Few matters in British politics are as pungent as fish. The industry represents just 0.1 per cent of the economy, but it stirs deep passions.

Nigel Farage of the Brexit party understands this: he once threw a crate of dead fish into the river Thames and vows that the “acid test” of any Brexit deal will be its impact on Britain’s fishing waters.

Nationally, the fishing vote may be insignificant, yet it is potent in parts of Scotland, where there are tens of highly marginal seats and small voter interest groups exercise a disproportionate influence. One such place is Peterhead, a town on the blustery north-east coast. Last year, £200m of fish passed across the floors of its vast new market.

Peterhead is in Banff and Buchan, one of the few Scottish constituencies that backed Brexit. While 62 per cent of Scotland voted Remain in the referendum three years ago, 54 per cent of voters in this Aberdeenshire seat backed Leave. David Duguid, its Conservative MP, says the local fishermen yearn for the UK to be “an independent coastal state” once more.

“The whole seafood sector, including processing, has a huge impact on the local economy,” Mr Duguid says. “First and foremost, the industry wants to come out of the [EU’s common fisheries policy]. Before I was elected, the Scottish National party had been banging on for 30 years that they’re the best bet for Scottish fishermen, yet they are doing everything possible to keep us in the EU. Yes, they may say that they have always been critics of the CFP, but only leaving the EU actually takes us out.”

Mr Duguid won his seat from the SNP in the gentle Tory tide that washed over Scotland at the 2017 general election. Banff and Buchan went blue for the first time in three decades, in part thanks to Brexit. But this patch is complex: the local electorate contains a forgotten political tribe — the “double Leavers”, who dislike both the UK and the EU.

Many of these voters supported the Tories in the previous election, when Brexit led the agenda. These double Leavers had nowhere else to go. They once accounted for more than a third of the SNP’s vote, but now the party is squarely behind Remain.

Mark Diffley, a pollster, is fascinated by the psyche of voters who “want to leave everything”. “The pro-independence Brexiters dislike and mistrust Brussels and Westminster in equal measure,” he observes. “Ideally they want Scotland to run its own affairs with no interference.”

Which way these voters jump in the next general election will determine the Conservatives’ chances of retaining seats in Scotland. Mr Diffley predicts that “when it comes to the voting crunch, the SNP and the Yes movement hope and think they will prioritise Scottish independence [over] EU independence, and that the union is still the issue that defines everything in Scotland.”

It is obvious why the double Leavers are fleeing the Tories. In part it is due to Boris Johnson, a very English prime minister who has limited appeal north of the border. Then there is the renewed salience of the secession question. The SNP can once again glimpse the light of independence through the fog of events in Westminster.

One crotchety Leave-supporting fish seller explains why he (and his family) are backing the Scottish Nationalists again. “What you don’t understand is that the SNP are the strongest voice for Scotland. They are the only ones who speak up for us and for our country. It’s about independence, but other stuff too.”

Conservative strategists believe they can hold on to between two and 10 Scottish seats, depending on circumstances. “If there’s an election before Brexit, it becomes about Remain and Leave in Scotland, not the union question,” said one electioneer. “The SNP will be battling the Liberal Democrats for the Remain vote and Brexiters will stay at home.”

Which Tories can cling on in Scotland? Former Scottish secretary David Mundell in the borders, his constituency neighbour John Lamont and Mr Duguid could survive. But elsewhere Scotland is turning Nationalist again. The double Leavers who helped bring the Scottish Tories back to life may yet prove to be the authors of their political demise — and that of the United Kingdom, too.

sebastian.payne@ft.com

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