California blue whales are back to pre-whaling levels

Conservation science is an endless cycle of hope and despair. But there’s a little bit more hope now, because blue whales off the coast of California appear to be doing quite well: they’ve recovered, it appears, from a century of systematic whaling.

Between 1905 and 1971, almost ten thousand individual blue whales were harvested from the Pacific Ocean. But then, starting in 1971 when whaling was outlawed, the population should have recovered. But abundance estimates in the eastern Northern Pacific (ENP) – that is, off the western coast of the United States – suggest that there hasn’t been an increase in blue whale numbers. What could be preventing the ENP population from increasing, now that it is no longer under the threat of hunting? There are two possibilities, according to University of Washington researchers Cole C. Monnahan, Trevor A. Branch, and André E. Punt. One is that there’s another source of mortality, likely of human origin, which is killing off blue whales. Another is that the population hasn’t grown simply because there isn’t any extra room for more of Earth’s most massive animals ever. In other words, the blue whales’ ecosystem could be at or approaching its carrying capacity.

The only known direct source of anthropogenic mortality for blue whales is collisions with ships, which are called ship strikes. (Other, indirect sources include pollution, overfishing, and noise.) But there have only been an average 1.8 ship strikes each year in the most risky area for ENP whales, off of Santa Barbara, California, plus an additional two ship strikes off the coast of Washington for the quarter century leading up to 2005. “These are all the known blue whale ship strikes recorded on the west coast of the United States during this period,” write Monnahan, Branch, and Punt. However, there are other strikes that are likely unreported or misidentified. Or sometimes a whale might be injured but not killed by a strike, only to later die of internal injuries. Sometimes a carcass might sink before even being noticed. The massive mammals, after all, are tiny in comparison to some of our largest ships. Another study on North Atlantic right whales found that carcass are only ever detected after ship strikes 17% of the time. If that applied to the Santa Barbara Channel, then we could actually be killing an average 10.6 blue whales each year – far more than the legal “potential biological removal” of 3.1 whales per year.

So which is it? Have we simply replaced whaling with unintentional ship strikes, or are blue whales at their maximum capacity already? That’s what Cole, Branch, and Punt wanted to find out.

They discovered that while ship strikes are probably occurring at rates above the legal limits, they “do not immediately threaten the status of the ENP blue whale.” Despite ship strikes, the researchers found that the population has probably increased since at least 1993, and that growth has slowed due to density dependence. Other environmental factors are limiting the growth of the population, such as the amount of food available. Still, that doesn’t mean the blue whales are home free. If shipping fleets expand and ship strikes increase, that could still pose a problem.

But the story is overall a hopeful one. It suggests that despite seventy years of systematic commercial whaling, at least one population of blue whales has recovered to the extent that its ecosystem allows, and appear to be under “no immediate threat from ship strikes,” write the researchers. (Which is not to say that there aren’t other economic or moral reasons to minimize ship strikes.) The Antarctic population had an abundance less than one percent of its carrying capacity according to a 1996 estimate, and the Chilean population was doing slightly better at 7.2 percent. Therefore the ENP blue whales are the only ones to have recovered from whaling, with abundance around 97% of its capacity. What it means is that “an exploited population of blue whales can recover given appropriate conversation measures and time.” – Jason G. Goldman | 10 September 2014

Source: Monnahan C.C. & André E. Punt (2014). Do ship strikes threaten the recovery of endangered eastern North Pacific blue whales?, Marine Mammal Science. DOI: http://dx.doi.org/10.1111/mms.12157

Header image: California blue whales — the cow is 76 feet long and the calf is 47 feet — swim near the California Channel Islands. Gilpatrick/Lynn/NOAA

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