The BBC's Richard Wilson: "Action to reduce destructive fishing practices"

The prince will tell the conference, which officially closes the World Wildlife Fund's International Year of the Ocean, 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 will say.

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

Prince Philip: "Prevent irreparable damage"

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 commericially unviable areas.

The prince is likely to endorse the WWF's plan of action, calling for major fishing nations to ratify 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.

Bycatch: A dolphin drowned in fishing nets

He will also press for a reduction in 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 is also likely to express concern about dangerous fishing practices like poisoning and use of explosives and will call for sustainable fishing by removing government subsidies and supporting the work of the recently-formed international watchdog, the Marine Steward Council.

Speaking at the launch of the International Year of the Ocean in December 1997, he said: "It may seem that the problem is simply a matter of reducing the tonnage of fish taken from the wild stocks in the oceans.

"The fact is that, as in all ecosystems, the viability of any one species is dependent on the viability of most - if not all - the others, and on the condition of their habitat.

"Any delay in taking action will amount to passing the buck to governments to prevent irreparable damage being done to one of the world's major renewable sources of food."