Now, scientists are racing to figure out exactly what's causing the record-breaking die-off.
Researchers say they need to quickly find the answer to better understand the changing threat this species faces, the risk to the wider ecosystem and humans, and what can be done to manage it.
The first dead calves appeared on the beaches of Península Valdés in June, at the start of the breeding season, which runs through to December. Península Valdés is located in Chubut province, on the coast of northern Patagonia.
In September, 409 calves were counted among a total of 1,375 southern right whales during a survey conducted by CESIMAR's Marine Mammal Laboratory. With more than 300 deaths already recorded, the die-off represents an exceptional proportion of these calves. The final figure will be released by the Southern Right Whale Health Monitoring Program in December.
"Samples were sent for analysis earlier than usual — typically, they are analyzed at the end of the year — because the higher-than-usual mortality is a cause for concern," said Víctor Fratto, the director of the Native Wildlife Rescue Program and a contributor with the whale monitoring program. "What those samples are showing is that the calves died from erysipelas," he told Live Science.
A known bacterium full of unknowns
A person suffering from a case of facial erysipelas. (Image credit: CDC/Dr. Thomas F. Sellers/Emory University, Public domain, via Wikimedia Commons)The bacterium that causes erysipelas, E. rhusiopathiae, is not exclusive to whales. It can be found in marine environments and in a wide variety of animals, including fish, crustaceans and birds. Pigs are one of its natural reservoirs.
The presence of E. rhusiopathiae does not necessarily mean an animal is diseased. Yet many questions remain about E. rhusiopathiae in whales.
Calves are especially vulnerable because their immune systems are still developing and they spend more time at the surface than adults do, exposing them to more attacks by gulls, which can transfer the disease.
Attacks by kelp gulls (Larus dominicanus) on southern right whales were first recorded at Península Valdés in the 1970s. The behavior began with just a few individuals in Patagonia's San José Gulf and became increasingly frequent over time, Ricardo Bastida, a marine biologist and professor emeritus at the National University of Mar del Plata who has studied these cetaceans for more than four decades, told Live Science. The birds also began scavenging the carcasses of stranded whales, Bastida said
The working hypothesis is that the gull is the vector: Pecking causes wounds on the back, allowing the bacteria to enter.
Fratto said that swabs will be taken from gulls to check for the presence of E. rhusiopathiae and assess whether the results could help develop a management strategy for the species.
What changed this year?
The major question for scientists is what might have changed this year to cause a bacterium already present in the ecosystem to trigger a die-off of this magnitude. Experts told Live Science they believe the phenomenon could be the result of several factors acting simultaneously.
One factor could be the environment. Changes in water temperature, salinity, pH or rainfall could alter the conditions under which infection occurs. There could also be changes in food availability, affecting the mothers' body condition and the vulnerability of their calves. Another possibility is that the pathogen itself has changed — for instance, through the emergence or increased circulation of a more virulent strain or a different serotype, Fiorito said.
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Coscarella is more cautious. Southern right whales are extremely long-lived, and calves represent only a fraction of the population. Therefore, a single season of exceptionally high mortality does not necessarily mean the population will go into decline, he argued. "What would be concerning is if there were two, three or four episodes like this," he noted.
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