If you look out to sea from the Christian Dior museum on the cliffs above Granville, you see the grey outline of what appears to be another part of the Norman coast.

It is. But it isn’t.

The island of Jersey, straggling along the horizon to the north-west, is one of the last fragments of the ancient dukedom of Normandy. Over there, 40 miles away, the people don’t speak French. Over there, the people are wealthier (twice as wealthy on average) as the people over here.

The great fashion designer Christian Dior was born in the grey, unglamorous town of Granville in 1905. He rapidly moved to Paris. His birthplace now lives from tourism and, for the time being, from fishing. If the Norman cousins over the water (and their millionaire-migrant fellow citizens) have their way, the town’s fishing industry is doomed.

“They want to take our fishing but what does fishing mean to them?” asked Baptiste Guenon, 34, the skipper of Cap Lyhan, which was one of 100 Norman and Breton boats that protested inside and outside St Helier harbour last Thursday. “It means 5% of their economy. What they really live from is banks and tax avoidance. So why take our fish?

“It makes no sense. It wasn’t just French boats protesting on Thursday. Ten of the boats with us were from Jersey. They’re also disgusted with what their government is doing. If the Jersey boats can’t sell their fish in France, they can’t work.”

Granville boats take around 70% of their catch, 100% in some cases, in the waters around Jersey – as they have done for centuries. A decision by the Jersey government late last month to limit those catches caused a political, and electoral, chain-reaction that led to an ill-considered threat by Paris last week to turn off Jersey’s lights. More than 90% of the island’s electricity comes by cable from France.

Cue indignation and much misleading reporting in the British media. On Thursday, according to the tabloid headlines, French trawlers “fled” the might of the Royal Navy.

“We scarcely saw the two British patrol boats,” said Frédéric Yann, 52, the skipper of Grosse Yann. “We stayed in St Helier harbour for four hours and then we left to go fishing – as we always said we would. What we did see was a launch with millionaire Jersey residents aboard throwing empty bottles at us.”

Seen from the French side of the water, what is the France-Jersey “whelk war” of 2021 all about? It is an attempt, in miniature, to replay the failed UK government campaign to make Brexit an El Dorado for British fishermen. According to French fishermen’s leaders, the effort is just as ideologically driven and just as misconceived.

New fishing licences were issued on 30 April by the Jersey government as part of post-Brexit fishing arrangements in UK and EU waters. When local fishermen examined them, they found they had been gutted and filleted.

Some of the licences covered only a few hours a year. Lobster or scallop boats were given licences to fish only for whelks. Parts of the Jersey coast were banned to French boats – against the terms of the fisheries protocol attached to the EU-UK Brexit deal signed on 24 December, as the European Commission confirmed on Friday.

Guenon usually fishes for whelks and scallops at least 100 days a year within the Jersey six-to-12 miles zone. His licence awarded him, without explanation, 22 days.

Marc Delahaye, the director of the Normandy Regional Fisheries Committee, told the Observer: “These licences were not the result of a misunderstanding or a mistake as the Jersey government now says. They were a deliberate provocation, part of a long campaign by an extreme nationalist group in Jersey politics to sabotage centuries of cooperation between the islands and France.

“They have sold a kind of ‘mini-Brexit idea’ of a Jersey freed from the shackles of French cooperation. They accuse us – falsely – of over-fishing and damaging the environment.”

Delahaye says that the British government – and media – should confront an obvious question: why is this dispute only between France and the Bailiwick of Jersey? Why is there no problem between France and the Bailiwick of Guernsey [which also covers Alderney and Sark]?

“The answer is that the Guernsey government is run by reasonable people, who understand the importance of cooperation with France,” he said.

For centuries, fishing rights around the Channel Islands were governed by bilateral deals with France. The Îles Anglo-Normandes are not part of the UK and were never part of the EU or its common fisheries policy.

On 24 December last year, in the last-minute dash for a post-Brexit trade deal, it was announced that the Channel Islands would be included in new arrangements for EU fishing in UK waters, and vice versa. Licensed EU boats could continue to fish up to six miles from the southern English and Channel Islands coasts.

Jersey issued some licences on 30 April but they bore little relation, French fishermen say, to the historic pattern of fishing. French fishermen’s leaders say the restrictions were never raised beforehand and therefore breach the terms of the Christmas Eve UK-EU trade treaty.

The French government’s “lights out” threat may also be illegal – and is certainly over the top. But politics is politics. France has regional elections in six weeks’ time. Presidential elections are 11 months away.

What happens now? With goodwill, all could be solved easily enough. The Jersey government appears alarmed by the way that things have spiralled out of hand. French fishermen’s leaders fear, however, that Boris Johnson’s government is enjoying gunboat diplomacy as a distraction from other broken fish promises.

“If it was left to local people – even the present Jersey government – this might be resolved easily,” Delahaye said. “Now that Paris and London are involved, especially London, it could get very complicated.”