Sharks missing from deep waters
Ocean depths beyond 2000 metres are almost devoid of sharks, a finding that is grim news for these threatened fish, a study finds.
An international team of researchers publishes its findings in the journal Proceedings of the Royal Society B.
The 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 5900 metres.
They generally saw or caught sharks at up to 2000 metres, but there were few sharks beyond this depth.
The deepest specimen was the leafscale gulper shark, caught at 3280 metres.
This means that there are no reserves of sharks living in the abysses, rarely-explored depths that are beyond 3000 metres that comprise 70% 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 2300 metres.
"Sharks are apparently confined to about 30% of the total ocean, and distribution of many species is fragmented around sea mounts, ocean ridges and ocean margins," says the paper, which mentions other chondrichthyes, a category that also includes skates and rays.
"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."
Endangered or facing extinction
But 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 says.
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, and are also being increasingly targeted for their fins, which are used for the Chinese 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, by Professor Imants Priede of Scotland's University of Aberdeen.