Orcas may be to blame for some mass dolphin strandings

Hundreds of dolphins stranded in Argentina appear to have become trapped while fleeing hungry orcas in a tragic lose-lose situation.

Videos shared on social media and citizen science platforms have helped scientists unravel the mystery of two deadly mass strandings in recent years, says Magdalena Arias of Argentina’s National Council for Scientific and Technical Research.

“This does not mean that predators always cause mass strandings, but it does suggest that predator-prey interactions can sometimes trigger these events, especially when combined with factors such as coastal geography, tides and the strong social behavior of dolphins,” Arias says.

Marine mammals have been stuck in mass die-off events for millions of years, but scientists still debate why this happens. Possible explanations include human disturbance, toxic algae, infectious diseases, disorientation, overpopulation, natural disasters, and group hunting efforts that go in the wrong direction.

Some research teams have suggested that the stranded groups may have been fleeing predators. But this is difficult to document, because the hunts cover large spaces and last a long time. “Often, researchers only see the end result: a pod of dolphins washed up on the beach that otherwise appear healthy,” she says.

Over the past five years, common dolphins (Delphinus delphis) found themselves stranded three times in northern Patagonia – something that had never happened before – prompting Arias and his colleagues to investigate.

The researchers analyzed video footage from drone cameras and cellphones uploaded to the citizen science platform eWHALE and social media sites by tourists, guides, fishermen and local residents.

They saw that in 2021, around 350 dolphins were speeding toward San Antonio Bay in Rio Negro province, Argentina, with a pod of eight orcas (Orca Orcinus) about half an hour behind them. At the mouth of the bay, some dolphins headed into the shallow harbor and lay low and still as if hiding, while the orcas turned back toward the sea. The next day, dozens of dolphins were found dead in the harbor.

Similarly, in 2023, around 570 dolphins raced towards the bay at high speed, followed by a pod of orcas. Some dolphins rushed into the harbor, but local authorities and volunteers managed to rescue them after the orcas turned away.

Necropsies of 38 animals that died during the 2021 event showed that they were in good physical condition and had no significant illnesses or injuries. Their stomachs contained no recent meals, suggesting they were not pursuing prey. “This makes some of the most common explanations for strandings less likely,” Arias says.

Scientific and lay reports on orca sightings over the past few years helped the team build a timeline map of their presence in the region and revealed two confirmed reports of orcas hunting and killing common dolphins.

The dolphins likely sought refuge in shallow areas that interfered with the orcas’ echolocation and movements, but then ended up getting stuck between sandbars and tidal channels, Arias says.

Orcas, on the other hand, may deliberately chase dolphins toward bays in an attempt to corner them. In fact, two of the 2021 orcas have been observed repeatedly using coastal features to trap sea lions, she says.

The prolonged stress and disorientation caused by the chase could have made the dolphins less likely to find their way back to the sea. Such cases likely occur all over the world and simply haven’t been documented yet, Arias says.

“This study highlights the important contribution that citizen science can make to research and how understanding these processes not only helps us explain mass strandings, but also better understand how marine ecosystems change – and how species respond to these dynamics,” she says.