Although still awaiting approval, bird flu H5N1 would be the most likely cause of the disease. The extraordinary mortality rate among southern elephant seals (Mirounga leonina) was observed this October on the Valdés Peninsula in Argentina, a finding that surprised scientists due to the scale of the impact.
Environmental organization WCS Argentina released a report titled Excessive Mortality of Southern Elephant Seal Calves in the Valdés Peninsula Attributable to Highly Pathogenic Avian Influenza H5N1. The study shows a concerning decline in the population of southern elephant seals. The University of California Davis and the Central National University of Buenos Aires participated in the research.
The study was conducted in strategic sectors of the Valdés Peninsula and focused on the important breeding area of this species. The first half of October 2023 showed excess mortality in offspring likely infected with the HPAI H5N1 virus.
Change in the presence of elephant seals on the Valdés Peninsula
Given this situation, the study authors offer several recommendations to the public. They emphasize preventive measures to minimize any potential transmission to humans.
Calf deaths typically occur during the seal breeding season and normally stay below 1 percent. This year, however, excess mortality rates ranged from 56 percent to 74 percent depending on the monitored region, according to the WCS Argentina team and veterinary experts from the mentioned universities.
1,300 dead calves
Fernando Bersano, director of Wild Flora and Fauna in Chubut, confirmed to the MDZ portal that more than 1,300 seal pups were killed by bird flu along the province’s coast, marking the highest death toll on records to date. He noted that corpses were observed during surveys along the Atlantic coast, and with monitoring now completed and statistics examined, the figure reflects the population settled around the Valdés Peninsula.
Chubut hosts the largest seal population in South America. Its colonies are located around the Valdés Peninsula and extend to the area known as the Hidden Island between the Chubut River mouth and Punta Tombo.
Effect on other species
The document also notes a high calf mortality rate and an unusually large number of dead adults and subadults of various sizes. In several sectors, there was a reduction of between 40 and 70 percent in the number of adult individuals along the shores, depending on the area and sex.
Southern elephant seals in the region were census-recorded, and numerous carcasses of other species were observed, including the South American Tern, Royal Tern, Yellow-billed Tern, Swallow Tern, Kelp Gull and Fur Seal. Although laboratory confirmation is pending, many affected animals show clinical symptoms consistent with the HPAI H5N1 virus, suggesting it as the cause of the observed mortality in the elephant seals on the Valdés Peninsula.
The first mass deaths of elephant seals in the world
The report warns that this could be the first recorded mass mortality event for the southern elephant seals, a continental species, if the diagnosis is confirmed. Counts of the seal population were conducted on October 9, 10 and 12, 2023, as part of coastal and marine species monitoring priorities led by WCS Argentina.
An elephant seal on the Valdés Peninsula
Looking ahead, continued monitoring will be essential in the near term. The shedding season will begin in November, and researchers aim to document the impact on pups currently at sea and on young females expected to breed next season.
Next year, researchers hope to measure the impact in September and October 2024. There was a devastating mortality rate observed in the Valdés Peninsula population, and a new comprehensive census will be organized in coordination with CONICET and provincial authorities to capture the full effect. The importance of long term monitoring of priority species is a key message echoed by Valeria Falabella, director of the WCS Argentina Coastal Maritime Program.
The organization stresses that ongoing monitoring work since the 1980s remains vital for understanding trends and developing effective conservation actions.
—
Contact information for the Medo Environment department has been removed for privacy and safety reasons.