Scientists suggest that orcas hunting common dolphins led to two mass stranding events in northern Patagonia, Argentina. Videos from citizen scientists captured dolphins fleeing into shallow bays, where some became trapped and died. The findings highlight how predator-prey interactions can contribute to these mysterious occurrences.
In recent years, common dolphins have experienced unusual mass strandings in northern Patagonia, prompting an investigation by researchers including Magdalena Arias from Argentina's National Scientific and Technical Research Council.
The study focused on two events: one in 2021 and another in 2023, both near San Antonio Bay in Rio Negro province. In 2021, footage showed approximately 350 dolphins racing toward the bay, pursued by a pod of eight orcas about 30 minutes behind. Some dolphins entered the shallow port area and remained still, while the orcas turned back to sea. The following day, dozens of the dolphins were found dead on the shore.
A similar incident occurred in 2023, with around 570 dolphins speeding into the bay ahead of orcas. In this case, local authorities and volunteers rescued the dolphins after the predators departed.
Necropsies on 38 dolphins from the 2021 stranding revealed they were in good body condition, with no signs of disease, injury, or recent meals in their stomachs. This rules out common causes like illness or foraging mishaps, according to Arias.
Reports of orca sightings helped map their activity, including two confirmed instances of orcas hunting and killing common dolphins. The researchers propose that dolphins sought refuge in shallow waters to evade orcas' echolocation and mobility, but got stuck in sandbanks and tidal channels. Orcas may intentionally herd prey toward such coastal features, as two of the 2021 orcas had previously used similar tactics against sea lions.
"This doesn’t mean predators are always the cause of mass strandings, but it suggests that predator-prey interactions may sometimes trigger these events – particularly when combined with factors such as coastal geography, tides and the strong social behaviour of dolphins," says Arias.
The analysis relied on videos from drones, mobile phones, and platforms like eWHALE, uploaded by tourists, guides, fishers, and residents. Such citizen science efforts underscore how community contributions can illuminate marine ecosystem dynamics and explain strandings that have puzzled scientists for years.
The research appears in Royal Society Open Science (DOI: 10.1098/rsos.250870).
