One of the world's largest freshwater fish is on the verge of going extinct, BBC reported.
A three-year quest to find the giant Chinese paddlefish in the Yangtze river failed to sight or catch a single individual.
That means that the fish, which can grow up to 7m long, has not been seen alive for at least six years.
There remains a chance that some escaped the survey and survive, say experts, but without action, the future of the species is bleak.
The concern for the Chinese paddlefish is that its fate will parallel that of the Yangtze river dolphin, a large mammal species that was once abundant in the Yangtze river system, but has recently been declared extinct.
A number of fish species vie for the position of the world's largest freshwater fish, including the arapaima (Arapaima gigas) of the Amazon river and the Mekong giant catfish (Pangasianodon gigas).
At up to 7m, the Chinese paddlefish (Psephurus gladius) is much longer than either, though it may not exclusively live in freshwater.
The fish is suspected to be anadromous, meaning it spends some of its life in marine waters before returning to the river to spawn. But it is so rare that little is known about its behaviour, life history, migration habits and population structure.
It is endemic to the Yangtze river system in China.
"It has special characteristics such as its sword-like rostrum. Some people call it the 'elephant fish' and we found out it swims on the surface of the water like a whale," says Professor Wei Qiwei, one of the leaders of the research team from the Chinese Academy of Fisheries Science in Jingzhou, China.
The last confirmed sighting of a Chinese paddlefish was made in the river on 24 January 2003.
Now scientists have published in the Journal of Applied Ichthyology the results of a three-year survey to find and locate the fish.
Professor Wei and colleagues surveyed the upper Yangtze river between Xinshi, Sichaun Province and Chongquing, covering a distance of 488.5km.
Most of this stretch of water lies within the Upper Yangtze National Nature Reserve, a protected area.
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