REVENGE is a dish best served cold, so the saying goes, but the patience of the wronged party may be tried a bit far when there is a gap of nearly half a century between the original dirty deed and the payback. This is the period for which fishermen in Scotland and the rest of the UK will have suffered the tender mercies of the Common Fisheries Policy by the time of Brexit day.

Our hardy lads, from Lerwick to Kirkcudbright, have always detested the CFP. They may recall how only at the last minute did fishing reach the agenda of the negotiations which put together the accession treaty between the UK and the Common Market, as it then was. Its original six had realised that the four countries applying to join them (Britain, Ireland, Denmark and Norway) controlled the richest fishing grounds in the world. They drew up a regulation in principle giving all members equal access to all waters, and adopted it on the morning of June 30, 1970, a few hours before the new boys’ applications to join landed in Brussels. This was the start of the CFP. The incomers would have to like it or lump it.

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The UK tried to wriggle out of the bind in which it found itself, but the next year surrendered. Some members of its government still wondered whether, before signing the treaty, they might have done more. But in a memo to cabinet, a civil servant gave the game away when he said our fishermen were “expendable”. Only Norway felt strongly enough not to join after all. Mainly over the issue of fishing, Greenland, an autonomous part of the Kingdom of Denmark, also left the EU in 1985 once it got a new Home Rule deal with enhanced powers. Scotland, however, has been stuck with the CFP ever since.

There can be no doubt of the disastrous effects. Today the Scottish fleet is less than half the size it was in 1973. At least we have not suffered the total destruction of the old long-distance fishery from East Anglia, which used to exploit the high seas between Iceland and Norway. Meanwhile the waters round the British Isles are heavily fished by vessels from Spain and other continental countries. It is reckoned only 30% of the catch is taken by boats with home ports in the UK, and some of these are in reality foreign-owned. Recently things have got a little better. While measurement is difficult, our fishermen have gradually been able to improve their share. Still, many experts argue none of this does much to achieve one of the main aims of the CFP, the conservation of fish stocks. On that, the jury is still out.

Here is the story after half a century, but in 2019 it is all going to change. Or is it?

The Scottish Tory leaders, David Mundell and Ruth Davidson, pictured, have muttered about resigning over the issue of fisheries – only then, in their typically heroic fashion, to deny they had any such thing in mind. Now the other dozen Conservative MPs, awakening from their normal torpor, talk about abandoning Theresa May in her hour of need, if it should strike during this week.

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This is all on account of the Prime Minister’s hints she might need a longer rather than a shorter period of transition to iron out the horrendous complexities that Brexit will bequeath us after March 29, 2019 – not just up to the end of 2020, as already envisaged, but beyond that point too, perhaps far beyond it.

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The logical position would be for the UK to leave the CFP as it formally relinquishes its EU membership next year.

The EU, however, under the wily guidance of chief negotiator Michel Barnier, does not stick to the logical position. He has the interests of the remaining members to uphold, including Denmark, Holland, France and Spain with their big fleets fishing in our troubled waters. So he wants the CFP to continue through the whole period of transition too, however extended that might be.

And – who knows? – if the transition were to be extended far enough, it might even become the new normal, the reality of relations between the UK and EU. In international affairs, temporary arrangements do have a way of turning permanent. In that case, the CFP might remain part of the long-term partnership between the UK and EU that the Prime Minister urges on everybody. If she wants to exclude fish, Barnier does not. And who says he will never get his way? His score in his marathon match with her is already several dozen points to nil.

So it is easy to see why all this puts the wind up Mundell and Davidson and their Tory MPs. One of the biggest shifts in a period of unstable Scottish politics has been the SNP’s loss of every single one of its north-eastern seats at the last UK General Election, after holding them for 30 years. The change at the Westminster level has yet to be confirmed at the Holyrood level.

If the Conservatives did achieve a similar result in the next Scottish election, even commentators like me, who wish it least of all, would need to admit there was a solid element behind the pipe dream of their revival in Scotland. Without such a result, there will need to be a quick career review for Mundell and Davidson. Nothing would prompt it so swiftly as a further betrayal of the fishermen.

And then a second Brexit booby trap lies in wait, newly primed by the SNP’s Minister for Rural Affairs, Fergus Ewing, pictured. He has just reminded us that, even after March 29, there will remain the problem of what to do, or rather where to go, with the powers returning to this country from Brussels. The UK Government has been so studiously vague on the question as to arouse justifiable suspicion it wants to rob Scotland of powers that were always exercised from Edinburgh before 1973, so as to keep them in London instead. Brexit will not be just about leaving the EU but also about politically centralising the UK as never before.

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Fishing is a typical example. There has been public intervention in the industry right since 1808, when a Fishery Board for Scotland was set up. It carried on till absorbed into the Scottish Department of Agriculture in 1939, only to find itself largely Europeanised after 1973. Policy then became uniform over the whole UK, as over the whole EU, and this could be used to argue that after Brexit the entire industry should be run from London. Ewing sees this coming, and counters it like this: “Scotland is best placed to look after and manage our significant fishing interests. To that end I expect the UK Government to recognise this and our devolved competencies.

“I expect powers to be transferred in full to the Scottish Parliament on important issues like the setting of quotas and days at sea, for example.”

That is the right stance for the Fishing Bill now going through Westminster in anticipation of Brexit. But it will not settle all outstanding issues for fishermen, or for the Scottish Government. After all, we hope to become within a few years what the EU defines as an “independent coastal state”, aiming to take up membership again where we left off. In that case there is no chance of extracting from our partners any major change in the CFP, which by then will have been in place for half a century as part of the established order which every new applicant simply has to accept.

I don’t envy the First Minister who will need to go and tell the fishermen this, not do I envy them in their forced return to a regime they hate. If I were one of them, I would be telling my sons to find an easier way to make a living.