By Alex Kirby BBC News Online environment correspondent

Many turtle species are in trouble

The gift, by an anonymous donor, is conditional on CI raising $4m in matching funds.

It was announced at a conference in Mexico organised by CI and similar groups to plan an oceans campaign.

The conference, on the theme Defying Ocean's End, agreed on an ambitious plan of action.

It brought together more than 100 marine experts from 20 countries, for what is planned as a science-based international initiative.

Recommendations included:

• setting up a World Ocean Public Trust, to manage the 60% of the oceans which lie in international waters and are at present open to uncontrolled exploitation
• expanding the global system of marine parks, to give formal protection to critical areas like submarine mountains. Less than 1% of the world's oceans are fully protected
• assessing global priorities, to get a clear idea which species are most vulnerable to extinction
• starting an educational campaign "to shatter myths about the ocean's limitless ability to withstand human neglect and abuse".

Some whales are barely recovering

She said: "The health of humankind is directly related to the health of the ocean - and the ocean and the marine life that calls it home is in real trouble.

"We couldn't afford yet another meeting where we just sat around and created a wish list, so we formed Defying Ocean's End to take unprecedented and bold steps forward."

CI says the world's oceans and the marine life they harbour are collapsing. A recent study said 90% of large, predatory fish, including tuna and marlin, had disappeared, mostly because of over-fishing and destructive fishing methods.

No despair

Other threats, including coastal development, pollution and climate change, were also devastating marine life.

The other co-convenor of the conference, Gordon Moore, said: "It's stunning to consider that in the past few decades, we have done away with the vast majority of large fish in the ocean and significantly altered the way marine systems operate.

Overfishing is causing havoc

"By using sound science and implementing an achievable action plan, we still have a small window of opportunity to reverse these trends."

Graeme Kelleher, who chaired the conference, said: "The world's ocean is the last living frontier on Earth. Its diversity and productivity exceed that of any on land, but have barely been explored.

"We have an opportunity and obligation now to protect the ocean for the future welfare of humans, other animals, and marine plants... Prevention now is better than scrambling for a cure later."