Killer whales are not short of prey in southwestern Canada

Populations of resident killer whales inhabit the Salish Sea off the southwestern coast of Canada. There is a northern population of about 300 individuals that feeds during summer in the Johnstone Straight, and a southern population, numbering just 73 individuals, that forages in the Juan de Fuca Strait. Both straits are important foraging grounds because this is where the killer whales catch migrating Chinook salmon, their preferred prey.

Recently, scientists have become concerned that whales from the southern population are thinner than their northern counterparts and speculated that there may be less fish available to the killer whales in the southern population. In response, scientists from the University of British Columbia conducted an assessment of the availability of prey for killer whales in both the northern and southern areas of the Salish Sea.

“People have been talking about a prey shortage as if it’s a fact, but this is the first study to quantify and compare the amount of their preferred prey, Chinook salmon, available to southern and northern resident killer whales,” said lead author Dr. Mei Sato, a research associate at the Institute for the Oceans and Fisheries at UBC at the time of the study, and now an assistant scientist at Woods Hole Oceanographic Institution.

“Measurements from drone footage have shown the southern resident killer whales are thinner on average than the northern residents – which supports the common belief that the southern residents are experiencing a food shortage,” said co-author Dr. Andrew Trites, professor and director of the Marine Mammal Research Unit (MMRU) at the Institute for the Oceans and Fisheries at UBC. “Our findings suggest that this food shortage is probably not occurring during the summertime when they have traditionally fed in the Salish Sea.”

The researchers surveyed the two areas – the Johnstone Strait in the north and the Juan de Fuca Strait in the south – during the summers of 2018 and 2019. They used state-of-the-art fish finders to locate and count Chinook salmon in the two straits, and were also able to estimate the sizes of the fish using this technology. In addition, the team also captured sample fish to validate the acoustic signals from their fish finders and distinguish between different fish species, as killer whales prefer larger, older Chinook salmon.

They collaborated with sport and commercial fisherman, the Sport Fishing Institute of British Columbia, and whale watching companies to design the survey and conduct fish sampling. In this way they identified the best time to carry out the field work, based on peak salmon availability and killer whale foraging behavior.

“We got help from sport and commercial fisherman, chartered fishing vessels and used echosounders to detect the number and size of fishes in the water,” said Dr. Sato. “Killer whales use a similar technique to locate the fish – only it’s much better than the electronic one we use because they can distinguish between different fish species in the water.”

The results showed that, although the distribution and sizes of fish were similar in both straits, prey abundance was four to six times greater in each patch sampled in the Juan de Fuca Strait. This clearly debunks the myth that killer whales experience a food shortage in their southern feeding grounds during summer. However, the authors emphasize that their study did not look at other factors that may prevent the southern killer whales from eating the available fish – factors such shipping activity and noise.

“We don’t really know about prey availability outside of the Salish Sea and outside the summer season. To answer the question of whether there’s an overall prey shortage, we still need to figure out what’s happening during winter and spring, whether there are significant fluctuations year-to-year and in the different locations these whales travel to, like California,” said Dr. Sato.

The study is published in the journal Canadian Journal of Fisheries and Aquatic Sciences.

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By Alison Bosman, Earth.com Staff Writer