Update Note: This article reports new information involving the southern right whale mortality event at Península Valdés, Argentina, following Nature World News' earlier coverage. The latest verified information comes from MercoPress reporting published Oct. 21, 2026, at 14:44 UTC, and UPI reporting published Oct. 26, 2026. Official death totals have not been released.

Adults and Juveniles Join the Toll

The die-off of southern right whales off Argentina's Patagonian coast is no longer limited to newborn calves. Dead juvenile and adult whales have washed ashore near Puerto Madryn in recent weeks, UPI reported on Oct. 26, after months in which nearly all the carcasses were calves.

Scientists now suspect two separate causes. Calf deaths have been linked to the bacterium Erysipelothrix rhusiopathiae, while a toxic algal bloom known as red tide is suspected in the adult deaths, MercoPress reported. To test the second idea, the monitoring program is adding water and shellfish sampling in the Nuevo and San José gulfs.

The shift matters because adult females carry the population's future — losing a calf is painful but common in a species where many newborns die naturally. Losing breeding adults removes animals that would otherwise produce calves for decades.

Unofficial counts now exceed 300 dead whales this season, according to MercoPress. Researchers and Chubut provincial officials have declined to release an official figure until the calving season ends. The Southern Right Whale Health Monitoring Program is expected to publish final numbers in December.

Even the unofficial tally far exceeds earlier peaks. The program, run since 2003, previously recorded highs of 113 dead calves in 2012 and 28 dead adults in 2022.

Nature World News previously reported that scientists had detected bacteria in dead calves and were testing whether wounds from kelp gull attacks let infections take hold. That reporting focused entirely on calves. Since then, the bacterium has been named, and adult deaths have opened a second line of investigation.

Two Suspects for Two Groups of Whales

For calves, Erysipelothrix rhusiopathiae is the leading suspect. The bacterium causes erysipelas and can enter through wounds, such as gull bites, or through feeding, explained Mariano Coscarella of the Center for the Study of Marine Systems at Argentina's National Scientific and Technical Research Council, or CONICET. Kelp gulls peck at calves' backs as they surface to breathe, and calves cannot keep their backs underwater the way adults do.

For adults, investigators are looking at red tide. In 2022, about 30 southern right whales died in Golfo Nuevo over a few weeks, and government researchers found toxins tied to a harmful algal bloom in samples from some of them. Harmful algae can produce toxins that move up the food chain through the tiny prey whales filter from the water, UPI reported.

Scientists have not confirmed either cause, and they have not determined whether the calf and adult deaths are connected. Finding a bacterium in a carcass does not prove it caused death, so labs must show the organism causes disease in living tissue.

"It seems to be a natural cause," Coscarella told AFP in comments published by the Buenos Aires Times, though he added that scientists cannot completely rule out an influence from human activity. Chubut officials have said no evidence so far links the deaths to human activity.

Ocean Conditions and the El Niño Question

A key unanswered question is why this year is so much worse. Gulls and bacteria have been present in earlier seasons without comparable death tolls. Coscarella has noted that 2026 is an El Niño year and that climate change or El Niño may have influenced ocean conditions.

No analysis has linked this season's deaths to El Niño, warmer water or a new red tide. Researchers have raised the possibility, but there is currently not enough evidence to draw that connection.

The water and shellfish sampling should help. Shellfish concentrate algal toxins over time, giving scientists a record of whether a bloom has occurred even if it has faded from the water.

Marcela Uhart, a veterinarian with the One Health Institute at the University of California, Davis, leads the monitoring program. She has said the team's 23 years of records on this same population are what allow scientists to identify this season as an unusual event rather than normal year-to-year variation. That long baseline will also be essential for judging whether ocean conditions this year differed from past seasons.

A Long-Lived Species Feels Losses Slowly

Península Valdés is South America's largest nursery for southern right whales. The population visiting the area grew from about 500 whales in the late 1990s to a record 2,110 in 2025. A CONICET aerial survey on Sept. 6 this year counted 1,375 whales, including 409 calves.

Those numbers put the reported calf deaths in perspective — if the unofficial counts are confirmed, the losses would represent a large share of the season's newborns.

The Whale Conservation Institute warned that, for a long-lived species, the consequences of a die-off "are not immediate and cannot be measured from isolated figures." Southern right whales can live about 60 years, and females typically give birth only every few years.

"What would be concerning is if there were two, three or four episodes like this," Coscarella told Live Science, as reported by Smithsonian Magazine. Specialists do not currently believe the deaths threaten the species' long-term recovery, UPI reported.

Whale-watching tours from Puerto Madryn are continuing as usual. Visitors and residents should not touch stranded or dead whales, should keep their distance from carcasses on beaches, and should report strandings to local authorities. Anyone harvesting shellfish in Chubut should follow provincial advisories, which are issued when red tide toxins are detected.

The next major development will be laboratory results from necropsies and the new water and shellfish tests, followed by the official season-end count in December. Nature World News will continue to follow the investigation.

The bottom line is that a calf die-off has widened to include adults, and scientists are now chasing two suspected causes at once. The long-term effect on the population depends on whether adult females were lost and whether this year proves to be a one-time event.

What Readers Want to Know

What has changed since the first reports? Juvenile and adult whales are now among the dead, and scientists suspect a red tide in those deaths alongside a bacterial infection in calves.

How many whales have died? Unofficial counts exceed 300 this season. Official totals are expected in December after the calving season ends.

What bacterium is linked to the calf deaths? Erysipelothrix rhusiopathiae, which causes erysipelas and can enter through wounds such as gull bites.

Is climate change or El Niño to blame? That has not been established. Researchers say ocean conditions may have played a role, but no analysis has made that link.

Is human activity responsible? Officials say no evidence so far links the deaths to human activity, though scientists have not fully ruled it out.

Does this threaten the species? Specialists do not currently believe it threatens the population's long-term recovery, but repeated events or heavy losses of breeding females would raise concern.

What should visitors to the area do? Avoid touching stranded or dead whales, report strandings to local authorities, and follow provincial shellfish advisories.