What it's already done about it is massively increase fish stocks. I'm unsure how much of the good stuff that's happened is related to the EU, certainly some of the bodies that organise it are at least partly funded by the bit of the UK's EU contribution that gets spent in the UK. I'm not sure how much of the Marine and Coastal Access Act 2009 is from the EU, certainly some of it relates to enforcing the EU's rules and penalties involved for breaching them.
https://www.theguardian.com/environm...analysis-shows
My understanding of the fishermen's moaning is that a: it'd be better *for them* if the UK's waters weren't open to EU fishing (the Spanish) and whilst you can't really argue with that you may or may not think that that's a good thing, b: that the spanish boats largely ignore the rules and the spanish don't enforce them which may or may not be very true and c: some of the EU rules on fishing are shit and the fish and people would be better off without some of them.
I think that a is certainly true, that there is considerable scope for improving b within the EU and that c is probably true and the EU might get around to doing something about it one day maybe.