Argentina’s elephant seals face 100-year recovery after avian flu devastation: Study

Biologist Valeria Falabella’s voice breaks as she describes the devastating scene. It was October 2023 when she and her team climbed down onto Punta Delgada Beach, in the Valdés Peninsula, a remote corner of central Argentina. While they were aware that avian flu had made its way across the Pacific, nothing had prepared them for what they were about to find. In the first harem of southern elephant seals (Mirounga leonina) something was off: several pups lay lifeless.

“In that moment there weren’t many [dead pups], but as we kept walking we saw the first dead adult,” says Falabella, director of coastal marine conservation at WCS Argentina. “In all the time I’d worked on beaches surveying elephant seals, I’d never seen a dead one; it was shocking.”

A few meters away, they found several South American terns (Sterna hirundinacea) writhing in the sand, showing clear signs of the infection. The team withdrew immediately and, using their binoculars, confirmed the inevitable: this was only the tip of the iceberg. Two days later, they returned to monitor the situation and assess the magnitude of the disaster.

“When we went back to the beach to count and observe the seals in more detail, the picture was bleak. Seventy percent of the pups were dead or dying,” Falabella says. “By that time there were several dead adult females too. The impact was huge.”

That year, 17,500 pups and an undetermined number of breeding adults died as a result of the epidemic. From one year to the next, the species’ previously healthy population had been decimated and even its survival came into question. It may take 100 years for the colony to recover the 18,000 breeding females it had in 2022, according to the projections of a new study published in Marine Mammal Science by researchers at WCS Argentina, the National Scientific and Technical Research Council (CONICET) and the University of California, Davis.

“Firstly, we need to recognize that these kinds of events are here to stay,” Falabella says. “This year we got lucky: there wasn’t another incident in the Valdés Peninsula, although other colonies did experience mortalities. However, we are dealing with a virus that mutates constantly, that adapts, that develops new capacities and that, evidently, can spread from mammal to mammal.”

In addition to the highly pathogenic H5N1 virus, there are other factors, such as climate change, that add even more uncertainty and create doubts about how the population may recover in the long term, Falabella says.

“We should bear in mind that these animals, when they go out to sea to feed, they do so in a context in which they also interact with fisheries,” she says. “The size and distribution of these fisheries are probably impacted by climate change, and we still have a limited understanding of this. We don’t know how this could affect the recovery of this species.”

The researchers say they’ll continue to monitor the situation.

“The priority now is the recovery of this population,” Falabella says. “And this can lead us to give recommendations, in some cases, such as determining in which areas fishing pressure should be reduced.”

The Valdés Peninsula, a key site for the species

The Valdés Peninsula, a UNESCO World Heritage Site, spans 360,000 hectares (890,000 acres) of unique coastal ecosystems. Its 400 kilometers (250 miles) of coastline include a series of gulfs and rocky cliffs measuring up to 100 meters (nearly 1,000 feet) high. It also has shallow bays, coastal lagoons and beaches that are an important refuge for marine mammals and birds.

Situated in the middle of these landscapes is Punta Delgada, a 3-km-wide (1.8-mi) beach that constitutes one of the main breeding grounds for thousands of southern elephant seals.

“Punta Delgada is part of my training and my background. It was the first place we worked; we’ve been tagging and carrying out research in the area for four decades,” Falabella says. It was there that she trained with Claudio Campagna, a researcher at CONICET and a central figure in the study of elephant seals.

“I found out about what had happened, and I didn’t want to go, I couldn’t,” says Campagna, now retired. “I helped to coordinate the surveys and the interpretation of the data. Then I worked a lot on the results so that we could communicate them. But after so many years of working with a very healthy population — because it had been growing for a long time — I didn’t want to expose myself to something so dramatic.”

This is why he approached the situation through numbers, Campagna says. With the data from the avian flu episode during the 2023 breeding season, the researchers came up with five scenarios to predict the recovery of the population that, until 2022, had 18,000 healthy breeding females.

Analyzing historical and recent data, the scientists estimated a pup mortality rate of 97%: in other words, 17,500 of the 18,000 pups born in 2023 likely died as a result of the infection. But the full consequences of the pups’ deaths will not become apparent until 2027, when they would have reached reproductive age.

Grave scenarios

In 2023, it wasn’t possible to ascertain how many adult females had died. Only one 20-km (12-mi) stretch of coast was monitored once a week. However, this stretch included important beaches, where a large number of animals normally gather each season. In the recent study, researchers say they were concerned when they found several bodies of adult females in places where none had been found in previous years.

This suggested that something unusual was happening, as dead adult females are hardly ever seen on the coast during the normal breeding season. It’s also possible that many other females died at sea, where their deaths would have gone undetected.

So the scientists proposed a range of scenarios. For example, if the 2023 episode only affected pups — who already face a high natural mortality rate — the population could recover to 2022 levels between 2029 and 2051.

However, the scenario changes drastically if the virus also impacted a significant number of the breeding adults. In this scenario, it could take until 2091 to return to a population similar to before the crisis.

The study presents an even more somber scenario. Looking at a combination between the deaths of adult females and the rupture of the social system caused by the loss of alpha males along with a recurrence of outbreaks in subsequent years, it could take more than a century to reach pre-outbreak numbers. In the worst-case scenario, the elephant seal population wouldn’t recover until the mid-2100s.

“We believe recovery will be slow because elephant seal growth depends on their survival at sea, especially when they’re young. And this hasn’t changed much,” Campagna says, referring to the fragile life cycle of southern elephant seals. It begins with the adult female arriving on land to give birth and then nursing the pup for the next three weeks, during which it will more than triple in size.

The mother then returns to sea, leaving her pup behind. The pup remains on the beach for another five to six weeks, together with other juveniles, until it has developed sufficiently to dive into the ocean, despite never being taught how to hunt.

The life that awaits it is harsh: only six out of 10 pups survive their first year. Few females will reach the age of 4, when they can begin to mate. Many are killed by predators or die from lack of food. Those that reach sexual maturity may reproduce for a decade or more, until they die at the age of about 17 years — if they’re lucky. But it’s rare that they grow that old in the wild.

Males face another challenge: they must survive to the age of 10 to become dominant and get access to females, but very few achieve this. The recent avian flu epidemic drastically altered this delicate dynamic, killing both adult females and males. And although only a few males are needed to impregnate many females, the mass death of adults has left a worrying hole in the colony’s social structure.

“The only thing we can do as conservationists is prevent them from being disturbed on the coast,” Campagna says. He points to a law that was recently passed in Chubut with the objective of contributing to these animals’ recovery.

“But once these things happen, conservation doesn’t have many tools to offer. How do you prevent the spread of an epidemic? We couldn’t do it with humans, so it’s tricky,” Campagna says.

Today, the outlook for southern elephant seals in Argentina is bleak and alarming, conservationists say. The crisis in the Valdés Peninsula contrasts sharply with the situation of other breeding populations of the species, such as those in Chile, where the future seems more hopeful.

In Jackson Bay, in the Tierra del Fuego archipelago at the southernmost tip of Chile, an avian flu outbreak in 2023 slashed the population of the elephant seal colony there by half. However, in the season that ended in April 2025, its population had doubled to 200 individuals with 33 pups, according to a report by WCS Chile, which has been monitoring this colony since 2008 alongside the Chilean Ministry for the Environment.

“These are hypotheses. Firstly, the Chilean colony, in Jackson Bay, is very small and relatively isolated, at the end of Almirantazgo Sound, in an area of fjords,” Falabella says. “Opposite the elephant seal colony, there’s a very large albatross colony, which didn’t experience any mortality either. I’m led to think it was more sheltered, precisely because of its geographical location.”

Temperature may also have been an important factor, Falabella adds. Southern elephant seals primarily mate on sub-Antarctic islands, and the Valdés Peninsula has the only continental colony that’s geographically situated farther north than the rest of the species’ distribution.

“This exposes the colony to a higher ambient temperature,” Falabella says. One of the hypotheses for the high pup mortality is that, in addition to being very small, they were also very exposed to the sun. Their black fur absorbed greater solar radiation than that of the adults, leading to them overheating faster.

“This added to the typical symptoms of avian flu, which include fever,” Falabella says. “Some veterinarians have suggested — and it makes a lot of sense to me — that the combination of all of these factors probably overwhelmed the pups.”

Is there hope?

A third scenario proposed by the scientists simulated a mass mortality of 50% of adult females and 100% of pups born in 2023.

“The survey this year [2025] will be very important to see what proportion of females may have survived without getting pregnant,” Falabella says. “If this is the case, the outcome could be slightly better, because these females didn’t in fact die; they were at sea, and we couldn’t count them in the survey because they weren’t pregnant, they weren’t on the beach, and we didn’t see them. I hope the third scenario turns out to be true. We’ll see.”

Banner image of female and male elephant seals, courtesy of Adriana Sanz.

This story was first published here in Spanish on May 19, 2025.

Citation:
Campagna, C., Condit, R., Ferrari, M., Campagna, J., Eder, E., Uhart, M., … Lewis, M. N. (2025). Predicting population consequences of an epidemic of high pathogenicity avian influenza on southern elephant seals. Marine Mammal Science, 41(3). doi:10.1111/mms.70009