Dear MARMAM community, On behalf of my co-authors, I'm pleased to share our new article that has just been published in Marine Mammal Science: Tortolini, V. M., M. C. Marchesi, E. C. T. Formigo, K. Zaouali, and M. A. Coscarella. 2026. “Killer Whale (Orcinus orca) Prey-Handling and Harassment of Southern Right Whales (Eubalaena australis) in Península Valdés, Argentina.” *Marine Mammal Science*42, no. 3: e70218. https://doi.org/10.1111/mms.70218. Summary: We report the first video-documented case of cooperative prey-handling and feeding by killer whales (KWs, *Orcinus orca*) on a southern right whale (SRW, *Eubalaena australis*) calf carcass, together with subsequent harassment events directed toward live SRWs and their defensive responses. Using drone footage and shore-based observations conducted in the Natural Protected Area El Doradillo, Península Valdés, Argentina, we documented a group of six identified KWs refloating and feeding on a sunken SRW calf carcass. Individuals alternated in holding, lifting, and releasing the carcass, facilitating access for all group members and evidencing cooperative prey-handling behavior. Later the same day, the group engaged in 46 harassment events targeting other SRWs in the area, primarily mother–calf pairs. We describe SRW defensive responses including 360° turns and zigzag movements, which were more frequently elicited when KWs approached from a latero-caudal angle. These predator–prey interactions may have been influenced by recent ecological changes, including a local decline in traditional pinniped prey following the 2023 HPAI H5N1 outbreak, coupled with the record-high abundance of southern right whales observed in 2025. Our observations underscore the ecological role of killer whales as apex predators in northern Patagonia, capable of rapidly adjusting their foraging behavior in response to shifts in prey availability. They further contribute to our understanding of KW-SRW interactions in a key breeding ground of the Southwestern Atlantic and emphasize the value of long-term monitoring to evaluate the potential consequences of these dynamics for the regional southern right whale population. Please don't hesitate to reach out if you have any questions! With regards, Virginia M. Tortolini *Lic. en Ciencias Biológicas * *Universidad Nacional de la Patagonia San Juan Bosco (UNPSJB)* *Abuelas de Plaza de Mayo 3051, Puerto Madryn U9120, Chubut, Argentina.* *Ph.D. candidate * *Centro para el Estudio de los Sistemas Marinos (CESIMAR) - CONICET* *Abuelas de Plaza de Mayo 2915, **Puerto Madryn U9120, Chubut, Argentina*