The California blue whale was once hunted to near extinction with about 3,400 of these marine mammals caught from 1905 to 1971. A new study, however, has shown that the animal's population, which has significantly declined because of whaling, has bounced back to near its pre-slaughter level albeit there is one threat that appears to pose a problem with the whale's rebounding number.

For the new study published in the Marine Mammal Science on Sept. 5, Cole Monnahan, from the Quantitative Ecology and Resource Management at the University of Washington in Seattle, and colleagues wrote that the population of the Eastern North Pacific (ENP) blue whale, which is estimated at 2,200, is nearing its size before it was aggressively hunted and the species is the only blue whale population that is known to have recovered from whaling.

Despite the promising growth, there are factors feared to affect the California blue whale's rebounding number. Conservation groups, for instance, claim that there are at least 11 blue whales that get struck along the U.S. West Coast per year, which is almost four times more than the annual "potential biological removal" of 3.1 whales allowed under the U.S. Marine Mammal Protection Act, and the researchers wanted to assess whether this collision with ships poses problem to the recovery of the endangered species.

Using a population dynamic model, the researchers found that ship strikes minimally threaten the whale's long term population. An 11-fold increase in the number of vessels also poses only 50 percent risks that the animal's population will be reduced to what regulators consider as "depleted."

"Results suggest density dependence, not ship strikes, is the key reason for the observed lack of increase," the researchers wrote. "We also estimate future strikes will likely have a minimal impact; for example, an 11-fold increase in vessels would lead to a 50% chance the long-term population would be considered depleted."

The researchers, however, said that regardless that ship collisions do not pose serious threats to the animal's population, it is still preferable that the whales do not get killed because of ship strikes.

"Even accepting our results that the current level of ship strikes is not going to cause overall population declines, there is still going to be ongoing concern that we don't want these whales killed by ships," said study co-author Trevor Branch, from the School of Aquatic and Fishery Sciences at the University of Washington.

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