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Southern resident killer whales do not lack summer prey: UBC researchers

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UBC researchers have determined a shortage of salmon is not a significant factor in the population decline of southern resident orcas. Ted Chernecki reports – Oct 13, 2021

Researchers at the University of British Columbia say they’ve “debunked” a common belief that southern resident killer whales lack summer prey in Canadian waters.

The endangered orcas feed on Chinook salmon, long thought to have been scarcer during the summer in their feeding grounds than in those of their more populous cousins, the northern resident killer whales.

Scientists now say the numbers of Chinook salmon in the Salish Sea during summertime are four to six times more abundant for southern residents than in northern resident feeding grounds.

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“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, a research associate at the UBC Institute for the Oceans and Fisheries at the time of the study, in a UBC news release.

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The report published Tuesday in the Canadian Journal of Fisheries and Aquatic Sciences compared Chinook availability for southern residents in the Juan de Fuca Straight with availability for northern residents in the Johnstone Strait between 2018 and 2019.

Using sonar, researchers found that the sizes and distributions of the fish were similar in both regions, but there were many more fish “study areas” of the Juan de Fuca.

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As it stands, scientists believe there are about 73 southern resident killer whales left in the world, and a growing population of around 300 northern residents, which are threatened, but not endangered.

The thinner stature of the southern residents may have contributed to the belief they have less summertime prey, said Sato, but the new research suggests that food shortage is “probably not occurring” when they feed in the Salish Sea in the summer.

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The research did not evaluate other factors that might prevent southern residents from actually catching their prey, like increased vessel traffic and noise in their traditional feeding grounds.

Sato and study co-author Andrew Trites, director of the UBC Marine Mammal Research Unit, both say research efforts should now focus on other seasons and factors that impact Chinook availability.

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“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,” said Sato, “whether there are significant fluctuations year-to-year and in the different locations these whales travel to, like California.”

The research was funded by Fisheries and Oceans Canada, and conducted with help from sport and commercial fisherman, the Sport Fishing Institute of B.C. and several whale watching companies.

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