Town fishermen joined others from Scarborough, Whitby, and Redcar at South Gare, near the mouth of the River Tees, to last week demand a new investigation into the mass deaths of crabs and lobsters in the area.
Around 30 vessels let off flares and displayed flags and banners during the protest, while more demonstrators gathered on land.
A Government investigation previously concluded a harmful algal bloom was the most likely explanation for the mass deaths of crustaceans last October, which saw large numbers of crabs and lobsters wash up on the region’s beaches.
But fishermen have refuted that theory and say they believe the blame lies with 250,000 tonnes of dredged sediment in the River Tees which they say has created a “dead zone”.
With a further two million tonnes of sediment to be dumped imminently at the same spoil site, the fishing crews say they have “grave concerns” for the future of the whole industry.
Mr Mortimer, after meeting town fishermen, has now said: “Although there remains intense disagreement about the causes of these deaths, what’s important is protecting the livelihoods of our fishermen and ensuring they get the support they need through this exceptionally difficult time.
"That’s why I will be contacting the Environment Minister, Victoria Prentis, as a matter of urgency and keep you updated with any developments.”
A Defra spokesperson said: “We note that there are reports of poor catches and we are working with the industry and partner agencies to monitor this and are communicating regularly with the fishing community.”