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Two monster trawlers have again been spotted off Cornwall — sparking fears they are killing marine life and devastating local fish stocks.

Dutch factory trawlers Willem van der Zwan (9,500-tons) and Afrika (7,000-tons) are both believed to be fishing mackerel to be sold to Russian, Nigerian and Japanese markets.

In the past days, both trawlers have been tracked off Cornwall by the MarineTraffic website.

The trawlers, which are capable of plundering thousands of tonnes of fish from the ocean, are operating perfectly legally, although the Government will be able to ban them from British waters after Brexit.

Willem van der Zwan has come under particular fire in the past from protesters and activists.

John Hourston, of the Blue Planet Society pressure group, believes that dolphins, whales and seals are victims of the trawlers' supersized nets.

“These trawlers target fish specifically around our coast, mostly in the autumn and winter, just when fish are aggregating to spawn," he said.

"The trawlers are targeting them when they are together.

“It’s madness; they are hitting them at that point before they can reproduce.

“We are also worried about the number of cetaceans washing up since the trawlers started operating at the end of September.

“Areas off Cornwall like Mounts Bay are full of common dolphins, bluefin tuna and pilot whales. There’s just so much marine life.

“The deaths can’t all be attributed to the trawlers, but we know when they turn up that dolphins start washing up on our shores.”

Mr Hourston is also concerned about the impact the trawlers have on local fishermen.

He said: “These guys are ten times the size of the standard in-shore boat, or standard boat people from Cornwall are using, and they can stay out and hammer stocks, leaving nothing for local fishermen.

"We know how important mackerel in particular is to local Cornish fishermen."

The Dutch owned 6,200 tonne Margiris vessel, which is registered in Lithuania, was also spotted in the English Channel in recent weeks.

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