The demonstration at South Gare is in response to shellfish crews' dissatisfaction with a government investigation into mass crab and lobster deaths that first occurred around the Tees last autumn, spreading further south to Staithes and Whitby.
Flares were let off during the gathering this morning. Large crowds of supporters watched from the shore.
A DEFRA and Environment Agency probe attributed the deaths to a 'naturally occurring algal bloom' by testing samples against 'neutral' crustaceans taken from unaffected areas of the British coast.
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However, fishing unions funded their own independent analysis by a consultant who concluded that a chemical called pyridine was responsible and that water temperatures would not have been high enough to support the type of algae blamed.
He theorised that dredging in the River Tees as part of the Teesside Freeport development has disturbed sediment containing polluting matter.
The government agreed to re-open their investigation but have now closed it again with no new findings. Dredging had been ruled out as a cause.
The Whitby Lobster Hatchery and local trawler owners have claimed that their catches are still significantly down and that dead shellfish are still being found.
During the protest, the flotilla sailed from a dredge spoil site two miles offshore and called for a halt to activity that disturbs the seabed.