Sharks could be more vulnerable to the fishing industry than was previously thought, research has revealed.
There are not thought to be hidden reserves of sharks
Marine scientists led by Aberdeen University have discovered that the deepest oceans of the world appear to be shark free.
One possible reason, they tell a Royal Society journal, may be lack of food.
The researchers warn that the findings mean all shark populations are within reach of human fisheries and could be at greater risk than was thought.
In their paper published on Wednesday - The absence of sharks from abyssal regions of the world's oceans - the international team of researchers reports that sharks have failed to colonise at depths greater than 3,000m.
Sharks are found throughout the world's oceans and it had been hoped new species would be discovered as exploration went deeper.
However, 20 years of exploration combined with analysis of records over the past 150 years, has convinced the scientists that the world's oceans are 70% shark-free.
The average depth of the oceans is 4,000m and bony fish - relatives of cod - thrive down to around 9,000m depth.
The scientists do not know why sharks are absent there but suggest one possible reason could be a lack of food resources.
Professor Monty Priede, director of Oceanlab at Aberdeen University, said: "Sharks are apparently confined to around 30% of the world's oceans.
"All populations are therefore within reach of human fisheries, near the surface and at the edges of deep water.
"Sharks are already threatened worldwide by the intensity of fishing activity.
"But our finding suggests they may be more vulnerable to over-exploitation than was previously thought.
"As far as we can see, there is no hidden reserve of sharks in the deep sea. All we see is all there is - it's highly unlikely we are going to find any more."
The scientists based their conclusions on a wide range of data, which includes information gathered during a major expedition along the Mid-Atlantic Ridge between Iceland and the Azores in 2004.
The team also used findings built up over the last two decades when the university's Oceanlab started developing landers - remotely operated vehicles - which have been used in deep waters all over the world.
Expeditions using landers visited deep areas including the South Atlantic off the Falkland Islands.
The shark paper is published in Proceedings of the Royal Society B: Biological Sciences.