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 about mermaids, has been spotted off China’s southern coast for centuries.
Not lately though. A new study shows that the dugong has become the first large vertebrate to become 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 still live off the coast of China, their numbers are too small to maintain a viable population. Dugongs occasionally become entangled in fishing nets, and the sea grass they eat in the northern reaches of the South China Sea has degraded over the years.
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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 facing great pressure from overfishing, coastal development and other pressures.
About 100,000 dugongs still live in waters in about 40 countries, but the latest findings do not bode well for other populations of the animal in Japan and Southeast Asia, said Helene Marsh, associate professor of environmental science at James Cook University in Queensland, Australia.
“It’s a sad story and a saving story,” Marsh, who has studied the dugong for decades, said by phone Friday. “I don’t think it will be the last place where people can conclude that dugogs are functionally extinct.”
Marine mammals, including dugongs, first evolved in the Eocene Epoch, about 54 million to 34 million years ago, particularly in the vast shallow sea that stretches from the Pacific to what is now the Mediterranean, according to research by Annalisa Berta, professor emeritus of biology at San Diego State University.
Dugongs belong to 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 only fully vegetarian marine mammal in the world.
Over the centuries, sirens have inspired stories about mermaids and other mystical creatures. Marsh said many of the legends are based on the idea that sirens, like the sirens in Homer’s Odyssey, lure sailors with magical properties. (The shallow, prehistoric body of water where dugongs grew, the Tethys Sea, is named after a sea goddess from Greek mythology.)
Dugongs once had an even larger range that extended into the Western Atlantic and the Caribbean, and there used to be several species of dugong. One of them, the Steller’s sea cow, was a food source for 18th-century hunters and explorers in the North Pacific, but was declared extinct 27 years after its discovery in 1768.
Australia has the largest population of dugongs in the world today, largely thanks to a vast, sparsely populated coast surrounded by abundant seagrass, Marsh said. The population off southern China was never particularly robust because there wasn’t that much seagrass there to begin with, he said.
Now the dugong may be gone forever from China’s shores, 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 occur in a very short time.”
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 coastline of southern China. Only three reported dugong sightings in the past 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. In contrast, there were no verified field sightings of dugongs in China after the year 2000.
It is possible that a “remnant” dugong population has survived off the country’s southern coast, but the ongoing degradation of seagrass and other coastal resources in the South China Sea makes the prospect of 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 the study was limited because it relied heavily on interviews, in which sources could potentially misidentify a marine mammal. “Certainly it is better to accompany it with available direct evidence of the absence of the species,” he said.
But Marsh, a co-chair of the International Union for Conservation of Nature’s Sirenian Specialist Group, said she sees no problems with the study’s methodology. “There may be a very small number of animals there that no one has seen for a while. that is very likely,” he said. “But I think their conclusions are quite strong.”
In 2015, the group classified dugongs as vulnerable to extinction, but the latest study highlights how population health varies from region to region, said Gabriel Grimsditch, program manager for dugongs at the United Nations Environment Programme.
Relatively healthy populations can still be found in Australia and the Persian Gulf, Grimsditch wrote in an email from Abu Dhabi. “However, outside of these two areas, most dugong populations are in dangerous decline, and the study from China highlights the threat of extinction they face across their entire range,” he said.
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