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The fourth scientific expedition to survey the population and natural habitat of the critically-endangered Yangtze finless porpoise endemic to China's longest river, has concluded and entered the data collection and analysis stage. The scientists aim to construct a complete picture of the porpoise's status before offering guidance on conservation strategies. According to the World Wildlife Fund (WWF), the Yangtze finless porpoise is the only sub-species in the river. After its better-known cousin, the baiji dolphin, was declared "functionally extinct" in the same waters in 2007, experts believe the finless porpoise is the Yangtze's last surviving mammal.