My co-authors and I are excited to share our newest publication: Romero, M.A., Bartés, S., Grandi, M.F., Brogger, M., Svendsen, G., González, R., Crespo, E.A. 2025. Impact of the avian influenza outbreak on the population dynamics of South American sea lions Otaria byronia in northern Patagonia. Marine Mammal Science, 2025: 0:e70031. The full text is available here: *https://doi.org/10.1111/mms.70031* <https://doi.org/10.1111/mms.70031> Abstract Understanding the population dynamics of marine mammals is essential for assessing their resilience to historical harvesting pressures and current anthropogenic threats. The South American sea lion (SASL, *Otaria byronia*), widely distributed along the South American coasts, represents the largest biomass of northern and central Patagonia. This species faces threats such as incidental fishing captures, aquaculture, and tourism, with well-documented impacts on its population. However, the effects of Highly Pathogenic Avian Influenza (HPAI) H5N1 on its population dynamics are poorly understood. This study provides new insights into the impact of the 2023 avian influenza outbreak on the SASL in northern Patagonia. Population trends and age-sex class composition of colonies were updated, incorporating the HPAI outbreak in the Rio Negro rookeries to compare trends before and after the epizootic event. A positive trend in SASL abundance was observed from 2007 to 2022 across most age and sex classes. However, in the 2024 census, 7175 ± 145 individuals were counted across three colonies, with the largest, Punta Bermeja, recording a 27.6% decline. During the 2023 outbreak, 1476 deaths were reported, affecting all age and sex classes. These findings suggest the HPAI H5N1 outbreak significantly impacted the previously positive population trend. The article is not open access, so it is available upon request! Best regards, M. Alejandra Romero -- Dra. María Alejandra Romero Centro de Investigación Aplicada y Transferencia Tecnológica en Recursos Marinos "Almirante Storni" - CONICET, Güemes 1030 – (8520) San Antonio Oeste, Río Negro. Argentina. Escuela Superior de Ciencias Marinas - Universidad Nacional del Comahue (IBMP-UNCo), San Martín 247 – (8520) San Antonio Oeste, Río Negro. Argentina. Tel/Fax: 54 (2934) 421002/430764 Email: romero.ale@gmail.com, aromero@cenpat-conicet.gob.ar