The dugong, a species of so-called sea cow that roams the ocean floor in Asia and Africa and is said to have inspired ancient legends of mermaids, has been spotted off China’s southern coast for centuries.
Not lately, though. A new study suggests that the dugong has become the first large vertebrate to go functionally extinct in China’s coastal waters, the result of a rapid population collapse there that began in the mid-1970s.
“Functional extinction” means that even if some dugongs are still alive off China’s coast, their numbers are too small to maintain a viable population. Dugongs are occasionally entangled in fishing nets, and the seagrass that they eat in the South China Sea’s northern reaches has degraded over the years.
The study in the journal Royal Society Open Science, based on interviews with nearly 800 fishermen in southern China and published online this week, is a cautionary tale for other mammals in the South China Sea — a site of stunning marine biodiversity that faces heavy pressure from overfishing, coastal development and other stresses.
There are still approximately 100,000 dugongs living in the waters off around 40 countries, but the latest findings do not bode well for other populations of the animal in Japan and Southeast Asia, said Helene Marsh, an emeritus professor of environmental science at James Cook University in Queensland, Australia.
“It’s a sad story and a salutary story,” Professor Marsh, who has studied the dugong for decades, said by phone on Friday. “I don’t think it’s going to be the last place where people can conclude that dugongs are functionally extinct.”
A ‘sobering reminder’
Marine mammals, including dugongs, first evolved in the Eocene Epoch, around 54 million to 34 million years ago, particularly in the extensive shallow sea stretching from the Pacific to the present-day Mediterranean, according to research by Annalisa Berta, a professor emerita of biology at San Diego State University.
Dugongs belong to the Sirenia, a biological order that includes the dugong and three extant species of manatees. All four are known colloquially as “sea cows,” but the dugong is the only one that lives exclusively in salt water. It is also the world’s only fully vegetarian marine mammal.
Over the centuries, sirenians have inspired tales of mermaids and other mystical creatures. Ms. Marsh said many of the legends are based around the idea that sirenians, like the sirens of Homer’s “Odyssey,” lure sailors with magical properties. (The shallow, prehistoric body of water where dugongs developed, the Tethys Sea, was named after a sea goddess from Greek mythology.)
Dugongs once had an even larger range that stretched to the Western Atlantic and Caribbean, and there used to be several dugong species. One of them, Steller’s sea cow, was a source of food for 18th-century hunters and explorers in the North Pacific, but it was declared extinct 27 years after its discovery in 1768.
Australia has the world’s largest population of dugongs today, primarily thanks to an enormous, sparsely populated coast ringed with abundant seagrass, Professor Marsh said. The population off southern China was never particularly robust because there wasn’t as much seagrass there in the first place, she added.
Now the dugong may be gone from China’s coast forever, joining the Yangtze River dolphin and other species that have disappeared from the country’s rivers and seas. The recent study described the rapid collapse of China’s dugong population since 1975 as a “sobering reminder that local extinction can happen within a very short time.”
A ‘dangerous decline’
The study, by researchers at institutions in China, Greece and Britain, is based on interviews conducted in the summer of 2019 with 788 fishermen along the southern China coastline. Only three reported dugong sightings over the preceding five years.
The authors also analyzed dugong records for China, which showed that 257 dugongs had been hunted there for food between 1958 and 1976. By contrast, there were no verified field observations of dugongs in China after the year 2000.
It is possible that a “remnant” population of dugongs has survived off the country’s southern coast, but continuing degradation of seagrass and other coastal resources in the South China Sea make the prospect of a population recovery unlikely, the authors wrote.
One of the authors, Songhai Li at the Institute of Deep-sea Science and Engineering in China, referred questions to Samuel Turvey, a co-author based at the Zoological Society of London. He did not respond to a request for comment.
Harris Heng Wei Khang, a marine ecologist in Malaysia who has studied dugongs, said that the study was limited because it relied to a large extent on interviews, in which sources could potentially misidentify a marine mammal. “It’s certainly better to accompany it with available direct evidence of the species’ absence,” he said.
But Professor Marsh, a co-chairwoman of the International Union for Conservation of Nature’s Sirenian Specialist Group, said she saw no problems with the study’s methodology. “There may be a very low number of animals there that nobody’s seen for a while; that’s quite possible,” she said. “But I think their conclusions are quite robust.”
In 2015, the group classified dugongs as vulnerable to extinction, but the latest study highlights how the health of the population varies from region to region, said Gabriel Grimsditch, the program management officer for dugongs at the United Nations Environment Program.
Relatively healthy populations can still be found in Australia and the Persian Gulf, Mr. Grimsditch wrote in an email from Abu Dhabi. “However, outside of these two regions most dugong populations are in dangerous decline, and the study from China highlights the extinction threat they face across their range,” he said.