The colony of southern elephant seals (Mirounga leonina) at Península Valdés (Argentina) grew by 0.9% from 2000 to 2022, reaching a population of 18,000 reproductive females. In 2023, an epidemic of the High Pathogenicity Avian Influenza H5N1 virus led to the death of almost all pups and an unknown number of adults. We tested five scenarios that included complete pup mortality along with varying levels of adult mortality and reduced fertility. Newborn mortality had the smallest impact on the future population due to high natural mortality. Consequences of pup deaths will not appear until 2027, when those lost pups would have first reproduced. Scenarios including mature female mortality had more severe and immediate consequences, with a reduction in the breeding population in 2024 predicted to match the flu death rate. It took about 10?years for the population to readjust to the 2022 age distribution. In scenarios including adult mortality, it will take decades for the population to return to the 2022 level. The 2023 epidemic may thus reverse the conservation status of a population previously having no threats to continued growth.