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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.
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http://www.ecoearth.info/shared/reader/welcome.aspx?linkid=61883