This was not argued well during Brexit (what was?) but it was an issue there. As much as Brexit didn’t fix anything for anyone, I do believe the EU operates far too much in the interests of fishing boats and far too little in the interest of conservation and realistic fishery management.
So I think Iceland is right to be wary.
Yes. Ten years ago mackerel was cheap and abundant but the quota was set in excess of scientists' recommendations. Stocks are dwindling now (surprise!) and mackerel has gone from cheaper than tuna (for a tin) to nearly double. It's absurd.