AN UNDERWATER “dead zone” devoid of most life has been discovered on the ocean floor near the epicentre of the earthquake that triggered the Boxing Day tsunami.

The first scientific dives off the coast of Sumatra since the disaster have revealed that, while most marine life was not affected, one site appears to have become uninhabitable to large marine species.

A submersible that made an 11-hour dive to a feature known as the Ditch, 2½ miles (4km) beneath the ocean’s surface west of the earthquake’s epicentre, found no trace of fish or other marine “megafauna”. The absence of visible life was “unprecedented in 25 years of deep-sea sampling”, scientists from the international Census of Marine Life said.

Ron O’Dor, the senior scientist on the census, said: “The sea is rich in life and you would expect a site like this to be quickly recolonised, but that hasn’t happened. It’s unprecedented.”

The “dead zone” appears to have been created when the Boxing Day earthquake, or one of its aftershocks, caused the collapse of an underwater cliff that released vast quantities of sediment into the water.

The fine sediment would make it very difficult for larger marine species such as fish to thrive, though it may not have affected very small creatures that were not sampled during the expedition. “It is not a good environment for an animal to live in,” said Paul Tyler, of the National Oceanography Centre in Southampton, who took part in the study.

Scientists have been puzzled to find such damage at only one site, as seven other spots investigated by the expedition were largely unaffected by the earthquake and tsunami.

Professor O’Dor said: “Normally when you go to the bottom of the sea anywhere and take a sample or look around, there’s always something alive. But five months after the earthquake this entire plain, created by the collapse of this cliff, was essentially devoid of life.

“No one has ever got to a site like this so quickly before. It may just be that it takes a while for things to get back to normal. The sea is very cold at this depth, and typically the speed of life is proportional to temperature. Nothing happens very fast at 4C (39F).”

The “dead zone” is one of a string of remarkable discoveries made this year by the Census of Marine Life, which is now halfway through a ten-year project to unpick the mysteries of the world’s oceans. The census has identified dozens of new species, among them a carnivorous sponge, Asbestopluma, in the Southern Ocean.

Scientists have also launched several tagging projects, by which the migration patterns of fish such as salmon and tuna are tracked. Almost 2,000 animals of 21 species have been tagged so far, including sharks, birds, turtles, seals and sea lions. One individual bluefin tuna was tracked as it crossed the Pacific Ocean three times in less than two years, swimming more than 40,000 miles.

The Pacific Ocean Shelf Tracking Project (Post) also revealed the annual migration routes of young salmon from rivers in the US and Canada.

David Welch, the project’s lead scientist, said: “New developments mean that we will be able to monitor the return migration of adult (salmon), providing information that could help better protect endangered stocks.”

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