Dolphins stranded in shallow water in San Antonio Bay, Argentina
Hundreds of stranded dolphins in Argentina appear to have been caught escaping from hungry killer whales in a tragic lose-lose situation.
Videos shared on social media and citizen science platforms have helped researchers solve the mystery behind two deadly mass stranding events in recent years, says Magdalena Arias of the National Scientific and Technical Research Council of Argentina.
“This does not mean that predators are always the cause of 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,” says Arias.
Marine mammals have been stranded in mass die-offs for millions of years, but scientists still debate why they happen. Possible explanations include human disturbance, toxic algae, infectious diseases, disorientation, overpopulation, natural disasters, and group hunting that takes a wrong turn.
Some research teams have suggested that stranded pods may have been fleeing predators. But it is difficult to document, since hunting covers large areas and lasts a long time. “Often researchers only see the end result: a group of dolphins stranded on the beach that otherwise appear healthy,” she says.
Over the past five years, common dolphins (Delphinus delphis) has been stranded three times in northern Patagonia – something that had not happened before – prompting Arias and her colleagues to investigate.
The researchers analyzed video footage from drone cameras and mobile phones uploaded to the eWHALE citizen science platform and social media by tourists, guides, fishermen and local residents.
They saw that in 2021, about 350 dolphins headed for San Antonio Bay in Rio Negro Province, Argentina, with a pod of eight killer whales (Orcinus orca) about half an hour behind them. At the mouth of the bay some dolphins headed into the shallow harbor and lay low and quiet as if in hiding, while the killer whales U-turned back into 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 group of killer whales. Some of the dolphins rushed into the harbor but local authorities and volunteers successfully rescued them after the killer whales had turned away.
Autopsies of 38 dead animals from the 2021 incident showed that they were in good body condition and had no significant disease or injuries. Their stomachs contained no recent meals – suggesting they weren’t hunting prey. “This makes some of the most common explanations for strandings less likely,” says Arias.
Scientific and recreational reports of killer whale sightings in recent years helped the team establish a chronological map of their presence in the area and revealed two confirmed reports of killer whales hunting and killing common dolphins.
The dolphins probably sought refuge in shallow areas that interfere with the killer whales’ echolocation and movement – but ended up getting stuck among the sandbars and tidal channels themselves, says Arias.
Killer whales, meanwhile, may deliberately chase dolphins into bays in an attempt to sway them. In fact, in 2021, two of the killer whales had been observed repeatedly using inshore trawls to catch sea lions, she says.
Prolonged stress and disorientation from the hunt may have meant that the dolphins could not find their way back to the sea. Such cases probably occur all over the globe and just haven’t been documented yet, says Arias.
“This study highlights the important contribution that citizen science can make to research, and how understanding these processes helps us not only explain mass strandings, but also better understand how marine ecosystems change – and how species respond to these dynamics,” she says.
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