The study authors hypothesize an explanation: The reduced availability of krill during the summer feeding season is causing pregnant mothers to abort and calves to die.
Sea Surface Temperatures
In their study, scientists compared sea-surface temperatures in the southwest Atlantic to their index of the yearly calving success of whales that breed off the Argentine coast.
Researchers found a strong correlation between the number of right whale calves born and changes in sea-surface temperature in the autumn of the preceding year.
Other experts say the study results are convincing. Charles H. Greene, professor of earth and atmospheric science at Cornell University in Ithaca, New York, said: "The authors provide compelling evidence that South Atlantic right whale calving rates are correlated with climate variability, once one takes into account the appropriate time lags."
The study authors were able to chart the sea-surface temperature against whale calf output for the years 1983-2000, and the results were clear: As the water temperatures rise from the norm, calf output declines.
According to the scientists, it doesn't take much warming to affect the species. Even small changes in the oceanographic conditions in the Southern Ocean, the circumpolar sea around Antarctica, could affect southern right whale population dynamics.
While the authors concede that there is limited data on the diet of southern right whales, it is highly probable that their main food is krill.
But sufficient data probably don't exist to tie krill concentrations in the southwest Atlantic firmly to whale reproductive success, the authors say.
Greene agrees. "The key missing link is still prey abundance," the factor that will link ocean temperature changes and calving rates, he said.
"The role of krill in this story," he continued, "must be worked out to develop a truly predictive understanding of climate impacts on predator populations in the South Atlantic."
The authors strongly suspect, however, that there is an inverse relationship between krill density and sea surface temperature. The warmer the water gets, the less krill there are.
Species other than right whales may also be affected.
Previous data support a significant relationship between sea-surface temperature and the breeding success of krill-loving gentoo penguins. Antarctic fur sealsanother krill predatoralso demonstrate poor pup survival when sea surface temperatures rise.
The three species don't consume krill in quite the same way, notes Leaper, of the International Fund for Animal Welfare.
"During the breeding season, the fur seals and penguins are constrained to forage close to their land-based breeding colonies. So they are dependent on krill availability in a small area," he said.
"For whales, it may be more important to reach a certain density within swarms of krill at which feeding becomes energetically efficient." But in any case, he added, "to a large extent, things revolve around krill."
Human consumption of krill may be a problem as well, according to Jon Seger, professor of biology at the University of Utah.
"The Southern Ocean krill harvest is currently unregulated," said Seger, who was not involved in the study.
"And many biologists are becoming concerned that human exploitation of krill may negatively affect not just right whales, but also many other species that depend heavily on this special resource."
Cooked and frozen krill meat is marketed as a nutritional organic seafood product and is sometimes used in health food supplements.
Leaper says the new study, which is described in the current online issue of the journal Biology Letters, has implications for whaling policy as well.
Commercial whaling has been banned since 1986 under a moratorium imposed by the International Whaling Commission.
But Japan continues to hunt certain whale species, saying it's done for scientific research.
Quoting the Japanese government, Leaper said that their motivation for scientific whaling is "to take into account species interaction [ecosystem] effects in understanding the dynamics of the baleen whale species in the Antarctic ecosystem and predicting future trends in their abundance and population structure."
But if that is their aim, Leaper says, hunting and killing whales is not the best strategy. "Our results would indicate that long-term studies of live whales are much more likely to achieve this objective than the Japanese whaling program."
Seger, the University of Utah biologist, endorses the idea of tracking individual living animals. "You wouldn't see a thing if you were just counting mothers and calves, without knowing who they are," he said.
"But when you see some of the females you've been following for years suddenly shift out of their usual three-year calving cycle, then you can easily see the effect, even if most females carry on as usual," he said.
"So to my mind, this is a spectacular demonstration of the enormous, but underappreciated, value of long-term photo-identification studies of natural populations."
Free E-Mail News Updates
Sign up for our Inside National Geographic newsletter. Every two weeks we'll send you our top stories and pictures (see sample).
SOURCES AND RELATED WEB SITES