The endangered southern resident killer whale population is not getting enough to eat, and it hasn’t been since 2018, according to a new UBC study.
Animals have been energy-deficient, on average in spring, summer and autumn, for six of the past 40 years, meaning that the energy they get from food is less than they expend. Three of those six years occurred in the most recent years of the study, 2018 to 2020. The average difference in energy is 28,716 calories, or about 17% of the daily energy required for an average adult killer whale. , according to the authors.
“With the southern resident population at such a low level, there is a sense of urgency for this type of research,” says lead author Fanny Couture, a PhD student at the Institute for Oceans and Fisheries (IOF) and OceanWise. “Killer whales and Chinook salmon, the primary prey of the Southern Resident, are important and iconic species of Canada’s west coast. Studying what is happening to the population can help come up with solutions, both for southern residents and potentially for other killer whale populations in the future.
The southern resident population, which feeds primarily on Chinook salmon, numbered 73 individuals in October 2021, compared to an increase in the northern resident population of around 300. Studies have suggested that the growth of the southern resident population may be hampered by a lack of food. .
Researchers analyzed how changes in the abundance, age and size of chinook, coho and chum populations that southern residents feed on in the Salish Sea and on the west coast of the island of Vancouver influenced the daily food consumption of killer whales from 1970 to 2020 for three seasons each year.
The study used estimated declines in Chinook salmon abundance and size to show that low availability of these fish likely resulted in energy deficits in killer whales. “The years when southerners were energy-deficient are also years when other studies report a lower population growth rate and higher mortality rate for killer whales,” says the co- author, Dr. Villy Christensen, professor at IOF.
Previous research has shown correlations between the abundance of Chinook salmon and the survival rate and fertility of southern residents. The decline in chinook salmon abundance could be attributed to many factors, Couture says, including the effects of climate change, greater susceptibility to disease and predation by other animals.
The model also predicted that southern residents would consume more chum salmon than chinook during years when chinook was at low levels, showing that animals could switch to other salmon species when the abundance of their main prey decreases.
Lead author Dr Carl Walters, professor emeritus at IOF, said the commercial fishery for Chinook salmon in Canada was curtailed in the late 1990s following observations of a decline in plenty. “These declines have continued despite severe reductions in the fishery, and one very likely candidate for causing them is the massive increase in Steller sea lion abundance since the mid-1980s; these sea lions now consume more fish than all commercial fisheries in British Columbia for all species combined.
Still, the dire situation for southern resident killer whales may require a reduction in catches of larger chinook salmon, which predict if southern residents are low on energy, Dr. Christensen said. This could include promoting fishing techniques that increase the survival of larger fish.
Other factors that could influence prey availability for southern residents include underwater noise pollution from boats that could affect foraging, Couture says. This could be an area for future research. The researchers could also apply their model to the northern resident population to determine if they too suffer from an energy deficit.
The model did not include winter, as it is not known where southern residents are during the season. The study was published in PLOS ONE.
Language(s) of the interview: English (Couture, Christensen, Walters), French (Couture)