On many cans you will spot a "dolphin friendly" logo. In the 1990s tuna fleets were forced to clean up their act by fitting all nets with special hatches through which accidentally caught cetaceans could escape. These measures were successful, as far as they went, and have created the legend that tuna is a "green" food, healthy for us, healthy for the environment. Hence that happy dolphin.

Don't believe a word of it. Every chunk of tuna comes from a wild fish. Because tuna are wide-ranging, fast-moving ocean fish, fisheries have developed awesome techniques for catching them. Fleets use vast purse-seine nets to scoop them out of the sea, while Japanese vessels, in particular, trail lines of baited hooks many miles long.

Such methods are undiscriminating. The bycatch - that is, the non-target species - routinely includes sharks, turtles and albatrosses. The ratio is about four sharks caught for every tuna. According to the Shark Trust, longlines operating off New Zealand have snapped up 450,000 blue sharks in 10 years.

There are now ominous signs that the targeted catch is also in trouble. The fish everyone wants to find in their net is the bluefin tuna. There are two closely related species, one in the northern oceans and the other in the southern seas. Both are magnificent fish. They grow up to two metres long and can weigh 500kg. Yet, despite their bulk, they are among the fittest, fastest beings in the ocean: sleek, warm-blooded and the ultimate in fishy power.

Two things are combining to bring down the bluefin. One is their slow breeding rate -they take at least 10 years to become sexually mature, and so are vulnerable to overfishing.

The other problem is that bluefin are expensive. A full-sized fish can fetch tens of thousands of dollars. And a market that was once centred in Japan is widening by the year. Many countries, including Britain, have acquired a taste for sashimi - thin slivers of raw tuna dunked in soya sauce. Last year we imported 1,600 tons of the stuff, worth £8.6m. But that is small beer compared with the potential market in China, where a fast-growing middle class eyes bluefin sushi as the ultimate gastronomic status symbol.

This isn't sustainable. Although bluefin can be farmed, no one has yet worked out a way of rearing them from eggs. All farmed tuna are simply wild-caught from the sea and fattened up. But stocks are becoming dangerously depleted. Catches around the Balearic Islands are down to just 15 per cent of what they were a decade ago, and six Spanish tuna farms have gone out of business.

According to the Worldwide Fund for Nature (WWF) which is monitoring bluefin closely, fleets from the EU, as well as from Japan, Libya and Turkey, are routinely ignoring fishing quotas and failing to report their true catch (and thus avoiding paying tax). Hence no one knows how many bluefin are being caught in the eastern Atlantic and Mediterranean. But it is certainly enough to put the population in peril.

"The fishery is out of control," says Dr Sergei Tudela, the head of fisheries for WWF. "Bluefin stocks are on the brink of collapse." The hungry market has brushed aside weakly enforced conservation measures with contempt. For example, says Tudela, last year France admitted exceeding its quota by 60 per cent. Evidence indicates that 50,000 tons of bluefin were removed from the eastern Atlantic last year, despite an all-nation agreed quota of 32,000.

Tudela is lobbying the International Commission for the Conservation of Atlantic Tuna (ICCAT), which is responsible for regulating the fishery, to adopt a strict recovery plan for the northern bluefin. He calls for an immediate close season on bluefin fishing from May to July, when the fish spawn. Beyond that, he wants to raise the minimum catch weight from 10kg to 30 kg. He calls for the EU to reduce over-capacity by scaling down fishing fleets. And he insists on much better control and reporting, with observers being allowed aboard all vessels.

ICCAT will meet in November to consider these proposals. The British Government supports them. France, Italy and Japan, it seems, do not.

It is much the same story in the Antipodes, where the sinking species is the equally delicious southern bluefin tuna. Australia has a successful A$280m (£110m) operation based on capturing the fish live and fattening them up in cages. Yet, despite strict quotas on the catch, the southern bluefin has been declining year on year. Now there are barely enough left to sustain the fishery.

The man in charge of Australia's fisheries policy, Richard McLoughlin, is angry. Despite an agreed national quota of 6,000 tons, he claims to have proof that Japan has been catching "anywhere between 12,000 and 20,000 tons for the past 20 years and hiding it". By illegally taking A$2bn [£800m] worth of tuna, Japan "has probably killed off the stock" . According to the most recent estimate, only four per cent of the original biomass of southern bluefin survives.

This species is currently classed as "critically endangered". Without urgent intervention, the southern bluefin is probably doomed to commercial, if not actual, extinction. But so long as Japan continues to allow only Japanese inspectors on board its fishing vessels, and refuses to install satellite monitoring systems, there is no way of checking its catches. All scientists know is what that country imports. It looks like stalemate unless Japan can be persuaded to see reason.

Can we, as consumers, do anything to reverse what is fast shaping up towards a double whammy - the commercial extinction of two of the world's favourite edible fish? Wait until the outcome of the November ICCAT talks, says Sergei Tudela. If the talks succeed, there is a chance of saving at least the northern bluefin. If not, then it may be time to look deeply into our green souls. And to pass that sushi by.

For more about the bluefin crisis go to www.panda.org/marine/tuna

What not to eat

ENDANGERED:

Northern bluefin (north Pacific and Atlantic oceans, Mediterranean)

Caught by seine nets and long-line and cage-farmed (in the western Mediterranean). The world's most expensive tuna, eaten as sushi. Much of the 45,000-ton annual catch goes to Japan.

Status: Data insufficient, but considered endangered in Atlantic

Southern bluefin (mostly caught off Australasia and South-east Asia)

Stocks have fallen by 95 per cent since the 1950s, and there has been a supposedly strict quota system in force since 1985. Caught mainly by longline, but also farmed in South Australia. Eaten as sushi.

Status: Critically endangered

Bigeye (tropical and temperate seas, excluding the Mediterranean)

Smaller fish weighing 4kg to 16 kg. With the decline in bluefin, fisheries turned to this species, which is now also declining. Atlantic stocks are down 50 per cent in 10 years.

Status: Vulnerable. Pacific stocks endangered

LESS ENDANGERED OPTIONS:

Yellowfin (tropical and subtropical seas)

Up to two metres long (200kg) but more usually 7kg to 25 kg. Likes to swim with other large fish and dolphins, hence a large dolphin bycatch until escape hatches were introduced in 1990s. Probably overfished, but stocks are still fairly healthy. Sold frozen, canned or fresh as sushi.

Status Lower risk; conservation dependent

Skipjack (tropical and subtropical seas)

Smaller fish, 3kg to 7 kg. Often found in large schools near the surface. Caught in seine nets or with line gear. No stock assessments since 1999 but probably still fairly healthy. Skipjack is the main species of canned tuna, with a catch rate of 1.5 to 2.2 billion tonnes a year.

Status Not threatened

© 2006 Independent News and Media Limited

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