I missed your post above when I replied to Jack, I was about to go out.

Overfishing is the simplest explanation. see link posted by jack.
Overfishing is not the reason why you allow somebody else to fish particular waters. That doesn't make sense.

People don't suddenly start stuffing themselves with certain foods because their country can no longer export it due to tariffs.
We eat a lot of fish. I don't know how much we export, and who to, but tariffs are something we can negotiate after we leave. Maybe New Zealand will buy our fish and we'll buy their lamb. I have no idea, that's for when we actually leave. That's not a choice we made, we literally cannot negotiate these things until we leave.

Certain enough to support Brexit though, that's the problem.
You're the one who thinks leaving the EU is a problem. I don't. I agree it's a clusterfuck, but it's not a problem until it is clear that we are worse off. That won't be clear for a long time, there are still many obstacles to overcome, such as trade deals with other countries.

Best case scenario is we get a trade deal similar to one we have now, which seems pretty unlikely given we'd be giving them two fingers up. Worst is we get treated like every other riff raff country.
Your focus here is the EU. That's your problem, it's like you think they're the only people we can trade with. If we get a similar trade deal while not being politically integrated into their superstate, that's great,perhaps even optimal. If they treat us like riff-raff, fuck them. Hello USA, hello New Zealand and Australia, hello China.