Twot weeks ago, David Frost, the chief U.K. trade negotiator, told the British Parliament that fishing was perhaps the biggest impediment remaining to an agreement after the U.K. signaled it was prepared to give ground on the issue of state support for businesses, which had been another major stumbling block.

Boris Johnson, the British Prime Minister, has threatened to quit the trade negotiations completely if a deal is not in sight at a crucial EU Brexit summit taking place in Brussels on October 15.

The EU’s chief Brexit negotiator Michel Barnier, for his part, has tried to urge European politicians, particularly in France and Belgium, to moderate their demands for access to British waters after the end of the Brexit transition period on Jan. 1.

But so far, Barnier’s pleas seem to be falling on deaf ears. France’s Europe minister, Clément Beaune, said last week that France “will not accept a bad deal and a bad deal in fisheries in particular. We will have no weakness on this issue of fisheries, that is clear.” Belgium said some of its fishermen had been granted rights to fish in British waters by King Charles II of England as far back as 1666.

A drop in the bucket

Fishing represents a minuscule portion of both the U.K. and EU economy—just 0.1% of gross value added and about 0.05% of jobs for Britain and even smaller fractions for the EU. So how did fishing become the issue that might scuttle a trade deal?

One is that the issue is highly emotive and symbolic for both sides. While it represents a tiny and declining portion of economic activity, fishermen are still closely associated with English identity.

Fishing rights were one of the principal areas that Brexit campaigners pointed to when they said they wanted to “take back control” from Brussels. English fishermen made up one of the most vocal blocks of Brexit voters too. So Johnson, who was one of the leaders of the drive to get the U.K. out of the EU, is sensitive to claims that he’s now abandoning fishermen.

In addition, Johnson may see fishing rights as one of the few areas where the U.K. might be able to score a “win” that the government can then tout as evidence of Brexit’s success, says Matt Bevington, an analyst with the think tank U.K. in a Changing Europe, which is affiliated with King’s College London.

Meanwhile, while fishing represents just 0.06% of the French economy overall, the industry is important to some cities in Brittany and the region around Calais that are likely to be important battlegrounds for French President Emmanuel Macron in the 2022 presidential election.

And access to British waters is crucial for fishermen in those regions. A third of France’s entire North Atlantic catch comes from U.K. waters. The percentage is even greater for fishermen from Denmark, Belgium, the Netherlands, Ireland, and Germany. If they were to lose access entirely, many of these EU fishermen would struggle to stay in business.

But Bevington points out that the U.K. fishing industry, particularly the market for species such as mackerel and herring, is heavily dependent on market access to the EU too. Just two species—herring and mackerel—account for about half of all fish landed in British ports, and yet U.K. consumers hardly eat them. Two-thirds of British mackerel and most of its herring catch is exported to the EU. In the absence of a deal, these fish would be hit with an additional 15% tariff.

Holy mackerel!

And it is not just the U.K. that needs to worry about the effect of these tariffs. Much of the pain of them is likely to be borne by EU consumers, who eat a lot of the fish caught in British waters.

Given this dynamic, Bevington says, there ought to be plenty of room for compromise. But, he says, within the dynamics of the overall trade negotiations, tactical and political considerations make it difficult for either side to offer concessions—until the very last minute.

“The U.K. knows it is in a strong position when it comes to fish, which it is not in other areas, so it wants to leave fish for last to exert leverage in other areas,” he says.

The tactic, though, risks backfiring, as French EU minister Beaune points out. “The British want their waters back, and they believe this gives them leverage,” he told French newspaper Le Monde. “But they forget that for everything else they are negotiating; they have a lot more to ask than to offer.” He also reiterated that France would “not sacrifice” the interests of its fishermen for the sake of a deal.

So far, the EU has staked out an absolutist position, demanding continued access to U.K. waters for its fishing fleet on the basis of the EU’s Common Fisheries Policy, which set quotas largely based on each country’s historical catch totals from the entirety of EU territorial waters. The U.K. says it wants to move to a system of “zonal attachment,” where foreign fleets would still have some access, but with quotas based on the actual stocks of each species found in a particular part of the sea.

Bevington says he thinks the EU will eventually blink. “There is going to have be movement,” he said. “The EU is unrealistic, and the zonal attachment would still give the EU a quota. So there is room for movement, and it is just a matter of where they land.”

But he cautions the EU is distrustful of the U.K., and as in any game of diplomatic chicken, there is always a risk that the two sides will wait too long, resulting in the failure of the entire trade deal.