9/5/14
The blue whale, the biggest animal on the planet, was hunted with abandon in the Pacific Ocean until the early 1970s. The species has been rebounding ever since, but a slowdown in the growth of the population frequenting waters off the California coast was a concern. Now it turns out to be a promising sign of recovery.
Scientists at the University of Washington have just published research finding that the West Coast blue whale population of around 2,200 individuals appears to be approaching its pre-slaughter size, with the slowing growth a function of the carrying capacity of the marine ecosystem. Collisions with ships remain a problem, the scientists write, but should not affect the whales’ prospects.
The paper — ”Do ship strikes threaten the recovery of endangered eastern North Pacific blue whales?” — was posted online today by the journal Marine Mammal Science. Here’s the core of the abstract:
We used a population dynamics model to assess the trends and status of ENP [eastern North Pacific] blue whales, and the effects of ship strikes. We estimate the population likely never dropped below 460 individuals, and is at 97% of carrying capacity (95% interval 62%–99%). These results suggest density dependence, not ship strikes, is the key reason for the observed lack of increase. 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. Although we estimate ship strike mitigation would have minimal impacts on population trends and status, current levels of ship strikes are likely above legal limits set by the U.S. The recovery of ENP blue whales from whaling demonstrates the ability of blue whale populations to rebuild under careful management.
As the university news release noted, it’s important to keep in mind that the California recovery is a tiny bright spot given that researchers estimate (from other work) that the 3,400 whales killed in that population from 1905 to 1971 pale beside the 346,000 harpooned in Antarctic waters in the same span.
The National Oceanic and Atmospheric Administration estimates that the Southern Hemisphere population was 175,000 before the whaling binge and is about 2,000 today.
One of the paper authors, the doctoral candidate Cole Monnahan, has a fascinating piece on the North Pacific whale research on a great new blog he maintains with another author, Trevor Branch, called Blue Whale News. In the post, Monnahan stresses the big questions that remain about the more heavily hunted blue whale populations in the western North Pacific. Here’s the kicker:
So what do we know about the population structure in other areas? In the western North Pacific, we know there is at least one population that spends a significant portion of their time, including the mating season, too far west for our hydrophones to hear them. The truth is no one knows what populations exist (or existed) in the western North Pacific. Thousands of whales were caught off Japan early in the 1900s, but none have been seen since. Was that a population that was killed off forever? Where do the blue whales go during the winter months to breed? These are exciting questions for future researchers to tackle.
To get a sense of the mix of whaling-era data, tracking and modeling used to estimate past blue whale abundance, read this PloS ONE paper by an overlapping research team from last year: “Estimating Historical Eastern North Pacific Blue Whale Catches Using Spatial Calling Patterns.”
