The Franco-British fish feud: How it started

A Franco-British feud over access to prime fishing waters escalated on Thursday as the two countries deployed patrol and navy ships near the Channel island of Jersey.

Here's a look at the origins of the conflict and why the waters are so important to Britain and its neighbours:

Brexit fallout -

Access to Britain's rich fishing waters was a major sticking point in post-Brexit talks.

A transition period was agreed in which EU fishermen would give up 25 percent of their current quotas -- the equivalent of 650 million euros per year -- in 2026. The deal would then be renegotiated every year.

Until then, EU vessels have access to an area between six to 12 nautical miles from Britain's coast, but they have to ask for new licenses.

This is where things got complicated.

The French side says London acted outside of the deal by tightening conditions for access to UK waters.

In order to obtain a licence to fish the French vessels must be able to show British authorities they were already operating in British waters in 2012-2016.

That can be established easily enough for larger vessels through geo-localisation equipment but not so for smaller ships that are not required to have the technology, according to French fishing group CNPMEM.

In addition, a new vessel launched from 2016 must prove it is replacing one that was operating in UK waters.

'Inadmissible' requirements -

France say its fishermen have received just half of some 200 licenses they requested from London.

The Channel Island of Jersey, a British Crown dependency not part of the United Kingdom, has only doled out 41 of 344 licences that were applied for and the few have "inadmissible" requirements.

One fisherman who used to fish in Jersey waters some 40 days a year was given access for just 11 days this year, and only to catch scallops, according to a French lawmaker.

Jersey authorities said they upheld the EU-UK deal.

Tensions boiled over on Thursday as 50 French trawlers massed in protest in front of the Saint Helier harbour on Jersey.

British Prime Minister Boris Johnson responded by deploying two royal navy gunboats to the area and France followed suit with two of its own coastal patrol vessels.

The French trawlers headed home later in the afternoon.

Jersey fishermen have their own gripe.

"The French fishermen out there want conditions removed from their licences so that they can fish with no constraints in our waters, whilst our boats are subject to all sorts of conditions," Don Thompson, president of the Jersey Fishermen's Association, told the Good Morning Britain TV show.

Rich pickings -

British-Channel Island waters provide rich pickings.

Fish that lay eggs along the French coast and all the way up as far as Denmark soon leave the zone for the deeper and colder waters off Britain further north, according to the French Institute for Exploitation of the Sea, Ifremer.

Climate change has accentuated that fact, particularly for cod and plaice, both species which abound in the North Sea.

Ifremer notes that the area of the Channel off Jersey is a transitional zone between the Atlantic and the North Sea which offers "succession of specific habitats" for various species including lobster and crab.

Around 20 percent of the French overall catch comes from British waters but that rises to 28 percent for the Netherlands, 35 percent for Ireland and 50 percent for Belgium, according to 2011-2015 data.

Various other EU states are also affected by the Brexit fallout -- including Spain and Germany.

The United Kingdom exports between 60 and 80 percent of its catch and the EU last year absorbed some 70 percent of British fish exports.