Flora and Fauna :: What made 30 dolphins come ashore in Brazil?

It was just another day on the beach for holidaymakers off the coast of Rio de Janeiro, Brazil, when a pod of about 30 dolphins swam ashore on Monday. Beachgoers quickly came to the rescue, rushing into the sea and dragging the dolphins by their fins and tails into deeper water.

The whole episode was captured by Gerd Traue in a video (above), which has racked up over a million views online.

What can experts learn from the footage? The species involved, for one. These are common dolphins (Delphinus delphis), which typically live a long way off shore, says Mark Simmonds of the Whale and Dolphin Conservation Society, a global charity.

However the video does not reveal what caused the stranding – fishing boats or sonar are two possibilities.

Had experts rescued the dolphins, says Simmonds, they may have examined the individuals for damage, such as net marks, that may have provided clues. But he says the dolphins in the video appear to be healthy.

Out of their depth

The topography of the coastline may have disoriented the dolphins, says Michael Moore of the Wood's Hole Oceanographic Institution in Massachusetts. It would not be surprising if offshore dolphins like these had trouble navigating the sandbars and silty seabeds found in shallow waters.

Nor is it surprising that such a large number of dolphins would head for land together. Dolphins are social creatures, so it would take only one member of the pod to go astray – say, if it was diseased – and the others would follow.

This social behaviour is what makes mass strandings of cetaceans so common. In the last month, for example, 179 dolphins have reportedly been stranded in Cape Cod, Massachusetts.

Call in the experts

Understanding what's going on in such cases is especially hard because scientists must rely largely on postmortem evidence. Moore and others are trying to develop early warning systems to get to the dolphins while they're still alive.

Sensors deployed in regions where strandings are common, for example, could detect the sounds of the cetaceans nearing shore and send a text message warning officials of an imminent stranding.

The Brazilian video has widely been greeted as a good news story. But Simmonds is uneasy about this.

"There's a clock ticking, so it's important to respond quickly, but it's also important to move them in the right ways," he says. Typically it's best not to touch the animals and to call in experts instead, he says. "Pulling their flippers can dislocate their bones, or even pull a flipper right off."