Supermarkets may make unlikely environmental heroes, but they are leading changes in public behaviour, fishery practices and government policy in the fight against overfishing. Threatened by a future with no fish to sell, they are doing something about it.

When Waitrose did a survey last month it found that more than three quarters of us do not make any attempt to buy sustainable seafood, mostly thanks to a widespread ignorance about crashing fish stocks around the world. Talk to the guys behind the fish counter, and they will tell you the same thing - no one asks where the fish comes from or how it is caught.

Customer pressure is usually the main driver of supermarket behaviour. Cynics may say that in a world where environmental credentials are valuable, there is incentive to promote the next Fair Trade or organic standard. But the leading supermarkets - not just Waitrose, but Marks & Spencer, Sainsbury's, the Co-op, Asda and Tesco's among others - are pushing for change because doing nothing would soon mean they have no fish to sell.

The facts are often repeated. Only eight of the forty-seven fish stocks around the UK are in a healthy state. Some 80 per cent of EU fisheries are either fully or over-exploited. And if current practices continue, worldwide fishery collapse is predicted by 2048.

Between 80 and 90 per cent of all fish caught in the UK goes through supermarkets, making them the major player in fisheries behaviour. (This is a fairly recent role - in 1982 independent fishmongers ruled, and only 10 per cent went through the supermarkets.) Shifting such volumes allows them a perspective denied to the average omega-3-hunter at the fish counter. It is hard to tell when a cod from Newfoundland is replaced by a cod from Iceland, but the supermarkets notice when they have to change supplier because a stock has crashed.

That they have chosen to act is an indication of how serious things are. Of course, they have been helped to come to the right conclusions thanks to concerted campaigning by the marine conservation groups.

Waitrose, Marks & Spencer, Sainsbury's and the Co-op are leading the charge by stocking only fish from responsibly managed, sustainable fisheries. The gold standard are fish certified by the Marine Stewardship Council (MSC), but although this scheme is expanding rapidly there are still not enough MSC-rated fisheries to fill demand. Instead, supermarkets are avoiding stocks listed as endangered or critically endangered by the International Union for Conservation of Nature or as “fish to avoid” by the Marine Conservation Society.

Now they want to let their customers know what they are up to, and the response promises to be strong. The same Waitrose survey that betrayed the general ignorance of any problem showed a large swing the other way once an explanation had been given. And explanations do not get much more powerful than the film The End of the Line, which looks at the effect of overfishing, and which is being shown today, World Oceans Day, at cinemas nationwide.

Challenges remain, however. More than half the fish sold in supermarkets is salmon, almost all of which is farmed. A fish that was until recently the preserve of the rich (remember when smoked salmon was a rare treat?) is now available to everyone. Tesco sells farmed salmon from £1.14 per 100g, compared with £3.99 for the wild-caught Alaskan alternative.

Anyone who loves to eat fish will know there is a world of difference between eating farmed and wild-caught salmon. The same will doubtless be true when the first completely farm-reared bluefin tuna hit the stores in two years' time, but by then that may be the only bluefin tuna available; new reports appear to indicate that Mediterranean wild stocks have already collapsed.

Fish farming has a dirty secret: no matter how well cared-for and clean the farms (which too often they are not), they consume five kilos of wild fish to create one kilo of fish to sell. Even the “responsibly farmed” Tesco option (£1.97 per 100g) is fed using wild-caught feeds. Sourcing this sustainably is difficult. Half of all fish consumed by humans now comes from aquaculture, but the global supply of small fish for fishmeal is already stretched.

Major reforms of how we manage the sea are clearly necessary to return the seas to productivity, and here the supermarkets are wielding their power in the other direction: upwards. It took the Marine Conservation Society a year to gather just over 100,000 votes in its campaign for marine reserves. Today the Co-op releases the results of just one week of polling of customers at the tills of 2,400 of its stores. Shown a short film while queueing, 360,000 customers voted using chip-and-pin machines. In all, 83 per cent were in favour of the establishment of highly protected marine reserves.

The case for such reserves is clear, not just for biodiversity but for fisheries. A worldwide network of reserves covering 30 per cent of the seas would multiply productivity many times over, while costing less to establish and enforce than the estimated £8 billion to £16 billion spent on fishing subsidies every year.

Supermarkets are convinced of the potential for a win-win situation, and so, when given the facts, are their customers. The Government, however, has resisted the firm commitments necessary. The Marine Bill will come to the Commons in the next few weeks, but, as it stands, contains no references to highly protected marine reserves or to the network that is crucial to their function.

Huw Irranca-Davies, the Fisheries Minister, says this is to allow flexibility in planning, but his sympathies lie plainly with the 12,729 fishermen in the industry, not the 360,000 shoppers who registered their opinions with the Co-op.

Frank Pope is ocean correspondent of The Times

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