Scientists Discover Key Clues About the Cuisine of Resident Killer Whale Populations

A team led by researchers from the University of Washington and the National Oceanic and Atmospheric Administration (NOAA) has uncovered key insights into the diets of resident orca populations. Researchers have long known that resident orcas prefer to hunt fish, particularly salmon. But some populations are thriving, while others are struggling. Scientists have long sought to understand the role diet plays in these divergent fates.

“Orcas are incredibly intelligent and learn foraging strategies from their matriarchs, who know where to find the richest prey resources in their areas,” said Amy Van Cise, an assistant professor of aquatic and fisheries sciences at the University of Washington who began the study as a postdoctoral researcher at NOAA’s Northwest Fisheries Science Center. “So we wanted to know if all this social learning affects the food preferences of different resident orca populations, or groups within populations?”

In an article published in the journal Royal Society Open ScienceVan Cise and colleagues report on the culinary preferences of two resident killer whale populations: Alaska and southern resident killer whales, which reside primarily in the Salish Sea and off the coasts of Washington, Oregon and northern California.

Both populations show a strong preference for salmon, particularly chinook, chum, and coho salmon. But they differ in the timing of their hunting and consumption of different salmon species, as well as in the other fish species they hunt to supplement their diets.

Southern Resident orcas are critically endangered, while other populations are growing. This new study will inform conservation efforts for resident orcas from Northern California to the Gulf of Alaska.

“We know that lack of food is one of the main threats to the endangered Southern Resident orcas,” Van Cise said. “We thought if we could compare their diet to the eating habits of a healthy and growing population, it could help us better understand how we can manage and protect this vulnerable population.”

While rivers in Alaska, British Columbia and the Pacific Northwest have historically provided resident killer whales with abundant levels of salmon, humans have recently disrupted this food supply, both directly by polluting the waters and building dams that reduce salmon runs, and indirectly by generating noise pollution that interferes with hunting.

Additionally, in the second half of the 20th century, resident orcas, particularly those in the South, were captured and herded into amusement parks, disrupting their social structure and further reducing their numbers.

This human impact has left its mark. While Alaskan resident orcas number in the thousands and the northern resident population has been steadily increasing, the southern resident population has plateaued at about 75 individuals. Recent research has implicated noise pollution from cargo ships and higher rates of pregnancy failure as factors.

For the study, the team collected fecal samples from southern resident and Alaskan killer whales at different times of the year between 2011 and 2021. The researchers analyzed DNA from the fecal samples to determine what the whales were eating. They found that the Alaskan resident’s summer diet included more chum and coho salmon, in contrast to the high-chinook salmon summer diet of a southern resident killer whale.

“Chinook salmon are clearly an important resource for resident orcas, regardless of their population. They are large and high in energy, making them a delicious and nutritious meal,” Van Cise said. “But what we’ve learned from Alaska residents is that stable sources of other fish—chum and coho salmon, or even flatfish like arrowhead flounder—can be an important nutritional supplement to help this population thrive.”

In recent years, the team has collected more fecal samples outside of the summer months. These samples have revealed a surprisingly diverse diet among resident orcas. Sablefish, arrowmouth flounder, lingcod, Pacific halibut and shortnose skate all feature in the diet of these whales, which were previously thought to feed exclusively on salmon.

The two populations differ in the non-salmon species they select to supplement their diets and in the times when they switch species. These feeding habits reflect a delicate balance between the regional abundance of different fish species, as well as a matriarch’s knowledge of reliable foraging locations.

“The survival of her family depends on the reliability of the foraging sites she knows from year to year,” Van Cise said.

In both the United States and Canada, resident orcas have become notorious, particularly because of the high-profile plight of the Southern Residents. The team believes their findings and follow-up studies of their diet are critical to helping them recover.

“While protecting key populations of Chinook salmon will always be essential to supporting the recovery of the endangered Southern Resident killer whale population, this study taught us that we may need to think more holistically about how we can conserve the entire ecosystem of fish that together make up the annual diet of predator populations like this one,” Van Cise said.