VICTORIA, the Seychelles (Reuters) - Tuna catches across the Indian Ocean have fallen sharply in the last two years but experts are split over what is threatening the region's $6 billion industry.
Conservationists blame years of unchecked exploitation while processors say climatic conditions may be driving the fish deeper away from their nets.
Tuna catches in the Indian Ocean, which accounts for roughly a quarter of the global haul, dropped by about a third last year to their lowest level for more than a decade.
Early indicators for this year show catches to be markedly below recent averages, Alejandro Anganuzzi, head of the Indian Ocean Tuna Commission, told Reuters.
"We cannot rule out the possibility that overfishing has occurred," he said.
Other forces such as changes in wind patterns, currents or the impact of predators might also play a part, he said.
Similar falls in catches are seen in the Pacific, where environmental groups say decades of overfishing has slashed some stocks by as much as 85 percent. European fishing firms now chase tuna in the Pacific after numbers fell in the Atlantic.
Last month, EU fisheries regulators banned trawling for bluefin tuna in the eastern Atlantic and Mediterranean to stop overfishing of a species that was approaching complete collapse.
Rewards for fishermen remain high.
Market prices for the delicacy have roughly tripled since last year. In Japan, where there is huge demand for tuna to make sushi, top quality fish can sell for up to $100,000 each.
In the Seychelles, tuna canning is worth $180 million a year and accounts for more than 90 percent of export earnings.
One of the biggest canners in the region, Indian Ocean Tuna (IOT) Ltd, says its volumes have dwindled by about 18 percent to 70,000 tonnes processed annually for the last two years.
IOT's general manager, Alain Olivieri, said the Indian Ocean had seen a "terrible" fall in catches, which he blamed on higher water temperatures pushing fish deeper out of reach of nets.
Experts are divided over whether these warmer warmers are the result of climate change or of cyclical ocean conditions.
Olivieri said most of the fish had descended from their normal level of around 250 meters below the surface, where they could be caught, to depths of 400 meters, where they were safe.
"I believe the fish are there and they will not stay permanently down, so when the temperatures improve they will move higher up where fishermen can catch them," he said.
David Ardill, a Mauritius-based expert, said tuna fishing in the southwest Indian Ocean was worth up to $6 billion a year, with Mauritius alone earning nearly $400 million annually.
(Additional reporting and writing by Ed Harris in Port Louis; Editing by Daniel Wallis and Angus MacSwan)
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