The UN has warned the world's governments that humankind's exploitation of the sea could be passing the point of no return.

A report by the United Nations Environment Programme (UNEP) found that more than half of the world's fish stocks are being exploited to their full extent, with nearly a quarter suffering from over-fishing.

Meanwhile, pollution, litter and deep sea drilling are all reaching into the depths of a marine environment hitherto preserved from the hand of man.

In the Central Pacific, the study found, there is now up to 6lb of marine litter to every 1lb of plankton. Elsewhere, there are around 46,000 pieces of plastic litter for every square mile of the world's oceans.

Achim Steiner, UNEP’s new executive director, said that particular attention must be paid to the 60 per cent of the world's oceans that are beyond the reach of national jurisdictions and conservation efforts, where modern technology and a lack of regulation is combining to harm the environment.

"Humankind's ability to exploit the deep oceans and high seas has accelerated rapidly over recent years," he said.

"It is a pace of change that has outstripped our institutions and conservation efforts whose primary focus have been coastal waters where, until recently, most human activity like fishing and industrial exploration took place. We now most urgently need to look beyond the horizon and bring the lessons learnt in coastal water to the wider marine world."

According to the report, just 1 per cent of the world's 3.5 million fishing boats are thought to be large, industrial vessels, but the giant loads they trawl from the deep sea account for around 60 per cent of all the fish caught on the planet.

Industrial fishing has helped to drive down the world's stocks of tuna, cod, swordfish and marlin by as much as 90 per cent in the last century.

Adding to the strain on the oceans, the UN estimated that nearly $10 billion of fish are caught illegally each year, up to 30 per cent of which is taken from unregulated waters.

Illegal longline fishing also kills more than 300,000 seabirds every year, including 100,000 albatrosses. Nineteen out 21 albatross species are now threatened with extinction.

The report also described a range of activities, including energy development and scientific research such as "bioprospecting" - the collection of biological artefacts for new products - that encroach on waters up to 2,000m below the ocean surface.

"Throughout the oceans, shipping, military operations and seismic exploration have intensified with growing impacts on deep water and high sea ecosystems and biodiversity," said Kristina Gjerde, a UN High Seas Policy Advisor who wrote the report.

"The spectre of climate change and its impacts such as ocean warming and acidification underscore the need to reduce direct human impacts, because healthy ecosystems are better able to respond to changing oceanic conditions."

Reacting to the report, Tim Yeo, the Conservative MP who is chairing a study of the Government's plans for a Marine Bill, said that Britain should take the lead in devising a new regulatory framework for the world's open waters.

"For generations we have regarded the sea as a resource we can all deplete at will. Those chickens are now coming home to roost," he said.

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