US: The variety of species in the world's oceans has dropped by up to 50 per cent in the past 50 years, according to a paper published yesterday in the journal Science.
Overfishing, habitat destruction and climate change have narrowed the range of fish across the globe, write biologists Boris Worm and Ransom Myers of Dalhousie University in Nova Scotia with three other scientists.
In some areas, such as the ocean off northwest Australia where a wide variety of tuna and billfish used to thrive, diversity has declined precipitously.
"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," Mr Worm says. "That's a recipe for ecological collapse and disaster."
The study, which marks the first worldwide mapping of predatory fish diversity, identified five places 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.
"These areas are really of global significance," Mr Worm said. "It's really important to protect them now, because 20 years from now they may not be there."
Temperature can also affect diversity - the study found that in the Pacific, the variety of fish expanded when El Nino brought warmer surface water, but then contracted when temperatures dropped.