The worldwide fishing industry could benefit from a $50 billion boost annually if stocks were allowed time to recover, the UN said Wednesday.

Already 32 percent of the world's fish stocks have been depleted by years of overfishing and poor coastal management, according to a UN Environment Programme report released in the Philippine capital Manila.

"The potential economic gain from reducing fishing capacity to an optimal and restoring fish stocks is in the order of $50 billion per annum," a summary of the UN report said, without giving details on how the figure was reached.

The report said overfishing, pollution from land-based farming and industry, and the destruction of habitat, including coral reefs and mangroves, were all having an effect on fish stocks.

This was directly affecting the 540 people around the world who are dependent on the fishing industry, experts at the launch of the "Green Economy in a Blue World" report said.

Cutting pollution would help fish stocks and fishermen's catches to rebound, Amina Mohammed, deputy executive director of the UN programme, said.

"Many ocean industries and businesses stand to benefit directly from cleaner, more ecologically robust marine ecosystems," she said.

While overfishing reduces fish stocks, pollution from the overuse of fertiliser in farming is also a major problem, she said.

The fertiliser washes into the sea, resulting in runaway growth of algae which sucks up all the oxygen in the waters and causes fish to "drown".

Experts have said there are over 500 oxygen-deprived "dead zones" in waters around the world created in such a way.

Europe could save at least $100 million annually just through improvements in fertiliser use to stop it affecting the oceans, said Linwood Pendleton, an oceans and coast expert from the US's Duke University.

Marine specialist Raphael Lotilla said that as much as 49 percent of all fertiliser used in Philippine farms ended up being washed into the sea.

"Let's work with farmers to figure out what is the right amount of fertiliser so everyone wins," Mohammed said.

The UN report also said marine-based renewable energy sources like wind, wave and tidal power, have huge potential but are not yet cost competitive.

It called for "long-term policies and targeted financial support from governments" such as grants, subsidies and tax credits, to improve the technology and bring costs down.

It also called for more measures to curb destruction of coral reefs, mangroves, seagrass beds and other marine habitats, as well as measures to prevent the spread of "invasive species" carried by ships' hulls.

Such species cause an estimated $100 billion in losses each year, the report said, without giving further details.