Exploring mass strandings of common dolphins: a response to predator encounters?

Abstract

Mass strandings of common dolphins Delphinus delphis are rare and poorly understood in the southwestern Atlantic. Although numerous hypotheses have been proposed worldwide, specific causes often remain unclear. This study provides the first evidence linking repeated mass stranding of D. delphis in northern Patagonia, Argentina, with the presence of killer whales Orcinus orca, supporting predator evasion as key mechanism. Two mass stranding events of D. delphis were recorded in San Antonio Bay, Rio Negro. In 2021, 52 dolphins died; 38 of these were recovered for complete necropsy, revealing good body condition and no evidence of disease, trauma or anthropogenic interaction. In 2023, around 570 dolphins stranded; all were returned to the sea, with no carcasses found in subsequent days. In both events, dolphins exhibited atypical inshore movements, high cohesion and disorientation shortly before killer whales were sighted. These events occurred in a shallow coastal area with sandbanks and channels, increasing stranding risk during evasion. Findings suggest that mass strandings may be triggered by prey escape responses to predator presence, compounded by environmental and social factors. This study provides new insights into cetacean predator–prey dynamics and emphasizes the need to consider top predator presence when assessing causes of mass strandings.