Food shopping is full of ethical grey areas. Be careful what you buy – or fish will be a luxury

http://www.guardian.co.uk/commentisfree/2013/jan/27/climate-change-consumers-sustainable-fish

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It used to be straightforward: cod was being overfished, so you knew not to eat it. Bluefin tuna was out of bounds. And don't even think of ordering the marlin steaks, unless you have the filthy environmental conscience of a natural gas fracking mogul.

But these were edicts issued when the only certainty was the crisis in fishing stocks; at a time when sustainability was regarded as, at best, a fringe issue. Now questions of sustainability have entered the mainstream and curiously that has made everything more complicated. More vested interests are involved and it has all become heavily political.

Certainly it made last week's announcement that mackerel – for so long regarded as the last bastion of plenty in the seas – should come off the menu very complicated indeed.

Of course, the advice to avoid eating this lovely oily fish has consequences for the fishing industry. But spare a thought, too, for the poor consumer, caught in the middle desperately trying to work out what they should and shouldn't be eating. It isn't clear cut.

As has been reported, mackerel stocks are very healthy. But they have migrated northwards into Icelandic waters, apparently as a result of climate change. The Icelandic fleet is pulling mackerel out of the sea like it's the gold rush and they have hit a rich seam.

The Marine Conservation Society, which issued last week's advice, predicts that at current fishing rates stocks will fall below sustainable levels within the next couple of years. The society is trying to use its consumer reach to force the European Union and Iceland to the negotiating table to agree on quotas.

In the meantime, those wanting to eat mackerel should eat only those caught via traditional methods, for example using handlines, drift nets or ring nets. In short, it's both OK and not OK to eat mackerel. And all you were hoping for was something to cook for your tea.

This is not the only grey area. Late last year celebrity chefs and food writers were criticised for publishing recipes utilising monkfish, because stocks are no longer sustainable. It turned out not to be true. Some monkfish fisheries are imperilled; others are not.

Likewise, the prohibition on cod is no longer as black and white as it once was. There may be question marks over the North Atlantic fishery, but in the Barents Sea a small rise in temperatures has led to a biomass explosion. The fishery is in a better shape than it has been for 60 years.

How does the ethically minded shopper navigate these switchbacks and hairpin bends?

As with all food issues these days we have a responsibility to educate ourselves; shopping passively is a luxury we can no longer afford. That means going online and seeking out sites such as the Marine Conservation Society's Good Fish Guide.

But the retailers also have a part to play, especially the big supermarkets. We have given them a free run at the retail market, more than 80% of which they now control. With that comes a responsibility to educate the consumer. It is, of course, customary to knock the supermarkets, so credit where credit is due. It so happens that on Friday Sainsbury's held a "Switch a fish day". Customers asking for one of the so-called big five species – cod, tuna, salmon, haddock and prawns – were offered a free sample of another sustainable species, such as sardine (recommended by the society as a replacement for mackerel), coley or trout. It was an attempt to spread fish purchases across a wider selection of species, so as to relieve the pressure. We need more such initiatives.

By coincidence today's restaurant review, in the <em>Observer</em> Magazine, is of an excruciatingly expensive fish restaurant at a fancy London hotel: a plate of brill for £30; some humble mackerel for an eye-popping £19. It's ludicrous and dispiriting. But unless more of us become sustainability-savvy, unless we look seriously at our purchasing decisions, that restaurant will stop being a joke and become a fact of life. Fish really will become a luxury that only the wealthy can afford.