It's an industry that makes only a tiny contribution to the overall economy in the UK and the EU. And yet fishing has cast a huge net over the Brexit trade talks from the beginning.
As European leaders meet for Thursday's EU summit, there are signs that the ongoing tussle over fishing rights could blow up into a major conflagration that torpedoes the chances of a trade deal.
"Fisheries is the most difficult issues remaining for us," the UK's chief negotiator David Frost admitted to British MPs last week.
The UK wants to take back control of its own waters, while EU countries have been holding out for the same access to British fishing grounds as they've enjoyed for decades.
Under the terms of the negotiations, if there is no deal on fish, there can be no free trade agreement at all.
Why is fishing so important for each side?
According to one measure of national wealth, latest UK statistics this month say that in 2019 marine fishing produced a grand total of 0.04% of GVA (gross value added).
But in terms of symbolism, the industry is hugely important to an island nation seeking to assert its independence. Fishing ports dotted around the UK's four nations employ only 12,000 fishermen. But their communities are often highly dependent on the trade.
Equally, for several EU coastal states fishing is similarly important, and access to British waters particularly so. Under EU rules European boats have been able to take 60% of their landing in seas around the UK.
What does each side want?
As far as possible the EU is seeking to maintain existing fishing arrangements -- quota share and access rights -- under the bloc's Common Fisheries Policy (CFP). Quotas for catches are divided among member states and fixed annually.
But the UK is determined to flex its muscles as an "independent coastal state" after the post-Brexit transition period expires at the end of 2020. This will give the country an "exclusive economic zone" of up to 200 nautical miles.
It aims to reduce the access of European boats to British waters -- while gaining a greater quota share for British vessels. However, an important factor is that the European Union is by far the most important export market for the UK fishing trade.
UK fishing fleets feel hard done-by under the EU rules they've followed for more than 40 years. The two sides also have differing views on the method for calculating quotas in future.
"We know that the EU is putting intense pressure on UK negotiators but we urge them to hold firm and not trade fishing away. The UK, as an independent coastal state, is seeking a deal with the EU that mirrors the arrangements it has with other independent states with which it shares stocks, such as Norway," the UK National Federation of Fishermen's Associations said in a statement last week.
Are there any signs of compromise?
The UK has argued for an annual quota agreement with the EU -- rejected by Brussels -- as in the deal it has recently struck with Norway. But the British chief negotiator Lord Frost has said the UK is now ready to compromise on its demand, suggesting multi-year accords instead.
Meanwhile reports have suggested that Frost's EU counterpart Michel Barnier has been exploring the limits for compromise among EU coastal states for whom fishing is important: Belgium, Denmark, France, Germany, Ireland, the Netherlands, Spain and Sweden.
Yet several are said to have become more hardline -- concerned that the interests of their fishing fleets could be sacrificed in exchange for British concessions in other areas, such as state aid for industry.
France, in particular, is taking a strident stance against conceding ground to Britain.
"An agreement must respect our conditions, whether it's about fishing, terms of competition or governance. If not, there will be no agreement," French European Affairs Minister Clément Beaune tells Le Monde in an interview for Thursday's edition.
He stressed that fishing could not be separated from the rest of the negotiations, and the UK would be wrong to believe it had a strong hand. "No overall deal will be possible without a good deal on this issue. A deal which offers visibility over time to (EU) fishermen and guarantees them access to British waters. We will not sacrifice their interests," he added.
Irish fishing bodies have also expressed concern that reduced access to UK waters could lead to EU vessels encroaching on their territory, threatening vulnerable stocks in environmentally sensitive areas.
In the case of France, politics may also come into play. The northern region of France is important for Emmanuel Macron and his ruling party. With 18 months to go until the 2022 presidential election, he does not want to be seen to be abandoning its interests.
Equally, in the UK the Scottish political scene is also an important factor. Some 40% of UK fishermen are based in Scotland, official figures say.
A deal seen as bad for the UK industry would also be bad for Scottish fleets, playing into the hands of the Scottish National Party (SNP) at a time when calls for independence from the UK are growing louder and with elections due in 2021 for the new parliament at Holyrood.
Such regional factors threaten to complicate the possibility for compromise. Meanwhile, the EU's non-fishing states are said to be unified on the importance of the issue.
With both the EU and UK digging in on their positions and the UK still threatening to walk away from the talks altogether unless an agreement is struck at the summit, the scene is set for a showdown in Brussels.
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