By Juliet Eilperin
Washington
July 30, 2005

The variety of species in the world's oceans has dropped by as much as 50 per cent in the past 50 years, according to a study.

A combination of overfishing, habitat destruction and climate change has narrowed the range of fish across the globe, biologists Boris Worm and Ransom Myers of Dalhousie University in Nova Scotia and three other scientists wrote in the journal Science.

In some areas, such as the ocean off north-west Australia where a wide variety of tuna and billfish used to thrive, diversity has declined dramatically.

"Where you used to put out a fishing line 50 years ago and catch 10 species, now you catch five species for the same amount of effort," Dr Worm said.

"That's a recipe for ecological collapse and disaster."

The study, which marks the first worldwide mapping of predatory fish diversity, identified five remaining spots in the world that still have a rich variety of species, two of them in US waters.

The spots include areas off the east coast of Florida, south of Hawaii, near Australia's Great Barrier Reef, near Sri Lanka and in the South Pacific.

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"These areas are really of global significance," Dr Worm said.

"It's really important to protect them now, because 20 years from now they may not be there."

The total catch for tuna and billfish has increased as much as tenfold over the past 50 years, they found, prompting fish diversity to plummet. Overfishing is the main factor in these species' decline, Dr Worm said, as well as for other fish caught inadvertently.

"That's what's driving the pattern," he said.

But in an example of how shifts in temperature can also affect diversity, the study found that in the Pacific, the variety of fish expanded when the weather pattern known as El Nino swept in and brought warmer surface water, but then contracted when temperatures dropped.

Predatory fish appear to like temperatures about 25 degrees , Dr Myers said. "Like Goldilocks and the three bears, ocean animals don't like it too hot or too cold, they like it just right."

To do the study, Dr Worm and Dr Myers — along with Marcel Sandow and Andreas Oschlies of Germany's Leibniz Institute for Marine Science and Heike Lotze of Britain's National Oceanography Centre — used data from Japanese long-line fisheries going back to the 1950s, which they cross-referenced with scientific data from the US and Australia.

The researchers determined that tuna and billfish are indicators of wider ocean diversity, and that these species are disappearing in many areas. Mid-size predators — snake mackerel and pelagic stingrays — are taking their place.

Dr Myers said international authorities need to ban fishing in ecologically valuable sites if they want to preserve them.

- Washington Post