Every Monday from early September a French fisheries official would ring the police prefects all along the northeastern French seaboard.

The official was anxious to know the level of militancy among fishermen as the 31 October deadline for a no-deal Brexit approached.

By the middle of the month police were warning the official of advanced plans for French fishing boats to blockade the port of Calais and bring seaborne trade from the UK to a standstill.

"They were planning big action," says one EU source. "Whether they would have carried it out or not - my guess is they would. They have nothing to lose."

Although fisheries represent a small part of the economies of Britain and Europe, Brexit has generated an intensity of fear and loathing among fishing communities on both sides that is found in no other economic sector.

It has become a totemic issue. For Brexiteers it is drenched in the symbolism of taking back control, but the economic impact of Brexit for coastal communities on both sides will be more than symbolic, not least in Ireland.

Howls of betrayal from both sides will therefore potentially echo around the fish negotiations.

EU fleets face the potential of being shut out of UK waters at worst, or at best denied the share of fish quotas they currently enjoy.

UK fleets have been told leaving the Common Fisheries Policy (CFP) will open the hatch to a bountiful future in which an abundance of fish stocks would go to British vessels and British vessels alone.

Howls of betrayal from both sides will therefore potentially echo around the fish negotiations.

The talks will be intense and they will be front-loaded in the overall negotiations because of a deadline of 1 July to get an overall deal on how the EU and UK manage over 100 fish stocks that straddle their waters.

Going into those negotiations, the EU knows fisheries are its weakest card. As an independent coastal state, the UK will - on paper - be entitled to fish as much of the stocks in its waters as it can.

So Brussels has raised the stakes: if the UK wants a free trade deal, it will be linked to what access EU vessels get to both UK waters and the fish that live in them.

"Every head of government, from Angela Merkel to Emmanuel Macron to Leo Varadkar are all fully aware of the importance of that linkage," says Sean O'Donoghue, chief executive of the Killybegs Fishermen’s Organisation (KFO).

Fishing communities voted heavily to leave. They were spurred on by the UKIP-linked Fishing for Leave which promised that post-Brexit "there will be nothing to negotiate - all the resources defined in UK waters currently held by other EU member states and allocated under an EU system will be automatically returned to the UK".

....the big obstacle to the sea of opportunities is the fact that Britain needs the EU single market in order to sell its fish, shellfish and processed fish.

Campaigners boasted the catch value could be worth £2bn, increasing to around £8bn after processing, with potential employment rising to 100,000 people.

Even the more sober Scottish Fishermen's Federation (SFF) hailed a "sea of opportunities" outside the EU.

George Eustice, the then fisheries minister, said Britain could land an extra 100,000 tonnes of fish.

Brexit an opportunity 'to right a historic wrong'

Such claims are now tempered by a growing realisation that British and European fishing communities will be locked in a mutual dependence which will survive long after Brexit.

It’s a truism of the fishing world that Britain exports what it catches and imports what it eats. That means the UK needs EU markets to sell fish into; and if they want that access the UK will have to allow European vessels to keep fishing in British waters.

The extent to which those fleets have fished in British - and indeed Irish - waters has long been a source of anger. Fishermen in the UK regard Brexit as an opportunity to right a historic wrong.

But many dispute that interpretation.

In simple terms, before the 1970s anything beyond inshore waters was regarded as the high seas, and fair game for foreign trawlers looking for plunder.

By the mid-1970s, after the Icelandic Cod Wars, maritime countries declared a 200-mile Exclusive Economic Zone (EEZ) off their coasts.

That got rid of the high seas approach, but both European and British fleets argued that since they had fished in each others waters since medieval times they had a fair claim to continue to do so.

So, from 1976 until the mid 1980s the EU tried to reconcile these conflicting claims through the concept called Relative Stability: member states would be allocated an annual share of fish stocks based on their historic fishing traditions (with actual catches rising and falling depending on what the science said about sustainability).

The problem for Britain was that member states were getting more out of UK waters than vice versa. Around 58% of the fish caught there goes to European vessels, whereas Britain gets only around 15% from non-British EU waters.

So, out of the CFP Britain will become an independent coastal state, like Norway, entitled to lay claim to the fish in its own EEZ.

However, migrating fish don't recognise political borders. Different species might spend different stages of their life cycles in different waters, with spawning grounds located in one EEZ and the point at which they’re caught when mature in another.

EU officials believe the change in tone is due to the sobering realisation of what a no-deal Brexit might have done to the British food and drink industry in general, and fisheries in particular

The UN Convention on the Law of the Sea (UNCLOS) and the 1995 UN Fish Stocks Agreement will oblige the UK to jointly manage fish stocks it shares with the EU, as a free for all would simply drive down the species.

If Britain has the capacity to harvest the entire allowable catch it in its waters it's perfectly entitled to exclude other fleets, but if it doesn’t then other countries must be allowed to fish there.

But the big obstacle to the sea of opportunities is the fact that Britain needs the EU single market in order to sell its fish, shellfish and processed fish.

Fisheries' link to overall trade deal

In 2016, nine normally rival industry bodies formed the EU Fisheries Alliance (EUFA), while eight member states formed the so-called "group of eight" countries most affected (including Ireland).

They have lobbied fiercely to ensure a link between continued access for European vessels to British waters and access for UK fish exports to EU markets.

But the link goes further: fisheries will be linked to the overall trade deal.

That could mean that in the negotiations, Britain's demands for financial services to continue to access the single market could get tangled up in EU demands for fisheries access to UK waters.

In Michel Barnier, a former French fisheries minister, fishing states and industry bodies found a sympathetic champion, who quietly prioritised the fishing issue throughout the divorce treaty.

In 2018, former British Prime Minister Theresa May sought a temporary UK-wide customs union with the EU in order to solve the Irish backstop dilemma.

After the UK departed, there would be a transition period until the end of 2020 (extendable until the end of 2022), during which the future trade agreement (FTA) - including fish - would be sorted out.

However, if the FTA wasn’t complete by then, then the backstop would kick in, meaning the UK as a whole would be in a customs union with the EU.

This idea alarmed member states. In a UK-wide customs union, British products would be granted tariff and quota-free access to the EU’s single market with nothing in return.

Member states therefore hurriedly insisted on what become known as "level playing-field" provisions (LPF): if the UK wanted this kind of "free" access it would have to sign up to EU standards on food safety, the environment, social and labour protections and so on.

However, fish would not be part of that arrangement. If they were, then the UK would have been able to sell their fish to the single market tariff-free, but with no reciprocal deal for European fleets to maintain access to British waters.

At the time Theresa May was desperately hamstrung by a dwindling majority in the House of Commons.

Michel Barnier tried to help her keep 13 Scottish Conservative MPs on board by pledging that both sides would commit to getting an agreement by July 2020.

If they failed, however, the UK-wide customs agreement would not apply to fisheries, meaning tariffs applying to British seafood exports.

However, member states were still unhappy. Tariffs on seafood are low, between two and four percent. British fish producers would therefore enjoy relatively free access to European markets with still no reciprocal access for European boats to fish in British waters.

The EU then insisted on a link between the fisheries issue and the overall economic relationship, and the Political Declaration accompanying Mrs May's Withdrawal Agreement established this link.

"Within the context of the overall economic partnership," the Declaration stated, "the Parties should establish a new fisheries agreement on… access to waters and [fishing] quota shares."

A few weeks later, at the instigation of France, EU leaders issued their own declaration describing a fisheries agreement as "a matter of priority," which should build on "existing reciprocal access and quota shares".

President Emmanuel Macron of France went further, appearing to threaten to keep Britain in the UK-wide customs union backstop unless it agreed to grant access to British fishing grounds for European vessels.

"I can't imagine that the desire of Theresa May or her supporters is to remain for the long term in a customs union," he said, "but to define a proper future relationship which resolves this [fisheries] problem."

In other words, if the UK failed to agree to a fisheries deal as part of an overall trade relationship, the backstop would kick in and the UK might have to stay in it.

Downing Street fired back threatening to take the EU to court.

"If [the EU] had closed its mind from the outset to the UK position on fisheries," said a spokesman at the time, "that would put it in breach of its duty of good faith under the agreement, and we can refer this to independent arbitration."

Eurosceptic member's response

Under pressure in Westminster, Theresa May attempted to weaponise the fisheries issue herself.

Speaking in Scotland in December 2018, she said: "If we can't reach a new and fair agreement by the end of 2020 then the default is that EU vessels would have no access to our waters, so they have a strong incentive to reach one."

Amid the uproar, one response from a eurosceptic member of her party stood out.

"This is not what was promised to the people of this country," the MP raged, "let alone the fishing communities of Scotland. And if history teaches us anything, it is that our European friends will not desist until they have worked out a way to plunder Scottish waters for their fish."

May’s Withdrawal Agreement simply reinvented the Common Fisheries Policy, the MP said, but with the EU holding all the cards, bullying and blackmailing until it got what it wanted.

That MP was Boris Johnson.

One year on, the same Mr Johnson has signed up to to exactly what Theresa May did on fisheries.

In the Political Declaration of October 2019, agreed by both sides, the four paragraphs on fisheries, including the link between fish and trade, is word for word the same as the Political Declaration of November 2018.

"That was really important to us," says Gerard Van Balsfoort, EUFA chairman.

"This is the starting position. There will be a direct link to the wider economic and trade partnership. We wanted this from the start in the mandate and also in the text of the Political Declaration. The whole linkage of the wider partnership and fisheries is still there."

The UK position remains, however, that reciprocal access for European vessels to British waters be treated separately from the overall trading relationship.

"[The future trade relationship] is separate to the question of fishing opportunities and access to waters," states a UK white paper from July 2018, "which consequently will be addressed separately, founded on the UK's legal status as an independent coastal state."

Ireland is particularly exposed to any restrictions on access to UK waters. Of the 50 species Irish vessels target, 47 are shared with British fleets.

But the UK rhetoric has shifted.

Fisheries minister George Eustice now talks of getting a fairer deal out of UK waters.

"We really want to get a fairer share of the quotas in areas where we don't get a fair share," he told a House of Commons committee on 16 October.

"I think it’s unlikely we get to a situation where we say all EU vessels are excluded, but we will be saying there are certain conditions to the access we grant, and that will be a rebalancing."

EU officials believe the change in tone is due to the sobering realisation of what a no-deal Brexit might have done to the British food and drink industry in general, and fisheries in particular, with shellfish production being badly hit due to the risk of products being held up through paperwork and delays at ports.

"The rhetoric on fisheries has been toned down," says one EU official. "There’s a realisation it’s a hell of a lot more complicated. The shellfish sector has raised their voice in the past six months, saying that with no deal they would be wiped out overnight.

"Especially in the west of Scotland, the small island communities - all the exports go to France. It’s time contingent, it has to get there in 24 hours."

"The challenge for the smaller export sector is they’re increasingly aware of the problem of access to [EU] markets," says Professor Richard Barnes, a fisheries expert from the University of Hull.

"There’s a lot more awareness of the potential risks of the delays in exporting, the increased costs, the bureaucracy, the catch and health certificates, proving the goods are of a satisfactory quality going into the EU."

Ireland is particularly exposed to any restrictions on access to UK waters. Of the 50 species Irish vessels target, 47 are shared with British fleets.

According to June 2018 figures, some 34% of Irish fish landings come from British waters. While the industry believes the risk of Irish boats being excluded entirely has receded, they are still worried about losing quota share, especially when it comes to the two most valuable species - mackerel and prawn - sourced from the west of Scotland and the Irish Sea.

"Overall we need 30pc access for all the different species," says Sean O’Donoghue of KFO.

"But that belies the fact that two of our main economic drivers are mackerel and prawn. We need 60-70% access for mackerel and 40% for prawns."

The EU ambition is to keep the status quo as far as possible in terms of access to British waters, and keeping the quotas as they are, but they expect a bruising fight.

"The UK intention will be to use the access to waters tool to get a higher share of the quota," says EUFA chairman Gerard Balsfoort. "What quota exactly, we don't know."

Complex discard ban

Brussels also wants to make sure that both sides in future follow the same management methods.

For example, the EU operates under the Maximum Sustainable Yield approach, which permits the highest sustainable catch across the various species without putting pressure on those species’ survival.

Brussels also operates a complex discard ban. If the UK were to follow different approaches on both, it could make an annual share out of Total Allowable Catches (TACs) very complicated.

The EU will also insist that both sides reach a long-term agreement on how to share the UK’s fish resources. Otherwise there would be, according to one official, annual "crisis" talks.

By comparison, the arrangement between Norway and the EU has to be updated annually, and that is already a headache for the EU. But there, only a handful of stocks are shared. By contrast the EU and UK share over 100.

"If the foundation stone isn't laid at the beginning in the trade talks," says Sean O’Donoghue, "and the access and percentage sharing arrangements aren’t agreed and written into the deal, then we’re on a hiding to nothing. If the arrangement has to be rolled over annually, the UK could say at the outset, that’s fine, but they’d wait two or three years and say, we’re not agreeing with that any more."

Should Boris Johnson win an overall majority in the British general election, ratification of the Withdrawal Agreement on both sides should be complete by mid-January.

There will be pressure for the fisheries talks to begin right away because of the 1 July deadline.

"Exactly how the talks will work, who’s sitting at the table is unclear," says one EU official. "It will be front-loaded. There will be an awful lot of meetings in Brussels, but how they get into the parameters of what they’re talking about could take six months."

Those parameters will include what access to UK waters EU boats get, what quota share they will get, and how both sides will manage the resource in the years to come. Getting all that agreed by 1 July seems a very tall order.

That means that Boris Johnson may have to concede an extension to the transition period beyond the end of 2020 due to the pressures on the fish talks alone, if not the overall trade deal.

Because the UK is currently still a member, then the entire quota regime will stay as it is for another year, as determined by the annual fisheries council on 16-17 December at which EU ministers and the European Commission hammer out the TACs and quotas.

However, if the transition is extended then the fisheries council in December 2020 will set out the quotas for the following year, but crucially the UK will only be given a consultative role, and will not be in the room negotiating with other member states.

So, the stakes for both sides in these negotiations will be huge.

Boris Johnson will probably have to make painful concessions in the trade negotiations anyway, but any agreement which grants French, Spanish, Danish, Dutch and Irish trawlers continued access to UK waters as before will trigger a particularly hostile response from those communities who were promised a huge Brexit dividend.

For the EU, there will be one chance to get an agreement that is to the liking of their own equally boisterous maritime constituency.

"The UK think they're going to be sold down the river," says one Irish official. "Our guys think the same. One of them is going to be right."