The Yahoo News , 21 February 2006
Pity the shark
PARIS (AFP) - Ocean depths beyond 2,000 metres (6,500 feet) are almost devoid of sharks, a finding that is grim news for these threatened fish, a study says.
An international team of researchers used deep-sea trawls, baited hooks and baited cameras to see where sharks live, testing depths in the Atlantic, Pacific and Indian oceans and Mediterranean Sea from 471 to 5,900 metres (1,530 to 19,175 feet).
Sharks were generally seen or caught at up to 2,000 metres (7,500 feet), but beyond this depth, the numbers were negligible. The deepest specimen was a species called a leafscale gulper shark, caught at 3,280 metres (10,660 feet).
This means that there is no reserves of sharks living in the abysses -- rarely-explored depths that are beyond 3,000 metres (9,750 feet) and comprise 70 percent of the oceans' volume.
As a result, almost all sharks are within reach of modern deep-sea trawlers, which can net fish to a depth of up to 2,300 metres (7,475 feet).
"Sharks are apparently confined to about 30 percent of the total ocean, and distribution of many species is fragmented around sea mounts, ocean ridges and ocean margins," says the paper, which appears in Proceedings of the Royal Society B, a British journal.
"All populations are therefore within reach of human fisheries, and there is no hidden reserve of chondrichthyan biomass or biodiversity in the deep sea. Sharks may be more vulnerable to over-exploitation than previously thought." Chondrichthyes is a category that include sharks, skates and rays.
Of the 490 species of shark, 25 are endangered or facing extinction, according to the "Red List" of threatened biodiversity compiled by the World Conservation Union (IUCN).
However, this number is set to grow, in the light of new data assessed by the IUCN's group of shark experts, the Swiss-based agency said on Monday.
Many shark species are very slow-growing, which means that a sudden drop in numbers can threaten their survival.
Shark species have been ravaged by commercial trawling -- they are scooped up accidentally as "bycatch" by trawlers hunting more lucrative fish, but are also being increasingly targeted for their fins, which are used for the Chinese dish called shark-fin soup, and for their liver, which is rich in oil.
Some species of chondrichthyan fish have adapted to the extreme pressures and poor light of the deep ocean.
The shark, though, seems to be excluded from life in the abysses because of a lack of food down there and the relatively high buoyancy of its liver, says the paper, lead-authored by Imants Priede of Scotland's University of Aberdeen.