Killer whales (Orcinus orca) are demonstrating different responses to the changing conditions in the Southern Ocean, a pair of new studies found. Killer whales that rely on large expanses of sea ice to feed may be struggling to find enough food as climate change drives a decrease in annual ice cover, while those that feed primarily in open water appear to be less affected.
The findings, published in the journals Marine Mammal Science and Marine Ecology Progress Series, come from a long-term study in which scientists collected and analyzed thousands of photos of these apex predators to provide the first estimates of abundance, size, and body condition.
The data gives insight into how killer whale populations are faring in light of a rapidly warming environment and consequential shifts in prey abundance, and how the whales themselves as top predators may be influencing the broader ecosystem. This information could help support marine conservation decision-making in the area.
“The marine ecosystem of the Antarctic Peninsula is rapidly changing due to unprecedented warming, resulting in impacts on a number of species as they lose important habitat for breeding and foraging,” said Holly Fearnbach, marine mammal research director at SeaLife Response, Rehabilitation, and Research and an author on both papers.
Three forms, or ecotypes, of killer whales occur in the waters off the Antarctic Peninsula: Types A, B1, and B2. Each ecotype is genetically and physically distinct, with characteristic feeding habits. Type A killer whales have the typical black-and-white pigmentation of most killer whales around the world. They prefer open-water habitat and primarily feed on elephant seals and minke whales.
Both Type B1 and B2 killer whales have distinctive pigmentation with very large white eyepatches, largely gray bodies, and a darker patch, or “cape,” on their backs. The larger Type B1 killer whales prefer to hunt Weddell seals on pack ice, while Type B2 killer whales, which are smaller and depend less on ice cover, feed on penguins, Weddell seals, and likely fish or squid.
To get a first-ever look at the population trends of Type B killer whales in the area, scientists collected thousands of images of these mammals during austral summers between 2008-09 and 2018-19 from a variety of research platforms. Researchers used the photos to identify individual whales by their distinctive and long-lasting natural markings and track them across the study period.
The resulting decade-long dataset revealed that Type B1 killer whales had a smaller population size of roughly 100 individuals, with abundance declining at a rate of close to 5% per year because of reduced survival or possibly movement south to find ice in other regions. Type B2 killer whales had a much larger—and stable—population size of about 740 individuals. Previous research found that Type A killer whales had high survival and increased in abundance by about 25% over the same period to a recent high of around 150 whales.
“We found that both Types B2 and A killer whales, the two ecotypes that prefer open-water habitats, have fared well over the past decade in terms of abundance, which may be a response to locally plentiful prey and access to more ice-free foraging areas. However, the ice-dependent Type B1 killer whales appear to be struggling with the loss of sea ice, which is the habitat for their primary prey—Weddell seals,” Fearnbach explained.
Researchers used a drone to collect aerial images to assess the size and body condition of all three types of whales between 2016 and 2019. As large top predators that require a high caloric intake, trends in physical health are more sensitive indicators of prey availability and the health of the broader ecosystem and may serve as an early warning system of impending population changes.
The study found that Types A and B1 whales were relatively large, with the longest males being Type As and averaging 7.8 meters (about 26 feet), and the longest females being Type B1s and averaging 6.9 meters (about 23 feet). In contrast, Type B2s were diminutive in size, measuring over 1 meter shorter on average for both sexes. Type B2 whales were also found to be significantly leaner, even when accounting for their shorter length, and there were several whales measured to be in anomalously poor condition.
“This relatively poor condition of Type B2s, along with their relatively high abundance, may indicate that the population of this ecotype is close to reaching carrying capacity, or the maximum number of individuals that the marine ecosystem around Antarctic Peninsula is able to support,” said John Durban, a researcher with Southall Environmental Associates and a lead author of the health study. “It may well be that the warming waters and declining sea ice are reducing this carrying capacity, and the poor body condition we are seeing may be a warning of impending population declines. Similarly, the relatively robust body condition of Type B1s adds some support that their declining abundance may be due to movement south to find their ice seal prey, where sea ice still regularly occurs.”
The data in these studies offers decision-makers at the Commission for the Conservation of Antarctic Marine Living Resources (CCAMLR), the multilateral management body charged with conservation of the Southern Ocean, a more holistic view of the ecosystem as they consider proposals to create a marine protected area around the Antarctic Peninsula and implement an ecosystem-based management system for the regional krill fishery.
“We will continue to monitor changes in whale health in this changing environment in future years, to build on our research over the past decade, and to use whales as sentinels to better understand the changes in the ecosystem on which these top predators depend,” Fearnbach said.
Jim Palardy is a project director and Kathrynlynn Theuerkauf is a principal associate with The Pew Charitable Trusts’ conservation science program.