An underwater famine is posing the latest threat to one of Australia's most endangered marine species, the dugong, which lives entirely on sea grass. At least 100 have starved to death in recent months and many more are likely to follow in the absence of their only food source.

Torrential rain and storms, including Cyclone Yasi earlier this year, have destroyed vast swathes of sea grass from northern Queensland to the New South Wales border. More than 1,000 miles of coastline which once provided the perfect habitat for these oddly shaped and gentle creatures are now denuded of the dugong's natural foodstuff.

Known as sea cows because of their total dependency on sea grass, numbers have plummeted over the past decade as they struggle to cope with extreme weather conditions, escalating industrial activity, and hunting by indigenous fishermen. Turtles, too, have fallen victim to the seagrass famine with several hundred reported washed up dead along the coastline.

"This is a national environmental disaster," says Professor Ellen Ariel, a marine biologist at James Cook University in Townsville. "What's happening now is they have nothing to eat and it's not going to change in any way soon. Sea grass takes between two to three years to recover, if there are no other extreme weather events in the meantime."

The Great Barrier Reef Marine Park Authority is similarly concerned, recently launching a campaign to protect dugong and green turtles which it predicts will die in record numbers. Forced to stray from their regular foraging areas in search of food, the two species are much more vulnerable to disease, injury and death. A major industrial development at Gladstone on the mid-Queensland coast is also increasing pressure on the marine habitat.

A multi-billion pound gas processing plant on the edge of the Great Barrier Reef has already attracted criticism. Last month Unesco's world heritage committee expressed its extreme concern at the Queensland and federal government's backing of the project. For her part, Queensland Premier Anna Bligh has pledged to make a comprehensive assessment of the plant's environmental impact.

In addition to climatic and industrial threats to the dugong population, indigenous fishermen have also been accused of endangering the species. Next month a television campaign will be launched by animal activists who believe Australia's Native Title laws are allowing the "uncontrolled" and "unmonitored slaughter" of dugongs and turtles. Australians For Animals has accused some aboriginal groups of "appalling cruelty".

Campaign organiser Colin Riddell says: "We have a confirmed report of a dugong calf being tied to the back of a boat, its cries bringing in the mother so they can both be killed. We have reports in our office of indigenous groups going out in motor boats with a GPS to find dugongs. Once found, they radio their mates and entire pods of dugongs are slaughtered."

Dugong hunting has been an accepted part of Australia's indigenous culture for thousands of years. Their ivory and bones are used in traditional crafts and their meat, which is said to be similar to high quality beef, is regarded as a delicacy. The Native Title Act allows dugongs to be caught by aborigines for personal, domestic or non-commercial needs, but, according to Mr Riddell, some are being sold for profit. He claims the meat sells for nearly £100 a kilo and is even being exported.

Now he is urging the government to call a moratorium on dugong hunting until population numbers are established. "I don't have a problem with Native Title hunting if it's done sustainably," he insists. "But let's just see how many are left."