France'sbusiest port, Boulougne-sur-Mer, sits just across the English Channel from Britain, in the Calais region.

Seagulls glide above scores of brightly painted boats docking to unload the catch of the day — mainly sole but also cod, roussette, crab and scallops.

It's all sold at a bustling seaside market where Marie-Laure Fontaine sells seafood from a fishing boat called Providence.

"Sole and cod and turbot, we get these all from British waters," Fontaine says. "And this is a worry."

Up to 80 percent of fish caught by fishermen here comes from British waters, which are about a two-hour boat ride away.

French fishermen have been nervous since Britain voted to leave the European Union last year. That's because when the divorce is final, the U.K. will also leave what's called the Common Fisheries Policy.

"After that, the U.K. will be an independent coastal state, like Norway or the Faroe Islands or Iceland," says Barrie Deas, chief executive of the U.K.'s National Federation Fishermen's Organisations. "The U.K. will determine its own fishing quotas and access arrangements. So I think it's realistic for the French to be worried."

And Jeremy Devogel, a 31-year-old fisherman from Boulogne-sur-Mer, is really worried.

He nets 70 percent of his catch in English waters.

"I've fished for 10 years and recently bought a bigger boat specifically to work in English waters and in rough seas," he says. "I'm more than 300,000 euros ($357,000) in debt."

He frowns as he lowers metal crates, just emptied of ray and sole, onto his new boat for the next day's catch

"On a scale of 1 to 10, my level of worry is a 10," he says.

Devogel waves hello to Stephane Pinto, 52, vice president of the regional fishing committee and owner of a fishing company.

"Eighty percent of our fish come from the British side, so that makes up 80 percent of our revenue as well," Pinto says. "Take that away, and the regional economy takes a big hit."

Pinto and his crew load the day's catch — mainly sole and roussette, a type of dogfish — onto a truck that they drive to a cavernous dock. There, they sort the fish by type and size into yard-long containers packed with ice.

The fish are sold at dawn the next morning. Restaurant owner Laurent Wacogne makes sure to get the freshest ones for his restaurant, La Plage.

"My philosophy is to follow the sea," he says. "It's very important to buy the fish in Boulogne. Most of that fish — the sole, the turbot, scallops, whitefish — that's all from British waters. But it is still local. Sea bass from Greece is nice, and it's available, but it's not local."

If the stock of local fish was drastically reduced after Brexit, he says, then he could go out of business.

Deas of the National Federation of Fishermen's Organisation says he understands French concerns but "fish is a zero-sum game. The more they get, the less we get, and vice versa."

The east cost of England, he says, has seen a huge reduction of fishing fleets partly because of the Common Fisheries policy.

"EU fleets catch about four times as much in U.K. waters as U.K. vessels catch in EU waters," Deas says. "The most extreme example is eastern Channel cod, where the U.K. share of that cod quota was 9 percent and the French share is 84 percent."

But the British could find it tougher to sell their fish to the EU if there's what's called a "hard Brexit," where the U.K. gives up full access to the single market and full access of the customs union, says Christophe Collin, technical manager of Armement Bigouden, a fishing company in Le Guilvinec, western France.

"If this Brexit is a hard Brexit, we think that the European community will tax quite a lot the English product, the English fish," he says.

Le Guilvinec is one of several fishing villages in Brittany, France's main fishing region. The village is a popular tourist destination, and its seafood restaurants feature monkfish and lobster fished from English waters.

Soazig Palmer Le Gall, who runs Armement Bigouden, says about half of Breton boats fish in English waters.

"Unfortunately, fishing is a small activity compared to other economic sectors in France," she says. "So there's no plan B at the moment. The only we can do is inform our political representatives that if there is no agreement between Britain and the EU, then it will be a disaster for us."

She and others in the French fishing industry worry that, after Brexit, they will be left fighting to catch fewer fish, crowded into a narrower band of sea.

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