Deep-sea trawling in international waters has depleted the world's fish stocks, threatening many species with extinction and radically altering undersea habitats.

While 12% of the Earth's land surface enjoys some form of environmental protection, the same is true for only 0.5% of the world's oceans.

Roger McManus, senior director of Conservation International's global marine programme, said on Monday: "The alternative is you pay now or you pay later. Either you restrict the amount of fish taken from the oceans ensuring stocks for humans in the future or you continue to mine them and stocks are destroyed."

The problem, McManus has said, is that most of the ocean's waters lie beyond the 322km that countries claim as their sovereign territory.

He said: "Early in history, there was a discussion of how to divide up the oceans and the idea that prevailed was that everyone would have rights of passage. In some sense that's been misinterpreted as rights of use and abuse."

Sustainable management

Environmental groups are proposing that governments join together to declare protected areas, where fisheries would be managed in a sustainable manner.

Last week, the environmental group Greenpeace presented delegates at the eighth biannual Conference of Parties to the Convention on Biodiversity in Curitiba, 650km southwest of Rio de Janeiro, with maps showing which areas of the high seas most warranted protection.

Marcelo Furtado, from Greenpeace, said: "At prior meetings, governments agreed to protect 40% of the world's oceans by 2010 and so far they've done nothing. We're presenting them with a concrete plan. If you want to do this, this is how it could be done."

Greenpeace was calling for a moratorium on all deep-sea fishing until a sustainable management plan could be worked out.

McManus said he believed the idea of protecting the world's oceans is one that will eventually take hold.

He said: "These management regimes provide a mechanism to consider how to go about using this resource, without destroying features and wonderful aspects of ocean we haven't even discovered yet."