Cape Town - There is something symbolic about its name, even if it’s mainly an approxi-mation of its skin colour: the majestic blue whale is the largest animal still gracing the Blue Planet, and as far as is known it’s also the heaviest creature ever to have existed.
Weighing up to somewhere between 170 and 190 metric tons, it dwarfs its largest land cousins that have been found as fossils: the enormous herbivorous titanosaurs like the Argentinian giant, Argentinosaurus, of some 95 million years ago whose weight is variously estimated at between 73 and 90 metric tons.
The blue whale can reach 32m in length; its tongue alone can weigh as much as an elephant; and its heart is as heavy as a small car. An adult animal can consume 3.6 metric tons of shrimp-like krill a day.
But the staggering size and beauty of the iconic ocean creature didn’t stop humans from massacring it – in fact, its huge body mass just made it all the more desirable as a target, and during the commercial whaling era the species came close to being wiped out.
Now researchers have found that at least one small population of blue whales – off the west coast of the US – has recovered to nearly the full extent possible, coming close to the “carrying capacity” of this particular marine habitat.
“It’s a conservation success story,” says Cole Monnahan, a University of Washington doctoral student and lead author of a paper posted online recently by the journal Marine Mammal Science.
The species was a major target of whalers from about 1919, with hunting peaking between 1925 and 1935 and lasting until 1962, when the species was commercially extinct. Numbers worldwide could have dropped to as low as only between 5 000 and 12 000, located in at least five groups.
In 1932, whaling nations adopted the Blue Whale Unit as a measure for quotas awarded.
In 1963, the scientific committee of the International Whaling Commission (IWC) recommended a stop to the hunting of blue and humpback whales, but it was only two years later that the IWC formally protected the blues, and it was more than two decades later – in 1986 – that the IWC moratorium on all commercial whaling came into effect. (The Soviet Union and Korea stopped the following year.)
Before commercial whaling started, the blue whale population in the southern hemisphere has been estimated at about 239 000 (with a range of between 202 000 and 311 000), mainly around Antarctica.
According to the IWC’s website, blue whales here were hunted until their population had been reduced to just a few percent of the historical unexploited stock size: in 1997/98, to just 2 300 (the estimated range was 1 150 to 4 500).
Blue whales in the North Atlantic and North Pacific Oceans of the northern hemisphere were similarly also exploited heavily.
Now, the new research by Monnahan and colleagues has shown that the number of California blue whales has rebounded to near historical levels – the only population of blue whales known to have recovered from whaling.
And while the number struck by ships is probably above legislated limits for US waters, such strikes do not immediately threaten their recovery, they say in a press release from the university about their paper.
One of the co-authors, UW assistant professor of aquatic and fishery sciences, Trevor Branch, describes the blue whales as “an icon of the conservation movement”.
California blue whales are at their most visible while at feeding grounds some 30km to 50km off the California coast, but are found along the eastern side of the Pacific Ocean from the equator up into the Gulf of Alaska. Today there are about 2 200 of these whales, and the new research puts that figure at about 97 percent of the historical level.
That may seem to some a low number, Monnahan says, but not in the context of how many California blue whales were actually killed. According to data that Monnahan, Branch and other co-authors published earlier this year in the journal PLOS ONE, about 3 400 California blue whales were caught between 1905 and 1971.
This relatively low number compared to the tens of thousands harpooned near Antarctica gives an idea how much smaller the population of California blue whales was likely to have been, says Branch.
Blue whale catch figures from the North Pacific were unknown until scientists – in particular Yulia Ivashchenko of Southern Cross University in Australia – teased out numbers from previously secret Russian whaling archives that are now public.
For their work published in PLOS ONE, the scientists then used acoustic calls produced by the whales to distinguish, for the first time, the catches taken from the California population and those hunted in the western Northern Pacific near Japan and Russia. The two populations are generally accepted by the scientific community as being different.
These catch data were among the key pieces of information used in their latest research to model the size of the California blue whale population over time.
The scientists say their theory that the population is returning to near its historical level better explains the slowing population growth that’s been recorded in recent years, rather than the impact by a growing number of ship strikes.
Other research has suggested that at least 11 blue whales are likely to be struck by ships and probably die each year along the US west coast, which is above the “potential biological removal” of 3.1 whales per year allowed by the US Marine Mammal Protection Act. But the new findings suggest there could be an elevenfold increase in vessel strikes before there is a 50 percent chance that the population will drop below what is considered “depleted” by regulators.
Of course, the researchers don’t want any whales to be killed by ships, Branch says.
“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.
“California blue whales are recovering because we took actions to stop catches and start monitoring. If we hadn’t, the population might have been pushed to near extinction – an unfortunate fate suffered by other blue whale populations.”
Sunday Argus