European Union and British negotiators will face each other screen-to-screen as the latest round of Brexit talks get under way on Tuesday. During this four-day round, the most time has been allocated to one of the toughest issues: fishing rights. Why is such a small industry such a big problem? Fishing is a drop in the ocean of the UK economy (just 0.12% of economic output) but has become one of the most intractable issues of the Brexit talks. The reasons boil down to history, geography and politics. More than 100 categories of fish straddle EU-UK waters. Securing greater British control of these fish was a big promise of the 2016 Brexit campaign, one repeated by Boris Johnson ahead of his 2019 election victory. But the EU has eight member states with a lot to lose, including Belgium, Denmark and Germany, who say they can trace their exploits fishing in British waters back hundreds of years. What does the UK want? More fish is the short answer. The government believes the deal agreed on British entry into the European Economic Community in 1973 was unfair and needs to be unpicked. That agreement – later enshrined in the common fisheries policy (CFP) – is based on historic catches. The result is that EU-based fleets land about eight times as much fish in UK waters as British fishermen do in EU waters, according to UK government data. The UK wants to agree annual quotas with the EU based on the principle of “zonal attachment”, meaning agreeing shares based on the percentage of fish inside each side’s exclusive economic zone (a 200-mile area round the shore). British officials argue that this system would be fairer, more scientific and is already used by the EU in its annual quota-setting talks with Norway. What does the EU want? The EU wants the status quo, which allows member states to fish in British waters, with the most lucrative harvests for France, the Netherlands, Ireland and Denmark. The negotiating mandate drawn up by the 27 member states instructs the EU chief negotiator Michel Barnier – a former French fisheries minister - to “avoid economic dislocation” for European fleets, which have traditionally fished in British waters. For the EU this means “quota shares” that could be in place for as long as 25 years. EU officials say an annual negotiation is doable with Norway, where only six categories of fish are at stake. What does the fishing industry say? British fishing fleets were some of the most vocal supporters of Brexit. The Scottish Fishermen’s Federation is urging the government to stick to its guns and ensure “unfettered control over access” to UK waters, although it doesn’t rule out allowing in EU vessels. The shellfish sector, which is not covered by the CFP, wants to maintain access to EU markets and has warned that no-deal tariffs could cost £41m a year. Are there enough fish in the sea? Conservationists worry that Brexit is a big distraction from overfishing. Around 41% of commercial fish stocks in the north-east Atlantic, including the North Sea, are overfished, according to Oceana. The NGO thinks the British government’s fisheries bill does not go as far as the EU’s common fisheries policy in its intentions to prevent overfishing, although it and other conservationists have often criticised the EU for failing to meet promises to prevent the overexploitation of the seas. Who holds the cards? EU officials think that the UK probably has more leverage over fisheries than any other aspect of the Brexit talks. But the EU also has cards to play: 71% of British seafood exports go to the EU, while European fleets in UK waters often specialise in catching fish that are less popular with British consumers, such as herring. Former senior UK official Andy Lebrecht has warned that English fishing fleets risk losing access to French and Irish waters if talks sour. Will there be a deal? The two sides pledged to exert “best endeavours” to agree a fishing deal by 1 July to give crews time to adjust to changes from 2021. But hopes of an imminent breakthrough are low, despite hints of progress. Barnier has described both sides positions as “maximalist” and said fishing was “perhaps the only positive” outcome from the last round of negotiations. While the two sides are set to hold deeper technical discussions from Tuesday, the EU’s fishing member states have made it clear to Barnier that they are in no mood for compromise. What if there is no deal on fisheries? If the EU lives up to its oft-repeated threat, no deal on fisheries means no deal on anything. British-EU relations would fall into a more antagonistic phase that would stunt trade and cooperation, with tariffs levied on British fish being sold into the EU and many other goods. No deal on fishing could also lead to skirmishes on the seas similar to the 2018 “scallop wars”. Earlier this year, it emerged that the UK had been quietly increasing its maritime defences to prevent a repeat of the 1970s cod wars. Navy boats are just a small part of the armoury, which includes satellite tracking, although British officials concede that they cannot control every square kilometre of British waters. For now, both sides say they want a deal.