MANILA, Philippines - Rising wealth in Asia and fishing subsidies are among factors driving overexploitation of the world's fish resources, while fish habitat is being destroyed by pollution and climate change, UN marine experts said Tuesday.

Up to 32 per cent of the world's fish stocks are overexploited, depleted or recovering, they warned. Up to half of the world's mangrove forests and a fifth of coral reefs that are fish spawning grounds have been destroyed.

The UN Environment Programme says less-destructive ways of fishing that use more labour and less energy are needed to help restore the health of the world's oceans and coasts.

The agency is leading a five-day conference in Manila of experts and officials from 70 governments.

Jacqueline Alder, head of UNEP's marine, coastal and freshwater office, said the increasing ranks of rich Asians are driving demand for better quality fish that are often not abundant, adding pressure to their supply.

"People don't want to eat the little anchovies anymore when they can eat a nice snapper or grouper -- much nicer fish, shows much more of your wealth," she told reporters.

Alder said booming population, more awareness of health benefits from eating fish, fuel and boat-building subsidies in industrial fisheries, weak management and limited understanding of ecosystems' values are also driving fish overexploitation.

She said subsidies should be reduced or eliminated, fishing gears should be less destructive, and the number of boats and fishers reduced. Habitat management should also be strengthened and marine protected areas established.

Fish is the main source of protein for up to 20 per cent of the of world's population and some 180 million people are directly or indirectly employed by the fishing industry, she added.

Vincent Sweeney, UNEP's co-ordinator for the Global Program of Action to prevent marine environment degradation from land-based pollutants, said up to 90 per cent of sewage in developing countries is discharged untreated into rivers, lakes and oceans, posing one of the most serious threats to water resources.

Other pollutants from land including nitrogen and phosphorous from fertilizers and detergents result in hypoxia or "dead zones" where too many nutrients cause an undesirable growth of plants that compete with coral reef and other marine life for oxygen.

Jerker Tamelander, head of UNEP's coral reef unit, said healthy coral reefs can produce up to 35 tons of fish per square kilometre each year while there is a catch reduction of 67 tons for every square kilometre of clear-cut mangrove forest.

The global market value of marine and coastal resources and industries is estimated at $3 trillion per year or about 5 per cent of the global economy, he said. Non-market value such as climate, water, nutrients and carbon regulation is estimated at $22 trillion a year.

"We've lost a fifth of the world's coral reefs and 60 per cent are under direct and immediate threat and climate change plays an additional role in driving reef loss," he said.

Tamelander said the decline in coastal ecosystems' health and productivity can be reversed by shifting to greener and more sustainable strategies, addressing threats and better management that involves all stakeholders.

"The sooner we act, the easier it will be and the longer we wait the harder it will be," he warned.