BBC Environment Correspondent Richard Wilson's exclusive interview with Prince Philip

Speaking in London at a conference to close the World Wildlife Fund's International Year of the Ocean, the prince said that 60% of the world's 200 most valuable fish species are already overfished.

Even stocks of relatively common fish like cod are at the point of collapse in some areas of the world and may never recover, he said.

The BBC's Margaret Gilmore: "Prince Philip called on people worldwide to force politicians to act"

Prince Philip is president emeritus of the WWF and is a vocal supporter of its policy on oceans.

In an exclusive interview with BBC News after the conference, the prince said: "The catch is declining despite an increased effort. A lot of the commercial species are becoming at least commercially extinct.

"If you have an elastic band you can pull it and it seems perfectly all right, but at some point it's going to snap. We're pulling, in a sense, the resources of the ocean."

Prince Philip: "Conserve locally"

Fish stocks are widely held to be declining due to pollution, massive "bycatch" (marine species caught accidentally in net fishing), unregulated exploitation of the open seas and government subsidies which support the fishing of otherwise commercially unviable areas.

The prince used his conference address to endorse the WWF's plan of action, which seeks to pressure major fishing nations into ratifying international agreements which they have signed.

Chief among these is the recent United Nations Fish Stocks Agreement, which remains unratified by most of its major signatories, including the UK and the European Commission.

Bycatch: A dolphin drowned in fishing nets

This includes reducing bycatch - running to 20 million tonnes a year - which is believed to be having a devastating impact on populations of dolphins, sharks, turtles and juvenile fish.

None of these have any commercial value and are often thrown back into the sea dead.

Prince Philip also expressed concern about dangerous fishing practices like poisoning and use of explosives and called for sustainable fishing by removing government subsidies and supporting the work of the recently-formed international watchdog, the Marine Steward Council.

"Subsidies only work if they are applied very intelligently, which at the moment they are not," he told the BBC.

He suggested an "international enforcement agency" to enforce sustainable fishing practices.

"The living ecosystems are either in the oceans or in the forests, because that's where the reservoir is. Those obviously have a priority for conservation," he said.