“Black fins, everywhere.”

That evocative line comes from an account last month posted by Kristy Brown of Naturaliste Charters, describing a dramatic take-down of a good-sized blue whale – the largest animal on Earth – by a big group of orcas off the southern coast of Western Australia (Watch the video here).

The attack occurred over the Bremer Canyon system, a network of submarine trenches striking the continental shelf break about 70 kilometres southeast of the town of Bremer Bay. Such “shelf-incising canyons” are often associated with significant marine productivity, and the Bremer system is no exception: It’s well known for major congregations of cetaceans, from bottlenose and common dolphins to sperm whales and, yes, orcas, well more than 100 of which gather here during the austral summer and fall – the largest-known seasonal orca get-together in Australian waters.

According to Brown’s report, the Naturaliste Charters whale-watching vessel came across orcas on the morning of March 16. “The action started slow, we were seeing a couple of surges around, but it didn’t seem uniform,” she wrote. “Usually surging orcas are spread wide moving loosely in one direction when they hunt a beaked whale. But this was different, these surges were scattered.”

“They were literally everywhere, and they were working together.”

Eventually they realised the orcas were working a blue whale reckoned at about 16 metres long: either a juvenile blue, Brown told Live Science, or a full-grown pygmy blue whale, a subspecies. (Blue whales, particularly those of Antarctic populations, may grow to 30 metres and weigh 160 tons or more.)

More and more orcas arrived on the scene, including “at least six big males from different pods.” Ultimately, the observers estimated that anywhere between 50 and 70 killer whales were taking part. “They were literally everywhere, and they were working together,” Brown wrote. “They were driving this whale from the depths of the S bends within the Bremer Canyon system at 1,000m of water and gradually forcing it onto the shallower continental shelf. This was their strategy, and they all seemed to know it.”

The orcas relentlessly attacked the blue’s jaw, a common strategy when the predators hunt large whales. Orcas are known to prize the tongues and lips of baleen whales, and furthermore attacks aimed at the victim’s head may reduce the danger of being whacked by its potentially dangerous flukes.

The attack unfurled for hours before the blue finally gave out in the afternoon. “A bubble of blood rose to the surface like a bursting red balloon,” Brown noted. The orcas wasted no time feasting: “We saw some blubber, only one hunk of flesh, and it was gone.”

The kill site attracted not only seabirds but also a hammerhead shark and long-finned pilot whales, also known to frequent the Bremer submarine canyons in large numbers.

There are relatively few recorded cases of orca predation on blue whales, especially when compared with other baleen whales more frequently recorded falling on the killer whale’s menu. An admittedly outdated (1991) review of orca interactions with other marine mammals noted four cases of killer whales preying on blues, including a pre-1925 report of five orcas killing an adult blue whale in Antarctica, as well as a handful of observations of the two species mingling apparently peacefully. More common cetacean prey for orcas, according to that survey, included fin, minke, and humpback whales – among the blue whale’s relatives in the rorqual family – as well as grey and bowhead whales, narwhals, and Dall’s porpoises.

Nonetheless, humans have witnessed some notable cases of orcas preying on or harassing blues in recent decades. In 2003, killer whales were seen feeding on a blue-whale calf within the Costa Rica Dome, an upwelling area about 230 kilometres west of Nicaragua. In 2017, some feisty orcas in California’s Monterey Bay charged an adult blue, which rapidly escaped.

Even if the Bremer Canyon blue whale was a juvenile of the larger subspecies, it was still much bigger than the orcas that dined on it. This – and the sheer number of killer whales involved – makes the incident noteworthy, but it’s actually the most recent of several recent attacks on similarly sized blues in this orca hotspot. In March 2019, orcas killed a 20-metre-long pygmy blue whale here – during a thunderstorm, no less – with upwards of 50 killer whales digging into the carcass. A mere two weeks later, a 15-metre pygmy blue also fell to orcas.

As Ian Dickinson reported here at Earth Touch on those incidents, some experts believe attacks such as the two 2019 ones and last month’s could be more frequent than previously appreciated. “Given the slow but steady increase in the Southeast Indian Ocean pygmy blue whale population (approximately 2,000 whales) . . . there is a possibility the killer whales of the Bremer Canyon are taking advantage of this population,” Micheline Jenner of the Centre for Whale Research told Dickinson.

And John Totterdell of CETREC WA (Cetacean Research), who witnessed the first of those 2019 attacks, posted on Facebook: “Even though this is the first recorded encounter (off Australia) of a large baleen whale succumbing to an attack, it’s likely other large whales (including blues) often face the risk of predation from killer whales.”

It’s worth noting, too, that an older observation of an orca attack on a blue whale – a well-photographed instance off Cabo San Lucas in Baja California in the late 1970s – involved an 18-metre juvenile blue, harried by close to 40 orcas.

Just how frequently orcas actually kill full-grown baleen whales – also known as mysticetes or “whalebone” whales – remains a matter of some debate. A 2007 paper suggested that, in high-latitude areas, adult mysticetes bearing orca bite scars (rake marks) usually have them when they’re first identified by researchers, and rarely seem to acquire new ones, suggesting they’d likely received them as younger animals. (The minke whale, a relatively small rorqual that doesn’t max out much longer than an orca, is an exception: Killer whales often hunt adults of this species.)

Calves of large whales may be especially vulnerable to orcas when migrating from their nursery waters at low latitudes to high-latitude feeding grounds. The mammal-eating killer whales of the North Pacific known as Bigg’s (or “transient”) orcas, for example, frequently target grey-whale calves journeying with their mothers northward along the western coast of North America (the aforementioned Monterey Bay is a common site for such predation). Orcas also frequently prey on humpback calves migrating along the coast of Western Australia.

Mother grey and humpback whales will both vigorously defend their calves from orcas; in the case of humpbacks, other adult whales will do so as well. Indeed, humpbacks are known for seemingly coming to the aid of even other animals under assault by killer whales on occasion – a possible case of interspecies altruism

Top header image: jellybeanz/Flickr