Brexiteers and big fishing interests say that Britain “betrayed” its fishing fleets when we joined the EEC in 1973. For the Scottish Fishermen’s Federation, for instance, Brexit offers a “sea of opportunity” for struggling coastal communities. The first claim is largely a myth. The second is an exaggeration and, for a thriving section of Britain’s fishing industry, a lie.

If Boris Johnson pursues his hard line in trade talks with the EU27, he will betray many industries, from car-making to pharmaceuticals to farming. He will also betray – and genuinely betray this time – a large part of the British fishing fleet.

Much of UK fishing – broadly the small-scale, ecologically sound part – is dependent on frictionless, overnight trade in fish, especially live shellfish and crustaceans, with the EU. About two-thirds of the shellfish, lobsters, crabs and langoustines caught by British fishers are sold to the continent.

Quotas are something of a red herring. The real threat to the survival of a large, thriving, ecologically responsible swathe of the industry comes from Johnson himself

That trade only exists because of the paper-free EU single market. It is the larger-scale, rich, noisy part of British fishing that drives the strident demands for a much bigger share of catches in UK waters.

In their opening salvoes in the post-Brexit trade negotiations, the British government and the EU27 have drawn competing lines in the sea. Johnson says Britain will “take back control” of fishing in its “exclusive economic zone” of up to 200 miles from the end of this year. He is prepared to talk about access for EU boats but insists quotas must be “first and foremost” for Britain.

The EU says that unless agreement is reached on continued widespread access by the end of July, there will be no favourable trade deal for Britain – not on fish and not on anything else. In the annexe to its negotiating guidelines, Brussels said that a UK-EU deal must “build on” the existing deal which gives EU boats 60% by weight of landings from British waters. The EU wants a permanent deal; Britain wants annual negotiations.

These are the opening positions. Common sense, economic self-interest and international law may eventually enforce compromise on both sides. That could be awkward for the government, given how absurdly overblown Brexiteers’ expectations have become. Last month Peter Aldous, the Tory MP for Waveney in Suffolk, told BBC Radio 4 that Brexit would bring a “sevenfold growth of our quota stocks in the southern North Sea”.

Talk of the “southern North Sea” is misleading: it is not an especially rich or important fishing area. Taking all “British” waters within the 200 mile limit, we now catch 40% of the fish by weight (60% by value). A sevenfold increase would mean taking 280% of the sustainable catch. To reach the “Aldous quota” our boats would have to throw the fish back and catch them again – twice.

Here are some more reliable fishing facts. Under international law, Britain became an independent coastal state in “control” of its waters up to 200 miles last Friday. But that control is not absolute. Under the UN Convention on the Law of the Sea, the UK is legally obliged to manage shared North Sea and Atlantic stocks with Norway – and the EU.

‘EU boats were allocated quotas in UK waters broadly in line with their historic catches.’ Fishing trawlers in the port of Fraserburgh, Scotland. Photograph: Murdo Macleod/Guardian

The convention also requires – but does not oblige – Britain to consider the historical fishing rights of its neighbours. Despite the Brexit lie that European boats were “given” access to British waters when we joined the EEC in 1973, they have actually fished “our” waters for centuries – just as our trawlers fished off Iceland and Norway.

EU boats were allocated quotas in UK waters broadly in line with their historical catches. While it’s true that these were high in tonnage terms, the catching rights include large quantities of fish for which there is little demand in the UK, such as horse mackerel and saithe.

When Britain and the EU do sit down to talk about the political price of fish, an equable deal on access and quotas is possible. Brexit or not, there is a strong case to reboot the catch shares first set in 1983. As a result of the climate crisis and other factors, fish no longer swim in the same places they did 37 years ago.

Cod and hake have migrated farther north in the North Sea and Atlantic; tuna and anchovies are found in numbers off Cornwall. Some increase in British quotas for mackerel, herring, cod and hake is justified. Removing overnight the livelihoods of hundreds of French, Irish, Dutch and Danish fishermen is not.

Such a deal would fall short of the strident demands for a post-Brexit “bonanza”, which is driven mostly by a few, often foreign-owned, fishing companies in England and a dozen or so wealthy families that control fishing in Scotland. Too bad.

If this government is genuinely concerned about “left behind coastal communities”, it should give priority to increasing the tiny quotas for the thousands of British inshore vessels under 10 metres long.

All of this is possible, but it would require a long, sensible, technical negotiation that sets clear and achievable aims. Nothing in the government’s recent, vacuous fisheries bill suggests that such questions have even been considered.

In any case, quotas are something of a red herring. The real threat to the survival of a large, thriving, ecologically responsible swathe of the British fishing industry comes from Johnson himself. The kind of minimal trade deal envisaged by the government will cripple not only British farmers and factories, but sink a large part of Britain’s fishing fleet. No “sea of opportunity” for them.

• John Lichfield is a journalist based in France