The "mass mortality" of hundreds of southern right whale calves mystify scientists.
Surprisingly large numbers of southern right whale calves are dying off the coast of Argentina, sparking concerns among marine scientists and conservation officials.
Overall, southern right whales are doing much better than their endangered brethren to the north. But for one group of southern right whales that gives birth off Peninsula Valdés, Argentina, fate has not been so kind.
Hundreds of the Peninsula Valdés whales have died since monitoring of their population size began in 1971, researchers report in a study published this month in the journal Marine Ecology Progress Series. From 1971 to 2011, 630 of the right whales died—adults and young ones combined.
But 77 percent of those deaths occurred between 2003 and 2011. And of those recent deaths, 89 percent have occurred in the calves. Scientists are still struggling to understand why.
Changes in monitoring efforts over the years could probably account for some of the increase in recorded deaths, researchers acknowledge in the study. But they can't account for all of it.
Something else is going on that has so far defied the efforts of scientists to get to the bottom of the situation.
It's a real frustration, said Vicky Rowntree, a behavioural ecologist at the University of Utah in Salt Lake City and director of the right whale program at Ocean Alliance/Instituto de Conservación de Ballenas in Buenos Aires, Argentina.
No Common Cause
A meeting of marine biologists and veterinarians convened by the International Whaling Commission in 2010 to study the problem settled on three possible explanations: low food abundance, disease, or toxins such as domoic acid or saxitoxin produced by harmful algal blooms.
"A lot of our research has been trying to look at those [explanations]," said lead study author Rowntree. But so far, researchers and veterinarians have been unable to find a common cause of death in the hundreds of samples taken from the dead calves.
A small number of calves show evidence of disease while others contain low levels of toxins.
Figuring out whether the calves got enough food is harder to determine, Rowntree explained.
Blubber thickness is one way researchers look at whether these calves are getting enough food or not.
They also look to other markers of nutritional stress in body tissues or the whales' baleen - located in a whale's mouth which they use to strain food out of seawater.
The timing of the deaths has both confused and helped efforts to understand what's going on. Baby whales in certain years seem to have died shortly after birth, while in other years, they've died later in the May-December calving season. This suggests that more than one cause may be responsible for these "high mortality" events, the study authors write.
"But the timing of the mortality might give us some clue as to why they died," said Rowntree. If a calf died shortly after birth, that could indicate a wide-scale food shortage problem.
If a calf died later in the year, when the whales start to feed on krill and copepods, then perhaps the young ones are ingesting a toxin or pollutant, she explained.
Analyses of Northern Atlantic right whale populations indicate they ingest algal toxins produced during harmful algal blooms. Krill and copepods are grazers and take in toxins along with the algae they eat. Those toxins, in turn, are transferred to the whales when they gulp down their prey.
"The adults can deal with it," Rowntree said. "But maybe the toxins are too strong for the babies."
Blame It On Birds?
Another possible explanation - supported by Rowntree and discussed at an April 2013 international meeting of veterinarians in Sausalito, California - is harassment by kelp gulls (Larus domincanus).
"They're large gulls and they've learned to feed on the skin they peck from the backs of the whales," she explained. "They make a hole in the skin and they attack it over and over during the season."
Some enterprising birds started the strategy in the early 1980s, and it has since spread to other gulls all over the peninsula.
Although it seems that the mothers have learned how to avoid such pointed attention from the birds, the calves haven't. "So most calves have a chain of lesions along their backs from being pecked by the gulls," Rowntree explained.
It's quite possible that this harassment could contribute to calf deaths around Peninsula Valdés, the animal ecologist said. That's the only big problem these right whales seem to have in common that sets them apart from other southern right whale populations which aren't experiencing such large numbers of deaths.
Watching What They Eat
For now, Rowntree and her colleagues are looking at tracking where these whales feed. There is some evidence suggesting that low prey abundance, specifically Antarctic krill (Euphasia superba), could lead to increases in whale deaths.
Female southern right whales depend on krill to bulk up their blubber reserves before they give birth.
During their calves' first few months of life, the mothers don't eat.
If the mothers aren't storing enough blubber, they may not be able to provide adequate nutrition for their babies.
Whale diets leave specific signatures in their baleen, depending on where in the world the animal fed, Rowntree explained. "If the mothers eat in the Southern Hemisphere, that food [has] a unique signature."
If they can link increases in Peninsula Valdés right whale deaths with low krill abundance, then perhaps they can narrow in on what's causing so many calves to die.
Wait and See
It's still a little too early to tell how all these deaths will affect the population's overall numbers. But it doesn't look good.
Last year, researchers recorded 116 southern right whale strandings in the Peninsula Valdés population. That accounts for nearly three percent of the western South Atlantic stock, the study authors write.
"No other known single-year die-off of baleen whales is as large," the researchers note.
Female southern right whales are sexually mature when they turn nine, Rowntree said. Since the die-off began in 2005, females born that year will be able to start having babies in 2014.
Southern right whale pregnancies last a year. Then it takes a year for the mother to nurse her calf and then another year for her to build up her blubber reserves for the next pregnancy.
So it will be another three years before researchers have a clear picture of how badly the Peninsula Valdés population has been affected.