Fisheries make up a tiny proportion of the UK’s economic activity – roughly 0.1%; around three-quarters of that is on the processing side of the industry. But that understates the cultural significance of fishing to a nation surrounded by water. The freedom to make a living from the sea is vital to many communities’ sense of self-sufficiency and self-esteem. It reaches deep into a national sense of independence, which is why the common fisheries policy (CFP) has felt like an affront to sovereignty. Yet when the UK leaves the European Union at the end of January , few believe we will be “taking back our waters”. Fishing’s symbolic potency means it will loom larger than many industries in the coming awkward trade compromise.

The parable of the fish tells of a painful adjustment: from Brexit as abstract celebration of sovereignty to Brexit as realisation of limitations on UK power. There will be an 11-month transition during which the terms of the CFP still apply, but then the UK becomes an “independent coastal state” under UN maritime convention. Its waters are its own to manage within an “exclusive economic zone” extending 200 nautical miles from the shore. That should feel like taking back control, which is why fishermen were enthusiastic about Brexit. The reality will feel different. About 60% of the fish caught in UK waters is taken by EU ships. British boats get around 15% of their haul from non-British waters. Other coastal states have their own fishing communities with their own identities and interests to protect. They can also pool their negotiating power relative to the UK via the European commission. In theory, Britain has a strong card to play with its newfound legal rights of exclusivity. Europeans want to be able to catch UK fish. Come January 2021, Boris Johnson could supposedly banish EU boats and deploy the Royal Navy to enforce a British monopoly.

July deadline

That scenario exists only as nationalistic fantasy. The UK is as interested in selling processed goods to Europe as it is in catching fish. British consumers eat relatively little local seafood. We import cod from (non-EU) Norway and Iceland. We sell crayfish and crab to France and Spain. The UK exports 80% of its catch. The majority goes to the EU, facilitated by frictionless borders. After Britain leaves the customs union and single market, those goods will face a range of new checks, labelling requirements and hygiene inspections. Maintaining access to EU consumers will matter more to British negotiators than excluding continental boats from UK waters, especially when many of those vessels catch fish we do not want. Few diners begrudge the Danes their share of North Sea sprats for use in pig feed.

Before talks about a comprehensive trade deal have even begun, the EU side will demand ongoing access to UK waters and Mr Johnson will acquiesce because the clock will be ticking. The deadline for a fisheries deal is July, and if that cannot be settled quickly the prospects for a deal on anything else slip away. The prime minister’s climbdown can be predicted with confidence because it has been rehearsed already. From the backbenches he accused Theresa May of linking fisheries and a future trade deal in terms that were “not what was promised to the people of this country”. But the relevant clauses in Mr Johnson’s “new” deal, struck a year later, are a copy-and-paste job from Mrs May’s text. Brexit could end up denying UK fishing communities symbolic control of coastal waters and impeding exports to the single market. Such a double disappointment has the potential to aggravate other political fault lines. Scotland will be disproportionately affected, reinforcing old grievances against Tory governments elected by English votes. By contrast, Northern Ireland’s privileged access to EU markets under Brexit withdrawal protocols will give fleets located there advantages not enjoyed by mainland competitors. Since fishing is a devolved issue, UK exit from the CFP will encourage politicians in Edinburgh and Cardiff to demand more control over local waters.

These issues flow from the inconvenient fact that fish do not care in whose waters they swim. The problem of trying to carve up rights to the sea is centuries old. EU policy evolved from the need to codify what had previously been understood as historic, traditional fishing rights. That imperfect system has struggled to accommodate environmental concerns. There are requirements to follow scientific advice on sustainability, but expert voices do not always speak louder in EU capitals than fishing lobbyists.

Diplomatic reality

Every December sees a round of haggling over the coming year’s quotas, with member states pushing against limits set to meet a pan-European commitment to end overfishing by 2020. That target is not going to be met. Sea bass, hake, herring and cod are all still being depleted at perilous rates. What role the UK will have in those discussions after Brexit is unclear. UN conventions oblige an independent coastal state to cooperate with neighbours in managing stocks. The impact of a climate emergency on sea temperatures and fragile maritime ecosystems should force all governments to fundamentally reappraise the fisheries business. Ultimately it is the appetite for fish that is unsustainable, but managing transition to a different diet is a political task more daunting than the conundrum of balancing different nations’ sense of entitlement to fish in neighbouring waters.

In several EU countries, but most notably in Britain, big trawlers and large fishing interests have squeezed out the smaller, more environmentally friendly boats on which local communities depend. This had little to do with the CFP and almost everything to do with domestic policy choices. If a post-Brexit government had the will to change the relationship between British consumers, coastal communities and fish, it could not be done on a unilateral basis. Even more limited ambitions will quickly impose the logic of continental collaboration. The practical economic benefits of open borders will corrode the rhetoric of taking back control. The diplomatic reality of trade negotiations will expose the vacuity of promises that were made about life outside the EU. It might flatter Britain’s self-image to conjure the myth of an island nation, ruling the waves, buccaneering on the high seas. In truth, the challenges facing this country, like the fish in its waters, do not recognise national borders or yield to one nation’s jurisdiction. That is a lesson Mr Johnson would do well to recognise soon, while still borne aloft on a high electoral tide.