles of coastline - from Hartlepool to Whitby and beyond.
"There have been ongoing die-offs," said Dr Gary Caldwell, a marine biologist at the University of Newcastle who has been studying the causes of the initial event in October 2021.
The incident led to the "virtual extinction" of crabs and lobsters in the area immediately around the Tees estuary, Dr Caldwell added.
There are concerns it could be connected to a freeport there. Freeports are areas where specific tax and customs rules can apply. The government says they are designed to "create thousands of high-quality jobs in some of our most disadvantaged communities".
According to the local authority, the Teesside freeport is the largest in the UK, covering 4,500 acres, and is predicted to create more than 18,000 jobs over the next five years.
Almost 50 reports of dead fish, shellfish and marine mammals being stranded, as well as reports from fishermen of dead and dying catches, have been logged by the North East Fisheries Conservation Agency since December 2021.
Local campaigners have also photographed dead shellfish and seabirds found on beaches in numbers they say are not typical for the area. Fishermen have reported that catches in inshore waters have been much lower and include dead and dying lobsters.
Sharon Bell, who lives in the village of Marske-by-the-Sea, has been spotting dead marine life since 2021.
"We're more than a year on now and it was literally only two weeks ago when I was filming hundreds of mussels right along here - it hasn't gone away," she said as we walked along the shore.
Hartlepool fisherman Paul Widdowfield commented: "We may as well be putting our pots in the Sahara desert now because there's nothing out there."
In May, a report by the environment department, DEFRA, and other government agencies concluded that a toxic algal bloom was the most likely cause of the mass mortality event.
However, research by Dr Caldwell and others, commissioned by a group representing the local fishing industry and campaigners, concluded that an industrial chemical called pyridine was a more plausible explanation.
Pyridine is known to be present in sediments in the Tees estuary following decades of industrial activity in the area.
Sampling studies of sediment in the Tees estuary, and lab tests of the chemical, suggest a strong potential link between disturbed sediment and harm to marine life, according to Dr Caldwell.
"We're seeing that legacy of Teesside's industrial heritage long buried in the sediment being brought back up and released back into the environment," he said.
The initial death of marine life occurred shortly after almost 150,000 tonnes of sediment was dredged from the mouth of the Tees estuary and dumped a few miles offshore.
A dredging operation of that size is unusual. Daily "maintenance" dredging to keep the Tees shipping channel clear tends to be on a smaller scale.
The potential link to contaminated sediments caused campaigners to raise concerns about ongoing dredging work to construct the freeport.
In addition, a dredging campaign at a nearby location in the estuary is currently moving 145,000 tonnes of sediment from the estuary and dumping it at sea.
Following pressure from activists and MPs, Defra convened an independent panel of experts to review the potential causes of the incident. It is due to publish its conclusions later this month.
For the time being, DEFRA is dismissing the proposed link between dredging, pyridine and dead marine life.
"A comprehensive investigation was conducted where government scientists considered the evidence robustly and concluded a naturally occurring algal bloom was the most likely cause," the department said in a statement.
According to DEFRA, 10 "wash-ups" of marine life on beaches in the area were investigated by the Environment Agency in 2022. One, it concluded, was caused by outfall from a nearby power station, while another was due to higher than average sea temperatures in the summer of 2022 followed by cold temperatures in December.
"Any wash-ups which followed the incidents between October and December 2021 were on a much smaller scale and in line with what would be expected during stormy winter months," it said.
Campaigners insist they are not trying to put a stop to continued development of the Tees estuary freeport.
"This didn't need to happen," said Hartlepool fisherman Stan Rennie.
He added that "of course we want jobs", but if the freeport is connected to recent events, it should not involve "the sacrifice of the fishing industry and the ecosystem".