Scientists have found that 70% of the world's oceans appear to be free from sharks because the animals cannot live below 3,000 metres (9,000ft), raising fears about their long-term survival.
Being confined to the rest of the ocean leaves sharks at greater risk of coming into contact with humans.
Monty Priede, a marine biologist at the University of Aberdeen, said: "Sharks are apparently confined to around 30% of the world's oceans, and all populations are therefore within reach of human fisheries.
"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."
The average depth of the oceans is 4,000 metres and bony fish - relatives of cod - often live at 9,000 metres. Sharks are present throughout the world and scientists had hoped that they would discover new species deeper down in the oceans' abyss. But a 20-year survey has shown that this hope is unfounded.
"It's a fact that's been staring at us in the face but we've never really expressed it properly, one of the most important things about fish biology in the abyss is that there are no sharks down there," said Professor Priede, whose research was published yesterday in the Proceedings of the Royal Society. "It poses a lot of questions about shark evolution: why the shark, which has had plenty of time in the ocean to evolve, for some reason has failed to colonise the abyss.
"There are deep sea species of shark, for example the lantern shark, which has lights on its belly, specially adapted eyes to suit a deep-sea animal, but yet it does not cope deeper than 2,500 metres. But there are cod-like fish swimming around at 6,000 metres perfectly happy. This, for fundamental biology, is a big conundrum that has to be solved."
Scientists suggest that lack of food could be a reason for the absence of sharks at lower depths.
The discovery also raises concerns for the future of shark populations as in shallower depths they are more likely to come into contact with humans, putting them at increased risk of being over-fished.
The Portuguese dogfish, which can swim down to 3,700 metres , has already been severely reduced in numbers.
Prof Priede said: "There's no doubt they suffer very heavily from fishing, mainly because they produce relatively few young - you can't fish for shark in the way you might fish for herring or mackerel."