Despite being members of the largest species on the planet, young blue whales have long managed to elude human detection.

In fact, just two blue whale births have ever been documented — off South Africa in 1911, and off Sri Lanka in 1946.

And sightings of calves are remarkably rare, falling far below what should be expected based on pregnancy rates. For example, just 35 were documented in the Gulf of St. Lawrence over a 45-year period, making up less than 2% of total sightings.

What’s responsible for this puzzling phenomenon of “missing” calves? Are they spending their youth in the deep ocean, where researchers can’t detect them? Are they dying prematurely?

There might actually be a simpler explanation: human error.

In a study published Feb. 20 in the journal Endangered Species Research, University of Washington whale researcher Trevor Branch hypothesized that the calves go largely undocumented because scientists tend to conduct research in the summer, when births are not taking place.

While the whales gather in large numbers in the summer, they typically produce calves in the winter and fall, when few researchers are around to watch, he said.

“My conceptual model can explain the mystery of the 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,” Branch said in a university news release.

In order to make this hypothesis, Branch assembled data from longitudinal studies and old whaling records.

He also examined other hypotheses to account for the “missing” calves — including low birth rates and low calf survival rates — but these were not as compelling.

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

Blue whales, which can grow up to 110 feet long and live up to 90 years old, are found in every ocean except the Arctic, according to the National Oceanic and Atmospheric Administration.

Their populations have been significantly depleted by whaling, and there are now estimated to number between 10,000 and 25,000 individuals.

_____

©2025 The Charlotte Observer. Visit charlotteobserver.com. Distributed by Tribune Content Agency, LLC.