More than 300 southern right whale calves found dead at Argentina's Península Valdés

More than 300 southern right whale calves have been found dead since June along Argentina’s Península Valdés, scientists say, a preliminary toll that would make the 2026 calving season the largest calf die-off on record at the site if confirmed. Researchers caution that the figure is still being checked for duplicates and that authorities will publish a consolidated final count after the season, but it is already far above the previous high of 113 calf deaths in 2012.

The leading hypothesis for the calf deaths is bacterial infection. Tissue samples from stranded calves have tested positive for Erysipelothrix rhusiopathiae, the bacterium that causes erysipelas, according to the Southern Right Whale Health Monitoring Program and its collaborators. Scientists stress that the finding does not yet prove the bacterium was the primary cause of death in every case, and laboratory work is continuing to determine how central it was to this season’s mortality.

Investigators suspect the bacterium may be entering through wounds caused by kelp gulls, which are known to peck at the backs of right whale calves. That possible pathway has precedent: A 2016 peer-reviewed paper reported that E. rhusiopathiae had previously been isolated from gull-inflicted wounds on southern right whale calves in samples collected in 2013. Chubut environment secretary Juan José Rivera has said the bacterium identified is common in the water and that its presence does not currently point to a human-caused event.

Scientists say the calf deaths may not be the whole story. In addition to the newborns, dozens of juveniles and adults have also been found dead this year. Preliminary necropsies — animal autopsies — found that some of those older whales were in good body condition and had eaten recently, prompting investigators to examine harmful algal blooms and marine biotoxins as a possible explanation for those deaths. That would fit a different pattern from the suspected bacterial disease in calves. There is also precedent: In 2022, coordinated testing confirmed paralytic shellfish toxins in adult whales that died in the same broader region, and authorities concluded a harmful algal bloom was the most likely cause.

Taken together, the findings suggest two processes may be unfolding at once: a likely bacterial disease pattern among calves, and a possible algal-toxin pattern among juveniles and adults. Scientists have not concluded that all the deaths share a single cause.

Península Valdés, in Chubut province, is one of the world’s best-known southern right whale calving grounds, and that is one reason researchers can say this year stands out. The Southern Right Whale Health Monitoring Program, coordinated by Dr. Marcela M. Uhart, has run systematic post-mortem monitoring there since 2003, with collaborators including CESIMAR/CENPAT, Universidad Nacional del Centro, INIDEP, provincial authorities and park rangers. That long-running record provides the baseline for judging how unusual the 2026 season is.

“The death rate [among young whales] is quite high this season in particular,” Mariano Coscarella, a biologist at the Marine Mammals Laboratory and CONICET, told AFP.

Uhart said the long record is what makes the current investigation so important. “When something unusual happens, the value of long-term monitoring becomes especially clear,” she said.

For now, much of the picture remains unresolved. The final calf count is still pending, and scientists are still testing whether the bacterium found in calf tissues was the direct driver of the mass mortality, why so many calves died this season, and whether the deaths of older whales reflect a separate toxic bloom event rather than one single cause.