Researchers say orcas eat well in Salish Sea

University of British Columbia scientists say their research debunks a popular belief that southern resident killer whales swimming in Canadian waters in summer months aren’t finding enough Chinook salmon to feed on.

Their research, published on Tuesday, Oct. 12, in the Canadian Journal of Fisheries and Aquatic Sciences, reports that Chinook numbers in the Salish Sea in summertime are four to six times more abundant for southern resident killer whales than northern resident killer whales.

While measurements from drone footage have shown southern resident killer whales to be thinner on average than the northern residents, our findings suggest that this food shortage is probably not occurring in summertime when they have traditionally fed in the Salish Sea, said Andrew Trites, co-author of the study and director of the Marine Mammal Research Unit at UBC’s Institute for the Oceans and Fisheries.

Researchers spent the summers of 2018 and 2019 surveying two areas known to be important foraging habitats for resident killer whales, the Johnstone Strait for northern residents and Juan de Fuca Strait for southern residents, where the orcas intercept migrating Chinook salmon returning to the Salish Sea. By use of state-of-the-art fish finders, researchers were able to compare the number and size of Chinook salmon in the two water bodies.

“We got help from sport and commercial fishermen, chartered fishing vessels and used echosounders to detect the number and size of fishes in the water, said Mei Sato, lead author, who was a research associate at UBC at the time of the study and now an assistant scientist at Woods Hole Oceanographic Institute. “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 survey and fish sampling were designed through collaboration with sport and commercial fishermen, the Sport Fishing Institute of British Columbia, and whale watching companies. The collaboration identified the best timing for conducting field work based on peak salmon availability and killer whale foraging behavior.  Researchers also captured sample fish to validate the acoustic signals from their fish finder and distinguish between different fish species, since orcas prefer larger, older Chinook salmon.

In this, the first study to use acoustic surveys to estimate relative abundance of large salmon in the Salish Sea, researchers found that while distribution and sizes of fish were similar in both straits there were four to six times as many fish in each patch in the Juan de Fuca Strait.  The study highlights the importance of assessing salmon distribution at sea using hydro-acoustic methods, said Stephane Gauthier, a scientist with Fisheries and Oceans Canada, which funded the study.

While this research looked at the availability of salmon, it did not assess other issues that might be presenting the orcas from catching fish, including high vessel presence and noise in the Juan de Fuca Strait compared to Johnstone Strait.

Researchers said that now having evidence that southern resident whales are not facing a prey shortage in their traditional summer feeding grounds in Canadian waters that further research would be directed to other factors, areas and seasons.