By Alex Kirby BBC News Online environment correspondent

Scientists say the sea mammal which inspired the ancient belief in mermaids is under serious threat.

They say the animal, the dugong, is declining across most of its range.

Dugongs are regarded as key indicators of the health of their environment, highlighting threats to other species and habitats.

They are being affected by human pressures, including pollution and the loss of their food source.

Scattered strongholds

Details of the mammals' plight appear in The Dugong: Status Report and Action Plans for Countries and Territories in its Range.

The report's funders include the United Nations Environment Programme (Unep), the World Conservation Union and the CRC Reef Research Centre.

It was released at Unep's governing council, which is meeting in the Colombian city of Cartagena.

The report's lead author is Helene Marsh, professor of environmental science at James Cook University in Townsville, Australia.

Females usually have only one calf

She said: "Dugongs appear to have vanished or already become extinct in some places.

"These include the waters off Mauritius, the Seychelles, western Sri Lanka, the Maldives, parts of Japan, Hong Kong's Pearl River estuary, several islands in the Philippines, and parts of Cambodia and Vietnam.

"Elsewhere populations appear to be declining, with the possible exception of northern Australian waters and those of the Red Sea area and Arabian Gulf.

"The situation in East Africa is particularly alarming and it is possible that this will be the next place where the dugong becomes extinct unless urgent action is taken."

Scientists believe Africa's dugongs are far more endangered than its elephants.

The report says growing pollution from land sources, coastal development, boat traffic and fishing nets are the dugongs' main worries. They are also hunted for meat, amulets and trophies.

Grazing in peril

Nor do the animals do much to help themselves. Females seldom produce more than one calf in a lifetime, and do not give birth at all when food is short.

Dr Tim Foresman of Unep said: "Even under a perfect, pressure-free and pollution-free environment, a dugong population is unlikely to grow at much more than 5% a year.

Dugongs reflect the health of their habitat

"Even a slight reduction in the survival of the adults, as a result of habitat loss, disease, hunting or incidental drowning in nets, can cause a chronic decline."

Dugongs, which are herbivorous, depend almost entirely for food on beds of seagrass.

These need sunlight, but in many areas they are being smothered by silt or mud, or harmed by herbicides used on land.

Fast movers

The report wants more co-operation between countries with dugong populations, especially as they are turning out to be more mobile than anyone had realised.

Scientists had thought they stayed put in one area, but the report says they have been found to swim up to 370 miles (600 km) in a few days.

The dugong, known in the Caribbean as the manatee, was thought by sailors in antiquity to be a mermaid, half-woman and half-fish.

The legend perhaps stems from the females' habit of holding their calves with one flipper when suckling them, and diving suddenly when disturbed, flipping their fish-like tails.

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