Yesterday, we marveled over the recent sightings of two killer whales targeting sharks off the coast of South Africa. The recent uptick in shark hunting has scientists wondering how the loss of great white sharks will impact the ecosystem. But it also has some wondering why the orcas are targeting so many sharks when their diets typically consist of a variety of fish and marine mammals.
Clues may lie in other populations of killer whales. Off the coast of British Columbia there are three genetically isolated groups of killer whales: Alaska residents, southern residents, and northern residents. During the summer, the southern residents traditionally forage off the coast of Vancouver Island and northern Washington state whereas the northern residents forage off the coasts of Johnstone and Queen Charlotte Straits. While the southern population of killer whales only includes 73 animals, and is considered endangered, the northern population is comprised of approximately 300 animals.
A new study conducted by scientists at the University of British Columbia sought to determine why the southern population has been declining. They suspected that southern resident killer whales may not be getting enough food in their environment since at least 2018. The authors of the new study published in PLOS ONE report that the southern resident animals may be currently experiencing a loss of about 17% of the energy required to function daily. The typical diet of a southern resident orca is comprised of about 90% Chinook salmon during the summer months. When Chinook salmon abundance is low, the orcas consume other fish such as chum salmon. Because the population at risk is so small, the researchers were not able to gather enough data examining potential links between salmon abundance and the survival or reproductive costs to the orcas.
It should be noted, however, that not all agree that Chinook salmon populations during the summer explain the low counts of killer whales. Another study published last year in the Canadian Journal of Fisheries and Aquatic Sciences, also led by researchers at the University of British Columbia, found that Chinook salmon populations are 4-6 times higher in the region of the southern resident killer whales than for the northern region killer whales. Study authors Dr. Mei Sato and Dr. Andrew Trites pointed out that the southern resident killer whales are thinner than northern residents, which is what led some scientists to suspect summer food shortages. However, the findings from this earlier study raise doubts about that hypothesis and suggest other factors might be at play including variations in food availability during other seasons leading up to summer or variations in exposure to noises and boating.
Clearly, there is a need for a detective to get to the bottom of this case of the declining killer whales.
F Couture, G Oldford, V Christensen, L Barrett-Lennard, C Walters. Requirements and availability of prey for northeastern pacific southern resident killer whales. PLOS ONE. 17 (6): e0270523, 2022. DOI: 10.1371/journal.pone.0270523
M Sato, AW Trites, S Gauthier. Southern resident killer whales encounter higher prey densities than northern resident killer whales during summer. Canadian Journal of Fisheries and Aquatic Sciences. 78(11): 1732-1743, 2021. DOI: 10.1139/cjfas-2020-0445