We trade fish quotas within the EU.
I think each of the 4 nations get a proportion of the UK quota.
I spent a year working in DEFRA on a fishing project. What apparently makes sense (restricting fishing areas, reducing the number of inactive vessels to reduce the number of licences, restricting what can be landed) is negated by a number of things:
the lack of real selection in fishing: whatever you pull up is pretty much dead/going to die so the thought is "why should it go to waste?"
the gaming of the system: the deep sea fleet is different to the inshore fleet (< 10 m vessels). But while we might think the inshore fleet is full of little boats not landing much it's complicated by the fact there are super 10s. which are catamaran boats with (obvs) a double hull. these are much more stable than your traditional yorkshire coble. and can land more, get out in more difficult conditions.
the fact that not all fish is recorded. WELL I NEVER. and doesn't have to be under a certain size. i.e. a box of fish <25 kg does not count against a quota.
those lovely fishing trips you might go out with on a stag do, well, there's what 15 lines on there? all landing whatever they want and at whatever size/capacity? don't need a licence and isn't recorded against a quota.
Paul Joy has clearly got the ear of someone in Brexit land.