In the latest update of its closely watched Red List of threatened species, the environmental group also moved Australia's western ringtail possum by two notches from "vulnerable" to "critically endangered".
The re-evaluation of the status of the two aquatic mammals comes after the numbers of Irrawaddy dolphin more than halved over the past 60 years, and over the past 45 years for the finless porpoise, the group said.
"These species live in shallow waters near shore and both have populations confined to freshwater systems, and that makes them extremely vulnerable to human activities," Craig Hilton-Taylor, head of the group's Red List unit, told reporters.
"In the Mekong River, for example, the majority of Irrawaddy dolphin deaths in recent years have been caused by entanglement in gill nets. These nets hang like curtains of deaths across the river," he said.
As for western ringtail possums, Australia's increasingly dry and hot climate had led to a decline of more than 80 percent in their numbers over the past decade, the group said.
"The threats pushing species to extinction come from us, from humans ... This also means that we can do something about it," Hilton-Taylor said.
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