Research results published in the Canadian Journal of Fisheries and Aquatic Sciences show that the number of Chinook salmon in the Salish Sea in summer months are four to six times more abundant for southern resident killer whales than northern resident killer whales.
The research, published on Tuesday, Oct. 12, debunks a popular belief that there are fewer Chinook salmon during the summer in Canadian waters for southern resident killer whales compared to an abundance of fish for northern resident killer whales.
“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 Mei Sato, who was a research associate at the University of British Columbia’s Institute for the Oceans and Fisheries at the time of the study. Sato is now an assistant scientist at Woods Hole Oceanographic Institute.
The southern resident population numbered 73 as of this month, compared with the increasing northern resident population of about 300 orcas. In recent years, southern resident whales have also been returning later than normal to the inside coastal waters of British Columbia and Washington state.
Andrew Trites, director of the Marine Mammal Research Unit at the Institute for the Oceans and Fisheries, said that measurements from drone footage shows that the southern resident killer whales are thinner on average than the northern residents, which supports the common belief that southern residents are experiencing a food shortage. However, he said, the UBC study suggests that this food shortage is probably not occurring during the summer, when they have traditionally fed in the Salish Sea.
While the study looked at the availability of salmon, it did not assess other factors that might prevent the killer whales from catching fish, such as higher vessel presence and noise in the Juan de Fuca Strait, compared to Johnstone strait.
Now that they have evidence that southern residents are not facing a prey shortage in their traditional summer feeding grounds, Sato and Trites said the focus of research efforts should be directed to other factors, areas and seasons.
The study was funded by Fisheries and Oceans Canada, the Canadian government department responsible for policies and programs supporting the country’s economic, ecological and scientific interests in oceans and inland waters.