Given the variable climate and ecology of the waters where the whales live and a trend of higher sea-surface temperatures, "the survival of these right whales is uncertain," Tynan and her co-authors concluded in the Science report.
Another study, published this week in the journal Nature, looked at endangered North Atlantic right whales.
The study concluded that based on a statistical analysis of 10,000 sightings of individual right whales, saving as few as two female North Atlantic right whales each year could save the species from extinction and maintain the population at its current level. Saving more of the animals could put the species on the road to recovery.
Right whales, which currently number between 300 and 350 individuals, are one of the world's most endangered species. New mothers seem to be particularly at risk.
The major causes of right whale deaths in recent years have been collisions with ships and entanglements in fishing gear, as the animals struggle to navigate an increasingly "urban" ocean.
"This year six right whales have already been losttwo adult males and four calves," said Scott Landry of the Center for Coastal Studies in Provincetown, Massachusetts, and a member of the Whale Rescue Team.
The two adult whales died after becoming entangled; two of the calves were killed when hit by ships, and the remaining two were in poor condition when they died although the causes of their deaths are still being investigated.
The right whale migration route follows the North American coastline, extending from feeding grounds around the Bay of Fundy southward to calving grounds of southern Florida. The route intersects with busy industrial and military shipping routeswith dire consequences.
"In 1984 we recognized that there was a problem with whale entanglements and we formed a whale rescue team," said Landry. Since 1984, there have been 57 whale entanglements reported along the East Coast, about half of them involving right whales.
A previous study suggested that the population of right whales has plummeted because the tiny population was dispersed over a very large region and the females had trouble finding male partners. The new study refutes this hypothesis.
Masami Fujiwara and Hal Caswell of the Woods Hole Oceanographic Institute in Massachusetts think that females simply do not live long enough to have multiple calves. In 1980 the average life expectancy of a female right whale was over 50 years; by 1995 that figure had dropped to less than 15.
Female right whales take about a decade to mature sexually, and then they can produce a calf only every three to five years. In 1980 females lived long enough to reproduce more than five times. Now, most females rarely produce more than a single calf during their lifetime.
Mother whales and their calves may also be more vulnerable, said Furiwara, an author of the new report, which appears in the November 28 issue of Nature.
Females that have just given birth are strained by the energy requirements of motherhood and are more susceptible to infections caused by any kind of injury, Furiwara explained. Also, new calves are not able to dive deep, which forces a mother and her calf to stay close to the surface, and in the path of ships.
"Both Hope and Despair"
In the 10,000 right whale sightings on which the report was based, Fujiwara and Caswell used a statistical method to weigh birth and death rates in an effort to determine why the plight of right whales has worsened so dramatically in the last two decades.
Almost every member of the North Atlantic right whale population is listed in a catalog with a serial number and photograph showing the distinctive black and white coloring on the animal's body. This helps scientists track an individual whale's life history. Scientists have also compiled genetic profiles for about 75 percent of the species by collecting skin samples.
Peter Kareiva of The Nature Conservancy in Seattle, Washington, wrote in a related commentary in Nature that the study provided "both hope and despair."
"A population that was thought by many to be doomed because of terribly low numbers can probably be saved," he said, adding that a few human-induced deaths could tip the balance towards the species' demise.
Kareiva said: "The conservation of a single animal has never before been so compellingly clear."
Landry said the study is heartening "because people are getting so pessimistic about this species that they are willing to give up." On the other hand, saving just two whales is not enough to remove the animals from the danger zone, he warned. "We are going to need to save many more," he said, "if the population is to grow and expand."
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