The eastern North Pacific blue whale population has rebounded since being hammered by commercial whaling, according to a new study. And ship strikes, long feared a major obstacle to the recovery in blue whale numbers, likely aren't major threats, the authors conclude.
The study authors used a computer model to estimate the size of the eastern north Pacific blue whale population before commercial whaling decimated their numbers—as well as current abundances and future population trends. They then examined whether the population was more affected by ship strikes or by the environment's ability to support a certain number of whales.
The team found that "there's no immediate population threat to these whales from ship strikes," says Cole Monnahan, a doctoral student at the University of Washington and the study's lead author. Rather, the reason the blue whale population in the eastern north Pacific hasn't increased, he says, is that the ocean off the west coast of North America can't support any more.
There are now about 2,200 blue whales in the eastern north Pacific, Monnahan says. And that number hasn't changed appreciably since 1993. "That's why people thought something was going on and that maybe ship strikes might be having an effect [on blue whale numbers]," he explained. (See "Blue Whale 'Hot Spots' Overlap With Shipping Lanes, Raising Threats.")
The researchers point out that they looked at only the eastern north Pacific blue whale populations. Their results in no way reflect a global recovery of blue whales. In fact, a 2013 study pegged historical blue whale population numbers in the waters around Antarctica at nearly 300,000. Recent estimates suggest that there are about 2,000 left in that southern population.
Not So Fast
The problem, says John Calambokidis, a senior research biologist at the Cascadia Research Collective in Olympia, Washington, is that "these types of models are based on the assumption that populations aren't experiencing other threats, like ocean warming."
The study's conclusions are a bit of a stretch, Calambokidis says, because the data on whale numbers is shaky. Historical catch numbers from commercial whaling fleets are notoriously underreported, as are estimates of how many blue whales are killed each year by ship strikes, both data sets that researchers fed into their model.
The study acknowledges these limitations, he adds, but does a poor job of addressing them in the computer model. That could "potentially contribute to false conclusions about ship strikes not being a threat to blue whales," Calambokidis says.
Based on the numbers in the study, it is possible that blue whale numbers have recovered in the eastern north Pacific population, the biologist acknowledges. But he is hesitant to say that even this particular population of whales has recovered. (Read about tracking blue whales in National Geographic magazine.)
Bruce Mate, director of the marine mammal institute at Oregon State University, adds another problem with the study. Its assumption that the carrying capacity—an environment's ability to support a certain number of animals—hasn't changed from the early 1900s to now is also a problem.
He cites his work studying gray whales in the Bering Sea between Alaska and Russia as an example of how quickly a habitat's carrying capacity can change.
Gray whales used to be concentrated in the Bering Sea about ten years ago, feeding on seafloor creatures. But "today, it's turned around," Mate says. Now, they're all north of the Bering Sea, he notes, because "they ate themselves out of house and home."
"The environment [today] isn't the same environment it was a century ago," Mate says, and the way that "the animals repopulate this environment may be quite different."
Monnahan was also surprised when he saw the computer model's results, and he is anxious to see how the research and conservation community receives the study. "If someone wants to challenge [the paper], we'd be more than happy to share our data," he says.
For now, Monnahan and colleagues are aiming to broaden their blue whale studies to the entire north Pacific. The eastern north Pacific blue whale population is, in some respects, the easiest to study, he says. "We know much less about blue whale populations in the western Pacific around Hawaii, Japan, and Korea."
Commercial whaling fleets caught many more blue whales in the western Pacific, and those historical numbers have been harder to get a good handle on, Monnahan explains.
"Basically, [we want to] compile all the data on blue whales in the north Pacific to learn more about their ecology," he says. (Learn more about blue whales through this interactive.)
For now, in regard to this current study, "all we're saying is that there's no immediate population threat to [blue] whales from ship strikes," Monnahan says.
Follow Jane J. Lee on Twitter.