The Yangtze River and Pearl River estuaries have been listed as newly registered "dead zones," according to a study released by the United Nations Environment Programme (UNEP).

Dead zones, or low oxygenated areas in the world's seas and oceans, are places where nutrients from fertilizer runoff, sewage, animal waste and the burning of fossil fuels trigger algae blooms.

The algae blooms need oxygen and remove it from water, endangering other marine life.

The number and size of deoxygenated areas has risen each decade since the 1970s. Experts warn that these areas are fast becoming major threats to fish stocks, and to people who depend upon fisheries for food and livelihoods.

The major pollutants affecting seawater off China were inorganic nitrogen and active phosphate in 2005, according to a report on pollution of the marine environment released this week by the State Environmental Protection Administration (SEPA).

The report said 500,000 tons of ammonia nitrogen and 30,000 tons of phosphate from land-based activities floated into the sea last year.

Eighty-two "red tides" one type of algal bloom occurred in China's seas in 2005, down 15 per cent from the previous year, according to the UNEP report.

Confirming earlier Chinese research, the report found that large-scale red tides were concentrated in central Zhejiang Province, the Yangtze River estuary, Hangzhou Bay and North China's Bohai Bay .

For example, last June in the Yangtze estuary a red tide influenced more than 1,000 square kilometres of water, resulting in the death of more than 12 million fish. Residents in Shanghai were warned to eat fresh fish as a safety precaution. Tests later confirmed the presence of toxic algae.

"China is making efforts to combat pollution from land-based activities," Zhu Guangyao, vice-minister of SEPA, said yesterday on the sidelines of the second intergovernmental review meeting of the Global Programme of Action for the Protection of the Marine Environment from Land-Based Activities.

With about 80 per cent of pollutants in the sea coming from land-based activities, most notably sewage, China has vowed to increase wastewater treatment in coastal areas from the current 50 per cent to 70 per cent in the next four years.

"UNEP will help China in capacity, technology and funds to protect its marine environment," said Achim Steiner, UN under-secretary-general and UNEP executive director.

UNEP estimated that the number of dead zones worldwide has climbed to 200. A full list will be available in 2007.

As well as the two areas in China, other dead zones were found off Finland, Ghana, Greece, Peru, Portugal and Uruguay, as well as the Western Indian (Ocean) Shelf.