ne billion people around the world depend on fish for their main source of protein. But the harvest of the ocean - once thought to be boundless - is declining because it is being overexploited.

The seas are the last major part of the planet where 'modern man' still behaves as a hunter-gatherer - but pays far less attention to conserving stocks of prey for the future than supposedly 'primitive' hunting communities in the world's forests and deserts.

There are two and a half times as many fishing boats plying the world's seas and oceans as would be needed to catch the amount that they could harvest without depleting stocks. And governments subsidize them by a massive $15 billion a year, up to a quarter of the total value of the fish caught, to keep them in business.

So it is not surprising that three quarters of the world's fisheries are in crisis - either already overexploited or pushing their limits; only a tiny 4 per cent are underfished. As each area has been fished out, the fleets have moved on to others, depleting them in turn.

As a result - despite all the boats, subsidies and efforts of fishers - the amount of fish caught worldwide peaked at the turn of the millennium and has since been declining. As population growth has continued, the catch per person - at 14 kilograms - is at its lowest level for 40 years.

Catches of many popular fish - such as cod, flounder and hake - have been cut in half, even though the amount of effort put in to hunt them down has tripled. Over the last half century the number of large predatory fish in the oceans - like tuna, marlin, swordfish and sharks - has dropped by a staggering 90 per cent. And over the same period the average size of a blue shark has plummeted from 52 to 22 kilograms.

Yet huge amounts of the global catch are wasted. Every year the world's fishing fleets throw out some 20 million tonnes of fish and shellfish as unwanted 'by-catch'. About one in every 12 fish landed are returned, dead or dying, to the waters - because selling them is not profitable enough, or because the fishers have caught them in excess of conservation quotas.

Other wildlife is also killed as a by-product of fishing. Some 300,000 small whales, dolphins and porpoises are fatally entangled in fishing nets each year, and over 250,000 endangered loggerhead and leatherback turtles are caught on longlines set for fish like tuna and swordfish. The lines also threaten some 19 species of albatross with extinction.

At last there is some effort to promote better fishing. An independent agency, the Marine Stewardship Council (MSC), certifies sustainable fisheries so that consumers can choose to buy ethically; so far it has endorsed 12 fisheries as sustainably managed, and 263 verified products are on sale in 24 countries. And there is increasing interest in setting up protected areas where no fish can be caught, so as to allow stocks to recover.