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The decline of the Southern Resident killer whales has long been linked with Chinook salmon abundance. Chinook are also an endangered species, yet selective targeting by fisheries of the largest fish has resulted in this species—and others—reaching smaller sizes as adults, and so reducing their calorific value to a predator.

A research study released in June 2022 examined 40 years of data from 1979 to 2020, to evaluate how changes in Chinook salmon abundance—including their maximum age and size—could impact the daily energy needs of the Southern Resident killer whales, and therefore their survivability.

What was the study?

The scientists developed two computer models: one included abundance estimates for all Chinook populations with a migratory route through the Salish Sea, including their estimated age, and the size/weight of fish aged three and older—favored by these salmon-dependent killer whales. The second assessed the whales’ potential daily Chinook consumption. By estimating the body size of each individual in the population, they calculated their “daily prey energetic requirement” and concluded that each whale needed on average, 225 lb. of Chinook salmon a day.

They calculated that from April to October, Chinook consumption ranged from a high of 216,300 in 1993, to a low of 166,000 in 2018. This reflects that there were 97 whales in 1993, and only 75 by 2018—in addition to reduced prey. In fact the model showed that since 1979, Chinook populations declined across all age groups, with two substantial multi-year declines, including 2015-2020 with an 18.4% drop.

Unfortunately Chinook today are less likely to reach their maximum size than in previous decades. Yet older, larger fish are critical if the whales are to meet their daily energy needs. The model showed that when larger Chinook are less abundant, they are “significant predictors of energetic shortages.” Unfortunately the number of Chinook reaching 3-, 4- and 5-years of age is in decline—as is their size. In fact since 2005, five-year old Chinook have decreased by 2 inches. That’s a loss of 220 calories to a predator. The study found that this decrease in Chinook size is twice as significant as change in abundance, though of course both are of great concern.

© Wild Orca

According to the model, the majority (82%) of the whales’ diet from spring to fall was Chinook from the Fraser River, Columbia River, and Puget Sound, but their relative contributions have changed quite markedly over the last 40-years. In 1979, Columbia River Chinook formed 27% of their diet, but by 2014 had increased to 61%, whereas Puget Sound Chinook “decreased substantially between 1979 and 2020,” with a 15% decline in abundance since 1986.

These changes are reflected in the Southern Resident killer whales’ travel patterns; long spring and summer visits to the Salish Sea to feed on Fraser and Puget Sound Chinook have become noticeably less frequent and shorter, as they’re forced to search for prey on the outer coast. These changes come at a cost to the whales, who “consistently consumed less calories in the spring than during the fall” and this confirms that “they could undergo a more acute nutritional stress during the spring,” as shown in stress hormone analysis of their fecal samples.

Food shortages

The model predicted that the Southern Resident killer whales had insufficient food in spring, summer and fall in four of the study years: 2008 and 2018-2020. However, when using the high end of the range of their estimated daily energy needs, this rises to 19 years of seasonal shortages. The scientists noted that in most years when the whales did not meet their daily prey requirement, “the population exhibited a high calf mortality rate and low birth rate.” This reflects findings from other research that linked insufficient prey with a 69% pregnancy failure rate.

The scientists did note one limitation in their model: they could not account for prey accessibility—that is when noise and disturbance from vessels impedes successful foraging, as a number of other studies have shown. They also make no mention of effects from contaminants, as previous research has shown that when nutritionally stressed, these whales release harmful PCBs into their bloodstream.

Nevertheless, despite not accounting for cumulative or interactive threats, this study highlights that 2018-2020 showed a disturbing trend in prey shortages that will translate to poor health, lost pregnancies and less robust whales, as is evidenced by WDFW’s recently released list of 13 “vulnerable whales.”

As this study concluded in 2020, the fecal samples Wild Orca collected in May and June 2022 will be important indicators of the current health of the Southern Resident killer whales, especially as they returned to the Salish Sea earlier this year than has been the “new normal.” We will share timely results from the analyses of these samples with the appropriate agencies, to ensure that the most up-to-date science can be used to benefit this population.

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Endangered Killer Whales

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Tell government leaders that “Dams Must Go!”

"Salmon recovery depends on large-scale actions, including breaching dams, systematically restoring tributary and estuary habitats, and securing a more functional salmon ecosystem.” NOAA Fisheries, July 2022.

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Tell government leaders that “Dams Must Go!”

"Salmon recovery depends on large-scale actions, including breaching dams, systematically restoring tributary and estuary habitats, and securing a more functional salmon ecosystem.” NOAA Fisheries, July 2022.

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