The world's oceans are awash in riches, with output rivalling that of some of the world's largest economies, but over-fishing, pollution and climate change are rapidly eroding those resources, WWF warned Thursday.

In a new report, the conservation group said oceans each year generate goods and services worth at least $2.5 trillion, while their overall value as an asset is worth 10 times that.

If oceans were a nation, they would constitute the world's seventh largest economy, ranking just after Britain but ahead of the likes of Brazil, Russia and India, WWF said.

Their estimated asset value of $24 trillion would meanwhile dwarf the world's largest sovereign wealth fund, in Norway, which holds just $893 billion, it said.

The 60-page report, "Reviving the Ocean Economy", meanwhile stressed that those estimates were clearly an underestimate, since they do not include offshore oil and gas and wind energy or "intangibles such as the ocean's role in climate regulation."

"The ocean rivals the wealth of the world's richest countries, but it is being allowed to sink to the depths of a failed economy," WWF chief Marco Lambertini warned in a statement.

Thursday's report, produced in cooperation with Queensland University's Global Change Institute and the Boston Consulting Group, indicated that oceans are changing more rapidly today than at any other point in millions of years.

- 'Serious signs of failing health' -

Collapsing fisheries, mangrove deforestation and disappearing corals and seagrass are among the changes that are threatening the livelihoods of hundreds of millions of people.

A full two thirds of the economic value generated by oceans depends on healthy ocean conditions, the report said.

But unfortunately oceans are "showing serious signs of failing health," it said.

The statistics are frightening -- some 90 percent of global fish stocks are already over-exploited or fully exploited, and marine species declined by 39 percent between 1970 and 2010, it said.

Global warming could meanwhile prove to be a death sentence for delicate coral reefs, containing some of the most diverse ecosystems on the planet.

Already, half of all corals have vanished, and they are all expected to be gone by 2050 if temperatures continue to rise at the same rate, the report said.

Mangroves, considered vital to coral reefs and fisheries, and which serve as important buffers for coastal cities against rising sea levels and storm surges, are meanwhile facing deforestation rates three to five times higher than the average global forest loss, it said.

All is not lost however.

Thursday's report calls on the international community to take urgent action, listing eight measures that could help reverse the trends.

Among other things, ocean recovery should figure prominently in the United Nations sustainable development goals, to be decided on later this year, and action must be taken to address ocean warming and acidification, it said.

It also called for at least 10 percent of coastal and marine areas to be protected and effectively managed by 2020, and 30 percent by 2030.

These actions "can provide a sustainable future for the hundreds of millions of people who depend directly on the ocean for their food and jobs, and for all humanity, which depends on the ocean as an essential contributor to the health of the planet," it said.