- WASHINGTON (AP) -- The variety
of tuna, marlin, swordfish and other big ocean predators has declined up
to 50 per cent over the past half-century due to overfishing, scientists
say.
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- For the first time, ecologists and oceanographers mapped
the hot spots with the largest concentrations of many big fish species,
then and now. Their findings are reported in Thursday's on-line edition
of the journal Science.
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- Researchers who had previously reported an overall decline
in the abundance of big fish now say there has also been a significant
drop in the number of different types of fish such as tuna and billfish
being found in many areas.
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- They did find a few places where fish remain abundant.
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- "We found five large hot spots that are still remaining
today, and two of those are in U.S. waters," said lead author Boris
Worm, a marine ecologist at Dalhousie University in Halifax.
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- Those two spots are in the Atlantic Ocean, east of Florida,
and in the Pacific Ocean, south of Hawaii. Two of the other areas are in
the South Pacific, east of Australia's Great Barrier Reef and north of
Easter Island. The other is in the Indian Ocean, east of Sri Lanka.
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- The scientists examined the only global data set for
big fish species since large-scale fishing fleets began spreading globally
just after the Second World War. It is based on Japanese fishing between
1952 and 1999 with the most widespread type of fishing gear, longlines,
that are used in the open ocean. Those float for up to 110 kilometres on
the water's surface, with baited hooks reaching down to 300 metres below.
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- Based on those data, the scientists found average declines
of about 50 per cent in diversity in the Atlantic and Indian oceans, and
an average decline of about 25 per cent in the Pacific.
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- They also discovered that the sea-surface temperature
and level of oxygen in the water correlate with where the big fish concentrate.
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- "The peak in big fish diversity is at middle temperatures,"
said co-author Ransom A. Myers, a fisheries scientist also at Dalhousie.
"Ocean animals don't like it too hot, or too cold, they like it just
right" about 25 degrees Celsius.
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- That contrasts with the general distribution of species
on land, which is richest in diversity at the equator and wanes toward
the poles.
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- Nathan Mantua, a climate scientist at the University
of Washington in Seattle, who was not part of the research group, said
the study is useful for identifying the combined effects of climate variations
and fishing on the distribution of the ocean's largest predator fish.
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- "They highlight that there are biodiversity hot
spots that are tied directly to physical structure in the ocean,"
which includes temperature, dissolved oxygen and currents, he said. "Out
in the ocean, it's pretty clear that the primary cause of the long-term
declines in biodiversity is fishing."
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- They also found that concentrations of many big fish
lined up closely with the only other known global mapping of ocean life
that of single-celled zooplankton.
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- Steven D'Hondt, a University of Rhode Island oceanographer
and a co-author of that 1999 mapping, said "the smallest animals in
the ocean and some of the largest show the same pattern of diversity at
the global scale."
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- Callum Roberts, a marine conservation biologist at Britain's
University of York, said the study could help policy-makers and conservationists
determine where best to locate marine-protected areas on the high seas
an issue being debated by the United Nations.
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- "While some hot spots have disappeared, there are
still some very special places where species concentrate," Mr. Myers
said.
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- Two years ago, he and Mr. Worm used the same data to
show that commercial fishing had depleted the world's oceans of 90 per
cent of the overall abundance of big fish that flourished 50 years ago.
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- The research was funded by the Sloan Foundation, German
Research Council, Pew Charitable Trust and the Natural Sciences and Engineering
Council of Canada.
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