In the rivers of the Peruvian Amazon, manatees wallow in the murky water, making them very hard to spot. If you're lucky, says British ecologist Kat Bruce, you might see their nostrils poking out above the surface.

A few years ago, she joined conservationists from green group WWF who were traveling a 1,000-kilometer stretch of river to study manatees and other species.

"You basically can't survey them apart from by going to communities and asking if they've seen any manatees recently," said Bruce, who in 2014 set up a monitoring company called NatureMetrics.