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.
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.
Mackerel is a migrating fish which historically have caused difficulties in managing the fish stock. Between 2020 and 2024 there were no agreement between the nations that has been fishing mackerel in the north atlantic. Iceland was f.ex. fishing mackerel in their zone for 15 years without cooperating with other nations. I think there's an agreement between most of the relevant nations now, except Russia but Russia have a general fishing agreement with Norway which might include mackerel.
Anyway, managing fisheries sustainably is difficult as a lot of fish migrates across huge distances and a lot of fishing is done in international waters. Ideally you need binding agreements between all states bordering an ocean region, which can be difficult when the states are in geopolitical conflict with each other ...
So I think Iceland is right to be wary.