How bird flu is decimating elephant seals |
The sudden decimation by bird flu of the world's largest seal has sent shockwaves through the scientific community, and the ecosystems shaped by these majestic animals.
At up to 5.8 metres (19ft) in length and weighing as much as 3,700kg (8,157lb), southern elephant seals are the world's largest seal species. They spend most of their time alone, foraging at sea. But once a year, thousands gather to breed along the Patagonian coast of Argentina in a noisy, boisterous gathering of giants.
Julieta Campagna, a marine biologist with the non-profit World Conservation Society (WCS), grew up listening to the breeding season's calls: the gruff rhythmic sounds of fighting between males and the squeaky cries of hungry pups calling their mothers. In October 2023, however, she was met with silence.
That year, Campagna and other researchers discovered hundreds of dead adults and newborn seals scattered on the Patagonian beaches of Peninsula Valdés. "I was petrified," recalls Campagna. "We saw hundreds of dead pups being eaten by seagulls. It was a horrible scene to witness."
The culprit was a deadly strain of bird flu, known as H5N1. First detected in poultry in 1996, by 2020 this highly pathogenic form of the virus had mutated to spread easily among wild bird populations. By 2022, it also began to transmit directly from mammal to mammal. Now, research seen by the BBC is revealing the unprecedented extent of southern elephant seal decline in Argentina, which may have knock-on consequences for the wider ecosystem.
The evidence for mammal-to-mammal transmission initially came from marine mammals in South America, where the virus killed pinnipeds (seals and sea lions) on a large scale for the first time.
In Peru and Chile together, more than 30,000 sea lions died between 2022 and 2023. In Argentina, about 13,000 sea lions and fur seals were killed. But it was at the Peninsula Valdés that the most impactful die-off took place. Researchers estimate that nearly 97% of the site's elephant seal pups died, making it the largest mortality event ever recorded for the species.
The exact extent of the virus's global reach among seals and sea lions is not known. Researchers estimate it has affected around one-third of the world's pinniped species so far. However, they say it is possible the flu has been affecting a greater number of individuals and a wider range of species than has been documented.
"While mass mortality events are highly visible, they may not represent the full spectrum of the virus's effects on pinnipeds," says Elizabeth Ashley, a veterinarian pathologist at the University of California, Davis. "Many pinnipeds are not easy to observe or access, so it is plausible that clustered or sporadic mortalities may have gone undetected in some locations."
The mass die-off recorded among elephant seals at the Peninsula Valdés may therefore be just the tip of the iceberg of bird flu's full impact on the world's seals and sea lions – and on wider ocean life. Understanding why they were so susceptible, as well as the consequences of such dramatic declines, could now help researchers prepare for new viral threats still to come.
"When you remove such a big mass, you completely upset the balance of the ecosystem," says Marcela Uhart, a wildlife veterinarian at the University of California, Davis. "No other species can replace elephant seals in the ocean."
As soon as the mass die-off was discovered at Peninsula Valdés, researchers feared it would have significant long-term impacts on the southern elephant seal population. The site is the only mainland breeding ground for the species worldwide and is considered a site of global significance for the conservation of marine mammals by Unesco.
In 2023, however, it was impossible to know the full number of dead adults, as additional deaths could have occurred at sea. Or how many females had become pregnant after the outbreak. So at the end of 2024, in the following breeding season, Campagna and colleagues from WCS – along with scientists from the University of California, Davis and the Argentinian research agency Conicet – returned to survey the beaches once........