Do we really know... how many (asian) dolphins are there?

''Countering their expectations, biologists working in Bangladesh have found a thriving population of 6,000 Irrawaddy dolphins, a species restricted to brackish bays and rivers from southern Asia to northern Australia that marine mammal experts had worried was vulnerable to extinction.
...
Dolphin and porpoise species that have adapted to rivers and deltas around the world have long been considered some of the most vulnerable of marine mammals because of their restricted habitats. In 2007, experts concluded that the baiji, a river dolphin that thrived in the Yangtze River for 20 million years in what is today China, had been driven extinct by a variety of activities by the nearly half billion people now living in that watershed.
'' [source]