A fisherman working illegally off the Dutch coast has been caught with 40 tonnes of mussels on board his boat.

The boat, which was not licenced, was operating in the Voordelta, the shallow sea near the Zuid-Holland and Zeeland islands, where fishing for mussels is banned because it is a protected area.

He was spotted operating on the night of August 20 without lights and without the required satellite system, the food safety board NVWA said in a statement.

The shellfish have been placed back in the delta so they can continue to grow and a report has been sent to the public prosecution department, the NVWA said.