"The report shows that worldwide, manta and mobula rays are declining, particularly in India, Sri Lanka and Indonesia," says Frazer McGregor from Murdoch University's field station in the Ningaloo Marine Park, Western Australia. "Their gill rakers are dried and boiled as a health tonic, primarily for traditional Chinese medicine."

Each year large numbers of Manta birostris and M. alfredi, the world's biggest ray species with wingspans of up to eight metres, visit the marine reserve as part of seasonal migrations.

These migrations cover hundreds of kilometres, and may follow a similar path to Ningaloo's whale sharks, into Indonesian waters and beyond.

McGregor investigated the population dynamics of Ningaloo's rays as part of his PhD research that also examined threats to their survival, and contributed his findings to the report.

"Manta rays are filter feeders, and during daylight hours they tend to accumulate in large numbers around 'cleaning stations' where small fish species clean them of any parasites and algal growth," he explains. "This habit, combined with their large size, makes them highly vulnerable to being harpooned from above, or caught in fishing nets."

"While they are protected from fishing in the waters of Ningaloo, once they leave the Marine Park, they are at risk of being targeted by fishermen - even in Australian waters where they have no protection."

Slow growers

Professor Mike Bennett, a ray researcher from the University of Queensland says the report raises important issues about manta rays worldwide.

"It shows that as shark numbers have declined through overfishing, pressure on rays has increased as fishermen look around for other things to catch," he says.

"Manta ray fisheries are unlikely to be sustainable; they have very conservative life histories, reaching maturity at about ten years of age and producing a single pup every two to three years."

By comparison, a great white shark, which is widely considered to be one of the world's most vulnerable marine species, may produce as many pups in one litter as a manta ray does over its entire lifetime.

"Where females are being slaughtered, it will take a long time for their populations to recover, if ever, should fishing stop," he says, adding that hunting manta rays provides short-term benefits to coastal communities.

"While the value of a catch may be relatively high to a subsistence fisherman, it can only be killed once."

"If it is left alone, its value may be recouped over and over through the growing international interest in manta ray tourism."

Recognising their decline

Manta rays' gentle nature and predictable habits are one of the main attractions that draw more than 12,000 visitors to Ningaloo each year.

However, McGregor believes that more needs to be done to regulate the burgeoning industry.

"Even within their safe zones manta rays are facing increasing pressure from unregulated tourism," he says.

"We've been trying to get a code of conduct introduced for the tourism industry so there are strict guidelines about how to behave when interacting with them."

According to McGregor, the International Union for Conservation of Nature recently updated the status of manta rays from threatened to vulnerable due to overfishing.

"This fact needs to be acknowledged by state and federal governments, and see them given appropriate Australian protection before the next listing updates their status as endangered."