You’d think, given the hundreds upon thousands of blue whales that – before the advent of commercial whaling – once swam the world’s ocean unconstrained, humans would have managed to document more than just the two blue whale births.

While numbers since the start of commercial whaling have plummeted to around just 10,000 to 25,000 blue whales remaining in the wild today, it has struck many as odd that – at a time of increased research, improved tech, and a growing number of ‘whale spotters’ around the world – that those recorded figures simply haven’t budged.

With such a distinct lack of documentation and in a bid to start increasing our ‘blue whale calf-spotting success rates’, researchers at the University of Washington are seeking to shed new light on why. And it turns out, the problem may rest with us… not with the blue whale.

Trevor Branch, a professor of aquatic and fishery sciences specialising in the study of blue whales at the University of Washington, has a collection of hypotheses he believes could explain the mystery of the missing blue whale calves. 

His study – now published in the scientific journal, Endangered Species Research – all began by sifting through a collection of long-term field studies and historical whaling records, to establish the preferred summer and winter aggregation sites of blue whales.

“My conceptual model can explain the mystery of missing calves: Blue whales produce calves, or give birth, shortly after departing their summer feeding grounds; and wean their calves seven months later – just before they return,” he said.

As a species, the blue whale has a relatively high annual pregnancy rate averaging somewhere between 33 and 50%. Despite this, the sighting rate of mother-calf pairings are at a low average of just 3.1%. 

Researchers know blue whales travel to colder regions in summer to feed on krill, one of these feeding hotspots being off the California coastline. They also know that, by winter – when blue whales tend to give birth – they are off to warmer regions such as the Gulf of California or the eastern tropical Pacific. 

After being born and weaned by their mothers for around seven months, the 16-metre long calf will then sever the metaphorical umbilical cord and begin to make their own way in life.

After looking into many hypotheses, including low fetal survival, low calf survival, calf-mother separation rates, and low birth rates, Branch finally landed on his best hypothesis so far: that researchers are simply observing blue whales at the wrong time of year to actually spot them with their calves.

“This new idea provides an alternative explanation for why some blue whale populations appear to produce very few calves: It’s not a failure of calf production,” he said. “It’s because fieldwork in those populations is understandably concentrated in easily accessible summer feeding areas.”

Yes, it is Branch’s suggestion that researchers observing blue whales in the summertime, when populations gather to feed on krill, will not be spotting mothers with calves, because their timing is all off. It’s a hypothesis that could prove to be transformative for our approach to blue whale spotting and conservation efforts.

In fact, Branch’s research is now being used to coordinate a larger effort to collect field data month by month in each described region to determine whether or not his suggestion is on the money. If correct, we may well start to see those blue whale calf sightings increase.

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