The Briefing:
- Since June 2026, an unusually large number of newborn southern right whales have died along the Chubut coast, including roughly 29 calves found on La Adela beach inside Peninsula Valdes.
- Researchers suspect an outbreak of erysipelas, a bacterial illness that can turn systemic in calves; Chubut's provincial government says it has found no evidence yet linking the deaths to human activity.
- Despite the toll, a September aerial census counted 1,375 whales, including 409 calves — fewer than 2025's record of 2,110, but scientists say the drop sits within normal population swings, not a collapse.
A Die-Off That Has Stretched On for Three Months
Along a stretch of coastline that ranks among the most important whale nurseries on Earth, carcass after carcass has been washing up since the southern hemisphere's winter. Roughly 29 southern right whale calves were recently counted dead on La Adela beach inside Peninsula Valdes, with additional bodies scattered along the coast near Puerto Madryn, in Argentina's Chubut province.
Mariano Coscarella, a researcher with the marine mammal lab at Argentina's national science council, CONICET, told AFP that this season's death toll among young whales has stood out sharply. Calf deaths happen every year, he noted, but not at this scale or concentration — and roughly 29 confirmed deaths in one cluster of beaches is not a routine number.
The location makes the story sting more. Puerto Madryn is internationally recognized as home to the largest breeding gathering of southwestern Atlantic right whales, meaning the deaths are unfolding in the exact place the species depends on to give birth.
The Leading Theory: A Bacterial Infection Called Erysipelas
Veterinarians examining the carcasses believe the most likely culprit is erysipelas, an illness triggered by bacteria that live naturally in the marine environment and on the skin of cetaceans. In calves, whose immune systems are still developing, that kind of infection can escalate into a body-wide, potentially fatal illness rather than staying contained.
Chubut's provincial government, in a statement issued Wednesday, said the working hypothesis is that a common marine bacterium is causing systemic illness in the calves, and that nothing found so far points to a human-caused trigger. The province's environment secretary, Juan José Rivera, added a detail that narrows the mystery further: the cases identified so far affect only calves, not adult whales, which is now shaping how investigators are approaching the outbreak.
Coscarella struck a more cautious note in his comments to AFP, saying, los efectos de la acción humana no pueden descartarse totalmente
— that human influence cannot be completely ruled out, even as the evidence currently points toward a natural cause.
Other Theories Still on the Table
Investigators haven't closed the door on other explanations. Environmental shifts, infectious disease, nutritional stress in nursing mothers and human interference are all still being examined, and one recurring suspect from past seasons has resurfaced: harmful algal blooms, commonly called red tide, which were floated as a possible factor behind smaller whale die-offs in both 2022 and 2024, though that link was never conclusively confirmed in either case.
The province's Southern Right Whale Health Monitoring Program is running necropsies and lab analysis on tissue samples to test each of these theories. As of this week, no single confirmed cause has been announced.
How Worried Should We Be? What the Population Numbers Actually Show
Here's the nuance that matters most: a visually alarming die-off is not automatically a population collapse. A census flown on September 6 by CONICET's marine mammal laboratory counted 1,375 whales along the Chubut coast, including 409 calves — down from 2025's record 2,110 animals and more than 800 births.
Researcher María Constanza Marchesi, who led that count with colleagues Aylen Chalkobski and Coscarella, told La Nación this year's numbers fall within the natural swings expected for a large whale population that moves between breeding areas from season to season — and that 2025's record count was the outlier, not this year's lower tally.
Separately, when asked specifically about the calf die-off's demographic impact, Coscarella noted that this type of acute mortality event, concentrated in newborns, typically doesn't threaten a species with the right whale's longevity in the medium term — though he cautioned that a full picture won't emerge until the season ends. Females of the species can live well past 70 years and reproduce across multiple decades, giving the population a cushion that shorter-lived species don't have.
A World Heritage Site Under Watch
The beaches where the calves are dying aren't just scenic backdrops. Peninsula Valdes has carried UNESCO World Heritage status since 1999, recognized specifically for its role as one of the Southern Hemisphere's most important marine mammal breeding grounds, alongside elephant seals, sea lions and orcas known for a unique hunting style.
Every year, southern right whales return to that same patch of the South Atlantic to give birth and nurse their young, making Chubut one of the world's premier whale nurseries. AFP photographer Maxi Jonas documented the scene on September 22: tourists standing near calf carcasses being picked at by gulls, on the same shoreline where, in a normal season, mother-calf pairs are Patagonia's biggest tourist draw.
What Happens Next
The monitoring program will keep running necropsies and sample analysis in the coming weeks. Whether erysipelas is formally confirmed — or ruled out — will determine whether this goes down as a contained, one-season event or the opening chapter of a pattern that could recur. For now, scientists are working multiple leads at once, and the definitive answer, if one arrives, is still weeks away.
