Blue whales are the largest animals on earth, which made them a magnet for whalers in historical times. Since the international whaling ban in 1986, previously-decimated blue whale populations have been steadily increasing, but they now
face a new threat: orcas.
The blue whale is the largest animal to have ever existed, found throughout the earth’s oceans and feeding peacefully through a baleen—their filter-feeding system—on a diet of microscopic krill. Due to their large size, blue whales supplied
a huge volume of blubber to whalers, who hunted them throughout the 1700s and 1800s, causing an extreme decrease in their population to near extinction.
Blue whales are the largest animals on earth. | Jeff Hester / Ocean Image Bank
In 1931, international agreements were made to reduce the number of whales being hunted, and in 1986 whaling was completely banned. Since then, blue whale populations have increased. However, they are still listed as endangered, and illegal
hunting remains a threat.
In recent years, a new threat to recovering blue whale populations has arisen: orcas. Orcas, or killer whales, are the largest species of dolphin and top apex predators in most of the major oceans. They are extremely successful co-operative
hunters, using a variety of intelligent methods to hunt fish, seals, sharks, and even whale calves.
Competition between orcas and great white sharks is likely in regions in which populations, and therefore diets, overlap. This may cause increasingly creative hunting techniques to develop. However, predation of large whales by orcas is
extremely rare, and only a few reports of orcas ‘chasing’ blue whales have ever been recorded—until now.
‘In recent years, a new threat to recovering blue whale populations has arisen: orcas.’
The first-ever documented predation of a blue whale by an orca pod was on the 21st of March 2019, off the coast of Bremer Bay in Western Australia. Approximately 14 orcas were observed attacking a fully grown and apparently healthy blue
whale.
Scientists from CETREC WA (Cetacean Research Centre, Western Australia) recorded the attack, observing that the whale had already suffered substantial wounds, including missing chunks of flesh, bone-deep gashes and the loss of most of its
dorsal fin.
After 20 minutes of continuous attack, the whale was bleeding heavily and weakening, until eventually a female orca forced its way into the whale’s mouth and began feeding on the nutrient-rich tongue. Around 40 other orcas joined the group
to feed on the carcass, including 16 that were photo-identified and matched to those seen at further killings.
Predation of a whale by an orca pod. | Jeff Hester / Ocean Image Bank
A second predation event occurred in the same area within 16 days, on the 6th of April. A blue whale calf of approximately 10 metres in length—twice that of the attacking orcas—was observed being predated by a pod of around 30 orcas. Some
were underwater, driving the whale up to the surface, while others attacked its head to prevent it from surfacing for air.
Both juvenile and infant orcas were actively involved in the attack, and again the tongue was targeted before the savaged carcass was left to sink to the seafloor. This sighting is less astounding than the first, as the blue whale calf was
substantially smaller than the fully grown adult taken down a couple of weeks prior. However, it is still an undeniably astonishing and scientifically significant event.
The third predation of a blue whale occurred two years later on the 16th of March 2021. Whale Watch Western Australia operatives, on a whale-watching vessel 30 kilometres from the Bremer Bay coast, observed 12 orcas in pursuit of a blue
whale, a high-speed chase which lasted about 100 minutes and 25 kilometres.
‘The killing of a blue whale calf by orcas is undeniably an astonishing and scientifically significant event.’
The whale was estimated to be 13 metres in length and identified as a juvenile. Both the blue whale and orcas were travelling at 18 kilometres per hour and creating large sheets of sea spray and tall bow waves, with the orcas methodically
charging and taking bites of flesh out of the whale’s flanks.
The whale reportedly stopped every few minutes, likely to rest, and was bombarded with orcas attacking both its head and rostrum (the upper jaw) each time. The whale was forced underwater for a final time when eight orcas attacked from its
flank, driving it to depth, and it was not seen to surface again. By the end of the killing, over 50 orcas were present in the area, with 16 being identified as present at both previous blue whale attacks.
These sightings are the first confirmed attacks on blue whales by orcas worldwide, though other records of injury to blue whales by suspected orca attacks have been documented off the coasts of California and Costa Rica.
A pod of orcas. | NOAA / Unsplash
In most documented accounts, the orca pods use similar hunting methods to succeed in killing the whale; around six to 10 orcas flank the whale on either side, while others surround the fleeing whale from both below and above, preventing it
from deep-diving to escape, and limiting its ability to surface for oxygen.
This weakens the whale substantially, allowing orcas to use ramming and biting behaviours to inflict fatal damage to the whale, removing large chunks of tissue from the dorsal fin and head region, with individuals sometimes swimming over
the head in an attempt to cover the blowhole.
This raises the question: why have such killings not been observed as frequently in the past? The answer is simple; in the past few centuries, blue whale populations simply have not been large enough for orcas to notice them and hence,
attacks would be rare and very unlikely to be observed by humans.
‘In the past few centuries, blue whale populations simply have not been large enough for orcas to notice them.’
However, with the recent increase in blue whale numbers, orca pods may be encountering blue whales more frequently and subsequently developing creative ways to hunt them. It may also be a side-effect of the decrease in fish stocks which
make up a large part of orcas’ diets, causing them to switch to other prey sources.
It has been speculated that killings by orcas may threaten the recovery of blue whale populations, however as it is a natural occurrence, this is still up for debate.
Some argue that orcas should be deterred from the area to promote the conservation of the blue whales passing through the area on their migratory route. Others point out that this is an entirely natural phenomenon and we should focus on
reducing threats directly caused by our own actions, such as pollution and illegal whaling before we meddle with the natural marine lifecycle.
Totterdell J., Wellard R., Reeves I., et al. (2022) The first three records of killer whales (Orcinus orca) killing and eating blue whales (Balaenoptera musculus). Marine Mammal Science. Pages
1-16.
