Unravelling the mystery of the missing blue whale calves

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Only two blue whale births have ever been recorded in human history, both decades ago. This remains an extraordinary mystery given there used to be hundreds of thousands of blue whales before whaling started (even now they number around 10,000-25,000­)—and they give birth every two to three years.

Not only are births very stealthy, but calves are also only rarely sighted—far less than would be expected from their pregnancy rates. Calves closely follow their moms and are sighted as mother-calf pairs, but why are so few detected?

A new University of Washington study, published Feb. 21 in Endangered Species Research, proposes why. Its explanation hints at when and where the unseen births are happening and where blue whale calves spend their earliest months. The findings offer some hope for the health of the population.

Two blue whales - one adult and one calf - pictured from above in a blue ocean.
Diane Gendron/Centro Interdisciplinario de Ciencias Marinas - Instituto Politécnico Nacional
A blue whale mother and calf swim together in the Gulf of California in Baja, Mexico, one of the warm-water places blue whales spend their winter months.

Trevor Branch, a UW Professor of Aquatic and Fishery Sciences who studies blue whales, set out to unravel this mystery by looking at a range of hypotheses. He proposes that one in particular is the best explanation: it’s mostly because researchers prefer summertime research on feeding congregations of blue whales, but calves are born in fall and winter, and are weaned before they return to feeding areas.

In summer, blue whales migrate to feed in colder regions where krill is plentiful: for example, off California. In winter, when ready to give birth, they return to warmer regions like the Gulf of California and the eastern tropical Pacific. Around seven months after being born, and already at a whopping 52 feet (16 meters) long, the calves are weaned and stop associating with their mothers.

But across various blue whale populations, high pregnancy rates of 33-50% annually seem to contradict the average 3.1% rate of sightings of blue whales involving mother-calf pairs.

The very top of two blue whales are pictured side on in the ocean, using their blowholes. Low mountains can be seen on the horizon with white clouds above.
Leigh Torres/ Geospatial Ecology of Marine Megafauna Laboratory (GEMM) Lab, Marine Mammal Institute, Oregon State University
A blue whale mother and calf swimming together in the South Taranaki Bight, New Zealand, a rare summer feeding region with many mother-calf pairs.

When compared with other hypotheses to explain the mystery of why so few calves are observed, such as low fetal survival, low calf survival, low birth rates, or calf separation from mother, Branch discovered that the timing hypothesis best explained observed patterns.

“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.

This would explain why researchers – most of whom conduct blue whale field studies in summer months – seldom sight mothers with calves.

Branch compiled data from long-term field studies and combined this with biological information from historical whaling records to come up with this hypothesis, finding higher proportions of calves in winter regions, and lower proportions in summer regions. He is now coordinating a large collaboration to test the idea with field data by month in each region, combined with estimates of the size of calves by month.

Pictured from above are two blue whales - one adult and one calve - swimming close together through a dark grey sea.
Leigh Torres/ Geospatial Ecology of Marine Megafauna Laboratory (GEMM) Lab, Marine Mammal Institute, Oregon State University
Aerial footage showing the close association between blue whale mother and calf, in the South Taranaki Bight, New Zealand.

One concern about low calf sighting rates was that this might be a warning signal of low birth rates or low survival of calves. Instead, the new hypothesis offers up some hope that higher number of calves could be sighted from field studies concentrated in regions that blue whales travel to in winter and spring.

“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, it’s because fieldwork in those populations is understandably concentrated in easily accessible summer feeding areas,” Branch said.

The research was funded in part by the International Whaling Commission’s Southern Ocean Research Partnership (IWC-SORP).

For more information, contact Trevor Branch at tbranch@uw.edu.  

Ghostly flight species of baleen whales avoid attracting killer whales by singing too low to be heard

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Killer whales are the only natural predator of baleen whales – those that have “baleen” in their mouths to sieve their plankton diet from the water. More solitary than toothed whales, baleen whales face predatory attacks from killer whales, especially mother and calf pairs. When attacked, some species fight back, while others choose flight. But whale species also produce loud underwater songs…what stops killer whales from homing in on their calls and attacking them?

In new acoustic research conducted by Trevor Branch, a Professor in the University of Washington School of Aquatic and Fishery Sciences, he found that some baleen whale species call at such deep frequencies that they’re completely undetectable by killer whales—who cannot hear sounds below 100 Hz. These tend to be the whale species that flee in the face of attack. Meanwhile, their high-frequency singing brethren who fight back when attacked, also tend to be slower-moving and more maneuverable. The deep singers are in the flight club and include blue, fin, sei, Bryde’s and minke whales, while the fight club includes right, bowhead, gray and humpback whales. Branch’s research was published in Marine Mammal Science on Jan 31. 2025.

Two dorsal fins of killer whales pictured in the ocean, charging a blue whale calf with visible killer whale toothmarks like a rake and missing chunks of flesh.
P. Markovic, CETREC, Western Australia
Killer whales charging next to a blue whale calf with visible killer whale toothmarks like a rake and missing chunks of flesh. Location: Bremer Bay, Western Australia.

The fight or flight hypothesis is not new, but research into acoustics is shedding new insights into the behavioral, morphometric, and ecological adaptations of baleen whales. Could this so-called acoustic crypsis, where whales that call at such deep frequencies that they are acoustically invisible to killer whales, have developed as a defense mechanism from attack?

Killer whales are found in every one of the world’s oceans, and their prey ranges from small fish to the largest whales on Earth. The fight species of baleen whales usually migrate and calve closer to the coast in shallow water, a haven of sorts which provides easier defense against killer whale attacks—especially for group defense in aggregations. Combined with their slow-swimming and more navigable bodies, their communication with other whales is often at higher frequencies easily heard by killer whales—above 1500 Hz. In contrast, flight species have streamlined and slender bodies adapted for speed, and typically disperse across wider open ocean regions for mating and calving, where they are able to flee in all directions.

These behaviors also have implications for feeding and mating. Denser congregations in shallow coastal areas leaves less food for fight species, in comparison to the open ocean favored by flight species. However, the opposite is true for finding a mate—it’s easier when you’re all in a similar location, versus spread out over long distances. Where do acoustics fit into this picture?

Singing is a fundamental part of mate attraction and selection for whales. Males of the flight species sing in a way that maximizes the number of females that hear them, producing simple and repeated songs to attract a potential mate, and singing over prolonged periods to allow females to track them down. “But these super loud songs could expose them and their mates to killer whale attack. And this is where acoustic crypsis comes in: singing at low frequencies that are impossible, or very difficult, for killer whales to hear,” Branch said.

A close up view of the mouth of a blue whale is pictured, with the side profile of a killer whale shown entering the blue whale mouth (to feed on its tongue after an attack). Pictured in a blue ocean with choppy white water around the whales.
J. Daw, CETREC, Western Australia
After a killer whale pod kills a blue whale calf, one dives into the mouth of the blue whale to feast on its tongue. Location: Bremer Bay, Western Australia.

Branch conducted a review of aquarium experiments on killer whale hearing ranges, reviewed the source frequency and source level of populations of all baleen whales, and combined these with knowledge of how sounds move through the ocean, to predict which whale populations can be easily heard by killer whales. It turns out that flight species generally can’t be heard more than 1 km away by killer whales, unlike the calls of fight species.

The research shows that under the sea there is a sound landscape governed by fear, with some whale species choosing to sing their songs to their prospective Valentines at deep levels to avoid attacks; while other whale species compete to sing the most varied and interesting songs, and fight back when attacked. The fight vs. flight differences appear to drive all aspects of the lives of baleen whales, from where they are found, to their communication, to where and when they breed and feed.

Branch said: “It just never occurred to me that some whales sing low to avoid killer whales, but the more I looked at this, the more I realized that every aspect of their behavior is influenced by the fear of predation.”

For more information, contact tbranch@uw.edu

One or many? Exploring the population groups of the largest animal on Earth

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Hunted nearly to extinction during 20th century whaling, the world’s largest animal, the Antarctic blue whale, went from a population size of roughly 200,000 to little more than 300. The most recent abundance estimate in 2004 put Antarctic blue whales at less than 1% of their pre-whaling levels.

But is this population recovering? Is there just one population of Antarctic blue whales, or multiple? Why do these questions matter for conservation?

Zoe Rand, a PhD student in the UW Quantitative Ecology & Resource Management program (QERM), is tackling these questions in a new study, published in Endangered Species Research on November 14, 2024. Building on the last assessment of Antarctic blue whales in 2004, and using old whaling records which were surprisingly detailed, Zoe has been investigating if there are different populations or one big circumpolar population. Also involved in the study is UW School of Aquatic and Fishery Sciences (SAFS) Professor and Faculty for the QERM Program, Trevor Branch, and Jennifer Jackson from the British Antarctic Survey.

Paula Olson
Antarctic blue whales are the world’s largest animal, and are still recovering from being hunted nearly to extinction during 20th century whaling.

Antarctic blue whales are listed as an endangered species, and understanding their population structure is essential for their conservation. Conservation at the population-level increases biodiversity, and this diversity helps the species adapt better to environmental changes and increases chances of long-term survival.

During the whaling years, biologists began the Discovery marking program. Foot-long metal rods with serial numbers were shot into the muscles of whales. When these whales were caught, the metal rod was returned, and information about the whale’s size, sex, length, and where they were caught, was collected. Looking at where whales were marked compared to where they were caught can shed valuable insight into the movement of Antarctic blue whales, but this data has never been used before to look at population structure.

International Whaling Commission
Historical mark that was placed in the muscles of whales and then recovered during whaling.

In this new study, this historical data was used alongside contemporary survey data in Bayesian models to calculate inter-annual movement rates among the three ocean basins that make up the Southern Ocean (Atlantic, Indian, Pacific), which are the feeding grounds for Antarctic blue whales. They found frequent mixing among the ocean basins, suggesting that whales do not return to the same basin every year. This points to Antarctic blue whales being one single circumpolar population in the Southern Ocean.

These results are consistent with evidence from Antarctic blue whale songs, heard throughout the Southern Ocean. Only one song type has been recorded amongst the Antarctic blue whales. In comparison, pygmy blue whales have five different songs corresponding to five different populations. These results are also consistent with genetic studies which found that Antarctic blue whales are similar genetically.

Paula Olson
Antarctic blue whales are listed as an endangered species, and understanding their population structure is essential for their conservation.

This is the first time that historical mark-recovery data from the Discovery marking program has been analyzed using modern quantitative methods. This data exists for many other hunted whale species, such as fin and sei whales, so it could provide a framework for similar analyses for those whale species too.

There is still a lot we don’t know about the Antarctic blue whale. Acoustic data and their movement on the feeding grounds, suggests there is just one population in the Southern Ocean. Even though they do not appear to be separated geographically on their feeding grounds, they could still have population structure because of differences in breeding habitats or the timing of migration. However, almost nothing is known about Antarctic blue whale breeding behavior. Using historical data from whaling alongside contemporary data such as satellite tagging, and photo-identification is our best hope for uncovering the secrets of the largest animal on earth.

Global ocean fish populations could increase while providing more food, income

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“If reforms were implemented today, three-quarters of exploited fisheries worldwide could reach population goals within 10 years, and 98 percent by mid-century,” according to a report in PNAS co-authored by SAFS Professors Ray Hilborn, Trevor Branch, and Research Scientist Mike Melnychuk.

School of Fish – NOAA

Impacts of ocean acidification on marine seafood

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Impacts of ocean acidification on marine seafood

Trevor Branch (SAFS), Liza Ray (SAFS), Bonnie DeJoseph (SEMA), and
Cherie Wagner (SMEA)

A review of the effects of ocean acidification that arose from
graduate student participants in the 2011 Bevan Series on Sustainable
Seafood has just been published in Trends in Ecology and Evolution.

Abstract
Ocean acidification is a series of chemical reactions due to
increased CO2 emissions. The resulting lower pH impairs the senses of
reef fishes and reduces their survival, and might similarly impact
commercially targeted fishes that produce most of the seafood eaten by
humans. Shelled molluscs will also be negatively affected, whereas
cephalopods and crustaceans will remain largely unscathed. Habitat
changes will reduce seafood production from coral reefs, but increase
production from seagrass and seaweed. Overall effects of ocean
acidification on primary productivity and, hence, on food webs will
result in hard-to-predict winners and losers. Although adaptation,
parental  effects, and evolution can mitigate some effects of ocean
acidification, future seafood platters will look rather different
unless CO2 emissions are curbed.

http://www.sciencedirect.com/science/article/pii/S0169534712002625