Marine Mammals · Argentina Feature

Southern Right Whale Calf Deaths at Argentina’s Península Valdés Pass the 2012 Record

Scientists have found a bacterium in the dead calves and suspect wounds left by gulls let it in. The official toll is due in December, and nobody yet knows what changed this year.

By SEVENSEAS Media | Ocean Science

The first dead calves washed up on the beaches of Península Valdés in June, at the start of the southern right whale breeding season. By mid-October, Live Science was reporting that more than 300 calves had died in Argentine Patagonia since then. At the end of September, La Nación had put unofficial counts at more than 340 dead calves along the peninsula’s shores. Neither figure is official. Researchers and the Chubut provincial government have declined to give a toll before the season ends, and the final number is expected from the Southern Right Whale Health Monitoring Programme in December.

Even the provisional counts sit far above anything the programme has recorded since it began in 2003. Its worst year for calves until now was 2012, with 113 deaths, which La Nación, citing the programme’s data, describes as the highest number documented for the species anywhere. In September the Instituto de Conservación de Ballenas (ICB), under whose umbrella the programme operates, said this season’s preliminary records suggested an event of even greater scale, while cautioning that the count still has to be consolidated and set against the number of births before its effect on the population can be judged.

A nursery with a long record

Península Valdés, in Chubut province, is South America’s largest sanctuary for southern right whales (Eubalaena australis), MercoPress notes. Females come into the sheltered waters of Golfo Nuevo and Golfo San José to give birth and nurse. The ICB and Ocean Alliance have photo-identified the population since 1971, building a catalogue of close to 6,000 known whales, and the health programme, coordinated by veterinarian Marcela Uhart of the University of California, Davis One Health Institute, examines the whales that die. Uhart told AP that more than two decades of monitoring the same population is what allows her team to call this year unusual.

The living population gives some context. A September survey by the Marine Mammal Laboratory of CESIMAR-CONICET counted 1,375 whales, including 409 calves. That is down from a peak of 2,110 whales, 816 of them calves, in 2025, according to Visión Marítima, but the visiting population has grown from roughly 500 whales in the late 1990s. When the census was released, researchers said the drop from 2025 fell within the natural fluctuation expected for the population, the Buenos Aires Times reported.

Live Science reports that in a typical year 20 to 30 per cent of a calf cohort dies from various causes, and describes more than 300 deaths against 409 counted calves as an exceptional proportion. The two figures were gathered differently, one a single survey and the other a running tally of carcasses across the season, so they should not be read as a precise mortality rate.

The bacterium

The leading suspect in the calf deaths is Erysipelothrix rhusiopathiae, the bacterium behind erysipelas. In late September the provincial government described the likely culprit as a bacterium common in seawater that can cause systemic infection in calves. In a later statement quoted by the ICB, the Chubut government said samples analysed from calves had shown the bacterium, which is associated with septicaemia. Víctor Fratto, an environmental management specialist who works on the programme’s necropsies, went further, telling Live Science that the samples show the calves died of erysipelas.

That is a stronger claim than the investigation’s lead has made in public. In early October Uhart told AP that bacteria had been detected in tissue samples sent to the veterinary school in Tandil, but that her team was still culturing them to identify the microbes and determine whether infection caused the deaths. The ICB, in a statement on 16 October, asked the public to wait for integrated results before attributing causes.

The bacterium is not new to these waters. It occurs in marine environments and in fish, crustaceans and birds, and pigs are one of its natural reservoirs, Carla Fiorito, a CESIMAR researcher, told Live Science. In a 2016 study she and colleagues reported isolating it from gull-inflicted wounds on two calves, one living and one dead, sampled during the 2013 season, the first such record. The paper offered evidence that the wounds can act as a route of entry for pathogens. As the ICB has stressed, it did not establish how those calves became infected, or that infection killed them.

The gulls

Kelp gulls (Larus dominicanus) peck at southern right whales as they surface to breathe, tearing at skin and blubber. Ricardo Bastida, a professor emeritus at the National University of Mar del Plata, told Live Science the behaviour was first recorded at Península Valdés in the 1970s, beginning with a few birds in Golfo San José, and that gull numbers in the region have grown on food from fishing activity and discarded catch. Adults have learned to surface with only their heads exposed, Uhart told AP. Calves cannot manage the manoeuvre, which leaves their backs within reach.

Fratto told Live Science the working hypothesis is that the gull is the vector, its pecking opening wounds through which the bacterium enters, whether it was already on the whale’s skin or carried on a beak from another infected animal. Swabs will be taken from gulls to test for it. But the attacks have gone on for decades, and on their own they cannot explain 2026. Diego Rodríguez, a marine biologist at the same university, argued that the bacterium needs tracing through the wider fauna of the ecosystem, not only in the dead calves.

Adults, juveniles and the red tide question

In October the pattern shifted. Juvenile and adult whales began to be found dead along the Chubut coast, Visión Marítima reported, and the ICB confirmed recent adult cases. According to the provincial statement quoted by the ICB, the adults found so far show no preliminary signs linking them to the bacterial picture seen in the calves. For the older animals, suspicion has turned to harmful algal blooms, the so-called red tides, which produce natural biotoxins that can move up the food chain through plankton.

There is a recent precedent. In September and October 2022, adult and juvenile whales died during a harmful bloom in Golfo Nuevo, and the programme’s investigation, which took months, concluded that biotoxins had played a role. That year’s 28 adult deaths were the programme’s highest adult count before this season. This year Mariano Coscarella, a CESIMAR researcher who sits on the International Whaling Commission’s Scientific Committee, told Cholila Online that tissue from the adults had gone to INIDEP, Argentina’s national fisheries research institute in Mar del Plata, to test for red tide toxins. Sampling of water and shellfish in both gulfs is being added in stages. Until those results are in, Visión Marítima notes, red tide cannot be named as the cause.

What changed this year

Identifying the agent does not answer the bigger question. Coscarella told Live Science it was a major step forward to have a fairly clear idea of what caused the disease, “but we do not know what changed.” Fiorito put it in terms of balance: for an outbreak like this, something must have shifted between pathogen, host and environment.

The possibilities researchers have raised with Live Science include changes in water temperature, salinity, pH or rainfall that alter the conditions for infection; changes in food supply that affect the mothers’ body condition and so their calves’ vulnerability; and a more virulent strain or a different serotype of the bacterium. Uhart told AP that warmer seas, potentially linked to climate change or El Niño, could contribute to conditions in which the bacterium overwhelms the calves’ immunity. The researchers Live Science spoke to said there is not yet enough evidence to link El Niño to the die-off.

On human activity, what officials have offered so far is an absence of evidence, not a clean bill. The Chubut government said in September that it had found nothing attributing the calf deaths to human activity. Coscarella, quoted by MercoPress, said no evidence had been found that direct human activity caused them, while allowing that climate change or El Niño may have influenced ocean conditions, and he told AFP, as carried by the Buenos Aires Times, that human influence could not be completely ruled out. He told Cholila Online that understanding the event could take a long time, pointing to a virus-linked dolphin die-off in the Mediterranean that took three years to explain, and that if the causes lie in the whales’ Antarctic or sub-Antarctic feeding grounds, there is little that can be done locally.

We don’t want to endanger these whales again.

Marcela Uhart, Southern Right Whale Health Monitoring Programme, speaking to AP

What it could mean for the population

No one involved is describing a population collapse. Coscarella told Live Science that one season of very high calf mortality does not necessarily send a long-lived population into decline, and that what would concern him is a run of such episodes. Rodríguez was less reassured, saying this year’s cohort of live-born calves will be largely missing from future adult records. The ICB points out that most southern right whales reach reproductive maturity at around nine years old, so the consequences of a die-off are not immediate and cannot be judged from one season’s numbers. Recurring losses on this scale, AP reported, could slow the recovery of a population that whaling once pushed to the edge.

The strandings have eased for now. On 27 October Cholila Online reported about ten days without a new dead whale, with animals starting to leave for their feeding grounds, although some stay in Chubut waters until late November or December. Coscarella warned that the lull was no guarantee against further strandings before the season closes.

Authorities ask anyone who finds a stranded whale, alive or dead, not to approach or touch it and to keep pets away. Fiorito notes that E. rhusiopathiae can infect people, usually through skin wounds. In Chubut, strandings can be reported to the provincial wildlife directorate on 280-4670760, the number the ICB publishes.

Read the Instituto de Conservación de Ballenas statement of 16 October 2026 (in Spanish), and Live Science’s reporting on the investigation.

SEVENSEAS Media · Conservation Journalism for the Ocean