Blue whales rebound after near extinction, study finds

A new analysis suggests there are as many blue whales living off the coast of California as there were before humans started hunting them to near extinction 110 years ago.

There are about 2,200 blue whales swimming on the eastern side of the Pacific Ocean, from Mexico up into Alaska.

Researchers previously assumed that the pre-whaling population was higher than that. However, researchers at the University of Washington, using historical data to estimate the number of whales caught between 1905 and 1971 – when whaling became illegal – said the current population is 97 percent as large as it was before 1905.

"For us, this is a great conservation success story," said Cole Monnahan, a lead author of the paper published Friday in the journal Marine Mammal Science. "We caught way too many whales from this population. But when we left them alone, they recovered."

Blue whales are found throughout the world. But for this study, the researchers looked at the population ofblue whales that are most visible while at feeding grounds off the California coast. They took into account recently released numbers of whales caught by Russian whalers, and used acoustic calls produced by the whales to distinguish the California blue whale population from those caught in the northern Pacific near Russia and Japan.

They determined that approximately 3,400 California blue whales were killed during the period studied.

"Considering the 3,400 caught in comparison to the 346,000 caught near Antarctica gives an idea how much smaller the population of California blue whales was likely to have been," said Trevor Branch, an assistant professor of aquatic and fishery science at the university.

The researchers noted that even though around 11 blue whales get struck by ships a year along the West Coast, their population growth has leveled off in recent years. They suggest that means the population may be almost back to normal.

"Before this study some people thought that number should be going up. But if there were about 2,200 whales to begin with, then that is what the environment can support," Monnahan said.

The Monitor's Pete Spotts explored the researchers' methods in a recent article.

The team producing the recovery estimate for blue whales in the northeastern Pacific based their tally on previous research they conducted – the first study to analyze past catch reports and separate them into northeastern and northwestern Pacific populations.

To do that, the team, led by University of Washington PhD candidate Cole Monnahan, used whale calls unique to each broad group to trace the paths and timing of their migrations, then applied the results to the most recent updates to historical catch reports. They estimated that between 1905 and 1971, whalers took 6,362 blue whales from the northwestern population and 3,411 from the northeastern group. The team published those results in the journal PLoS One in June.

By deriving an annual average catch during that period and combining it in a population model, Mr. Monnahan and colleagues found that today's northeastern Pacific population represents 97 percent of their estimate for pre-1905 levels.

The study demonstrates “the ability of blue whale populations to rebuild under careful management," the team notes.