Dolphins have stranded in droves on the shores of Patagonia. Researchers believe they have found the culprit

Dolphins have stranded in droves on the shores of Patagonia. Researchers believe they have found the culprit

By Jackie Flynn Mogensen edited by Claire Cameron

A dolphin murder mystery is unfolding on Patagonia’s shores: several mass strandings, with many dolphins washing up for no apparent reason. In one such incident in 2021, for example, 52 dead dolphins turned up in San Antonio Bay, off the coast of Río Negro, Argentina – but apart from being dead, the animals appeared to have been in good health with no visible wounds or signs of illness. Then, about a year and a half later, several hundred dolphins stranded in shallow water in the same area, although thankfully no deaths were reported.

The reason was a mystery. But now scientists have identified a compelling suspect: killer whales. In a new study published in the journal The Royal Societyshow researchers how the presence of killer whales nearby in both cases may have spooked the dolphins, causing them to flee into San Antonio Bay’s dangerously shallow waters.

Dolphin strandings can be triggered by myriad issues, from changing tides to prey behavior — but the study provides “new evidence” that predators may also play a role, the authors write.

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“This study provides, for the first time, evidence to support the hypothesis that such coastal encroachment can be triggered by stress induced by the presence of predators, particularly killer whales, in the area,” write the authors. Killer whales are known to hunt dolphins (although there is evidence that some killer whales team up with other dolphin species to hunt as well).

To arrive at their findings, the researchers relied on interviews with local residents and fishermen, as well as video footage, to piece together the timeline of events that led to the mass strandings. “In both events, dolphins showed atypical coastal movements, high cohesion and disorientation shortly before orcas were sighted,” the authors write.

The results may help explain other mysterious mass stranding events at known hotspots with similar geographies such as New Zealand, Australia and Massachusetts, which often go unexplained.

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