A new diet study co-authored by NOAA Fisheries scientists sheds new light on the diet of the Southern Resident killer whales in fall and winter. This study confirms the whales’ year-round dependence on Chinook salmon and the astonishing diversity of rivers they came from, but it also offers insight into other species on the menu when Chinook salmon is less available over the winter months.
What was the study?
Researchers followed the whales periodically from 2004-2017 in their fall and winter foraging habitats, Puget Sound, and the coastal waters of Washington, Oregon, and California. They aimed to collect and identify fish species by collecting either prey remains floating at the surface after a successful feeding event; or floating fecal matter (yes, poop!) from an earlier meal.
What did they find?
The study demonstrated that the Southern Resident killer whales are true salmon specialists, even in fall and winter. In fact, the study found that “our estimate of 80% Chinook salmon in the diet in outer coast waters in winter and spring is nearly identical to that obtained for summer.”
Chinook salmon is indeed king year-round.
Which species were on the menu?
Nine species were identified in total: four salmonids—Chinook salmon, Coho, Chum, and Steelhead; three flat fish—Pacific Halibut, English Sole, and Arrowtooth Flounder; and in addition, Lingcod and Big Skate.
The total number of these non-salmon samples was small, and in fact, the researchers noted that Southern Resident killer whales preferentially chose Chinook salmon, even if other salmon species they’re known to eat is more abundant and Chinook salmon is scarce.
This study clearly demonstrates just how essential Chinook salmon is to these whales year-round. “Averaging approximately 50% of their diet in the fall, increasing to 70-80% in the mid-winter/early spring, and increasing to nearly 100% in the spring.”
By combining their data with previous studies, these authors now estimate that from January to August, Chinook salmon is the most significant part of their diet. It is consumed almost exclusively from April to August, and it’s now shown to play an important role throughout winter.
Two salmon species accounted for 71% of all samples collected in fall and winter, and with steelhead included (another salmonid), this figure rises to 88%. The authors noted that the increased diversity of species consumed in winter suggests Chinook salmon are harder to find.
What does DNA reveal about the secret lives of Chinook salmon?
The DNA from almost half of the Chinook salmon samples collected could determine their river of origin. This showed that the Southern Resident killer whales had eaten Chinook salmon originating from 19 different home rivers!
When the whales were foraging in Puget Sound, 20 Chinook salmon samples collected revealed nine different rivers of origin. While most originated in the North and South Puget Sound Rivers, others had traveled from much further including the Sacramento River in California and the Skeena River in northern British Columbia.
This demonstrates the remarkable distances that Chinook salmon travel to feed, some as far as 1,000 miles from their home river. It also reflects the genetic diversity of Chinook salmon these whales have evolved to rely on, both seasonally, with salmon runs from the same river, and geographically from Canada to California. This diversity is nature’s backup plan for the natural fluctuations in salmon abundance that are now being sorely tested by unnatural changes to their habitats and our climate.
The timing of salmon runs, as well as the knowledge of where to find food when some salmon are scarce, is crucial inherited knowledge passed down through the Southern Resident killer whale female lineage. After coexisting with Chinook salmon for centuries, salmon fishing is now far less predictable for these whales due to the enormous impacts that humans have had on salmon populations in the last 100 years. As a result, fewer females are surviving beyond menopause, leading to a decline in the collective knowledge base of this population. Where once a female might have 100 years of experience, today, it might be only 40.
What did we learn from samples collected on the outer coasts?
We know very little about the winter travels of the Southern Resident killer whales, but the outer coast is most used by K and L Pods, who may travel as far south as Monterey Bay in search of salmon. In fact, samples collected included Chinook salmon from California’s Klamath River.
Most samples were collected in the coastal waters off Washinton State. Of these, 33 were Chinook salmon that represented 15 home rivers, including the two most northerly rivers identified in this study, the Taku and Stikine, which run from northern BC into southeast Alaska.
The study showed that 90% of the Chinook salmon originated in one of four river systems:
1. The Columbia Basin—fed by rivers originating in Washington, Oregon, and Idaho.
2. Puget Sound, Washington State,
3. Fraser River, British Columbia, Canada, and
4. The Central Valley, California.
Previous studies identified the Columbia, Puget Sound, and Fraser River systems as essential sources of Chinook salmon for the Southern Resident killer whales; this study now highlights their year-round importance. The significant contribution to the fall and winter diet by California’s Central Valley Chinook salmon is important new information.
Of great concern is that nearly two-thirds of the fish originated from rivers where Chinook salmon populations are listed as “Threatened with Extinction” under the Endangered Species Act, i.e., Puget Sound, Klamath River, Sacramento River, Columbia River, and Snake River.
It was notable but not surprising that few Snake River Chinook salmon samples were found in this study, given they’re struggling for survival, as dams restrict access to spawning habitat. In fact, this river was just named “America’s Most Endangered River 2021″ by the environmental advocacy group American Rivers.
What are the key findings?
This important study sheds new light on the Southern Resident killer whale’s reliance on Chinook salmon year-round, reinforcing what has long been understood—Chinook salmon is the key to their survival. Yet, since this study was concluded in 2017, catch limits in the following four Chinook salmon fishing seasons in Washington State were set without change: all authorized by NOAA Fisheries, the federal agency responsible for fishing, but also for recovering endangered species—including Chinook salmon and the Southern Resident killer whales.
There are fewer whales in this population today than when listed as Endangered in 2005. Yet numerous studies show that the Southern Resident killer whales could thrive again if we increased the number of older and larger Chinook salmon in the ecosystem. Better-fed whales could live longer, with more calves surviving. Bigg’s killer whales in the Salish Sea are steadily increasing despite being subjected to environmental contaminants, vessel noise, and disturbance. The difference? Their prey is abundant. Actions to recover Chinook salmon can still turn this around. It’s not too late.
Read Wild Orca’s March Statement in response to the study’s release.
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