It's hard enough to manage a fishery stock sustainably when the fish stay put. Once they start moving around it's almost impossible. That's why the European Union and Iceland are heading into a mackerel war. It's a foretaste of things to come, as warming oceans cause fish to migrate in order to stay in their temperature comfort zones.
The conflict this time is quite different from the "cod wars" between Iceland and Britain in 1958 and in the early 1970s, as Iceland progressively extended its maritime boundaries in order to save its cod stocks from over-fishing by British trawlers. Back then, Icelanders were indisputably in the right. If they hadn't acted decisively, their cod would have gone the way of the world's richest cod fishery, on the Grand Banks of Newfoundland.
Newfoundland lost its cod because it was no longer an independent country, and the cod fishery ranked pretty low on the Canada's priorities. Ottawa wasn't willing to pick a fight with other countries over codfish when it had so many other trade issues on the table. Whereas the cod fishery was the biggest industry in Iceland, and so it fought hard to defend it: British trawlers' nets were cut by the Icelandic Coast Guard, there were ramming incidents, and there was much angry rhetoric. In the end Iceland won, as it deserved to - and it still has its cod stocks. (A president of Iceland once told me privately that she believed Newfoundland would still have its cod too if it had been free to fight for them).
But Icelanders are not saints, and this time they are in the wrong. The issue is the Atlantic mackerel, whose total catch went from about 150,000 tonnes in the early 1950s to over a million tonnes in 1975, and then fell back to about 700,000 tonnes by 2010. A smaller relative of the tuna, its flesh is much in demand in Europe, and it has become a mainstay of the British, Dutch and Scandinavian fishing fleets.
They know that the mackerel stock is being over-fished, and in recent years they have set quotas for the total allowable catch. This required complex negotiations between the European Union (representing the Britain Denmark, Sweden and the Netherlands) and Norway (which is not an EU member). The talks were successful, but last month the Marine Conservation Society removed mackerel from its "(safe) fish to eat" list anyway.