Thousands of dead crabs and lobsters have washed up in the Hartlepool, Redcar, Saltburn and Staithes areas since October, with the phenomenon being reported as far south as Robin Hood's Bay and affecting catches in Whitby.
Environment Agency lab testing of the crustaceans had already ruled out sewage contamination as a factor and the impact of undersea cables - such as the electricity 'pipeline' between Northumberland and Norway - has also been excluded alongside seismic activity and dredging.
Scientists have also conducted water surveying out at sea and have narrowed down the cause to either a disease affecting only crabs or lobsters, or a natural event.
Environment Agency operations manager Sarah Jennings said: “We understand how distressing this incident is for the local shellfish industry and for members of the public, so this investigation has been a top priority for Environment Agency and Cefas laboratories.
“We’ve used both traditional and innovative screening methods to analyse samples of water, sediment and crab looking for traces of contamination. We’ve screened for over 1,000 potential chemical contaminants but found no anomalies that could lead to an event of this scale.
“Our environment officers have also reviewed environmental permits and scrutinised industrial sites in the Teesside area, but again found no evidence of abnormal discharges that could lead to an event of this scale.
“In a bid to better understand the scale of the incident, our survey vessel the Humber Guardian has taken samples from the seabed, which show that only crabs and lobsters appear to be affected.
“By combining this evidence we have ruled out chemical pollution and sewage as likely causes, and the investigation will now focus more on whether disease or a natural event could have been responsible for the deaths.”