Bear in mind that fishermen cannot determine what they are going to catch. As reported by an MP..
"“Even the smallest under 10 metre trawlers
are having to dump about 150 kg of cod and 300 to 400 kg of
whiting per day, regardless of what gear they use. For that, they get as
little as 100 kg of prawns and 100 kg of haddock. The fish that the
fishermen are out to catch are effectively swamped by the fish that they
cannot land and have to discard. They regard discarding as an unethical
practice, damaging to fish stocks and to the housewife who could buy the
fish”
However say their quota is 100kg of prawns and haddock as described above.
Say the small trawler catches 100kg of Haddock but few prawns, it will then carry on fishing for prawns and have to discard all of the haddock as well as the cod and whiting that it catches.
They are banning discards however.... This merely means that the trawler has to return to port to unload it's non quota catch which cannot be sold as anything other than ground up fishmeal for livestock. The fish are still dead, and cannot be sold, the method of dumping is merely different.
The EU's position is simple, namely that by signing up to the EU that your waters and all the fish in them become theirs.
Indeed the CFP was a deliberate resource grab just prior to the UK joining. It was a non negotiable condition of entry which Heath told his mandarins not even to negotiate over.
It is likely the Heath knew exactly what he was doing and the consequences however the same could not be said for later leaders.
John Major for instance complained to other EU leaders about the CFP and was quietly taken aside by Helmut Kohl who suggested that he actually read the treaties. Major had no idea that he had no right to complain, that Heath had signed all of our fish away.
The same for Howard in the noughties, promised all sorts of reforms ( none of which it appears would have been possible under EU law) , which were quietly dropped by Cameron.
Then there is the European courts, most famously with the Factortame judgement.
en.wikipedia.org/wiki/R_(Factortame_Ltd)_v_Secretary_of_State_for_Transport
You might think we were being slightly generous in attempting to limit foreign ownership of a vessel of company at 75%, though the eventual ruling was that we could not discriminate on the basis of EU nationality or location at all.
In other words you have quotas, but your quota can also be used by foreign vessels or firms who don't operate out of or land the fish in the UK.
There is no incentive for our fishermen to return the favour as the vast majority of all fish in the EU are caught in our waters.
We are no longer allowed to keep statistics other than number and sizes of boats as that is an EU competency! However in 1983 our quota was about 37% by weight and 13% by value of our own fishing stocks - which themselves make up over 70% of the EU total.