Except for cod, the British don't eat fish or shellfish, the result being that at present most of the fish the UK catches is exported (with 75% of exports going to the EU) and most of the fish eaten in the UK is imported. (UK sea fisheries statistics 2019)

It's probably true that the extent to which British waters are fished by other EU nations is unbalanced but nevertheless, the situation the UK has created for itself is bonkers. The UK fishing industry is simply too large to sustain itself (a problem faced, one suspects, by many countries with large coastlines relative to land mass and population), unless it radically revises its approach to sustainable fishing - a task it's even less likely to be able to undertake without considered cooperation from its neighbours.