What Southern Alaska Resident Killer Whales Really Eat

For years, the answer to what killer whales eat in the chilly waters of southern Alaska seemed straightforward: mainly Chinook salmon. A substantial long-term study, however, reveals a far more varied and adaptable picture.

Scientists at the University of Alaska Fairbanks led a major research programme that tracked southern Alaska resident killer whales over several decades.

Using established approaches alongside modern DNA techniques, the team identified marked dietary changes linked to both season and location.

Southern Alaska killer whales shift between feeding grounds

The research examined three principal summer foraging sites: Kenai Fjords, eastern Prince William Sound and western Prince William Sound. Feeding was concentrated within a different period at each site.

At Kenai Fjords, researchers collected most samples between mid-May and mid-June. Feeding in eastern Prince William Sound was greatest from mid-June to July. In western Prince William Sound, the bulk of feeding took place from July to September.

These timings indicate that the whales travel between feeding hotspots as summer advances. Their prey choices shift as they move from one area to another.

Killer whales change salmon species

The researchers gathered 255 prey remains from 1991 to 2021, while also examining faecal samples collected from 2016 to 2021. Both types of evidence pointed to the same distinct trend.

Within Kenai Fjords, Chinook salmon accounted for roughly 77 percent of prey remains. Chum salmon was the leading prey in eastern Prince William Sound, representing about 62 percent of prey samples.

In western Prince William Sound, coho salmon was the primary prey, comprising around 77 percent of prey remains.

This consistent summer progression from Chinook to chum and then coho suggests the whales track the seasonal availability of salmon. When one species becomes harder to find, another takes on greater importance.

“Switching between these salmon species – with important contributions from groundfish – is a different narrative from the one we usually hear about the diet of fish-eating killer whales in the North Pacific, which emphasizes Chinook salmon as their primary prey,” said co-author Hannah Myers.

The diet combines salmon with groundfish

The investigation looked beyond salmon. DNA testing of faecal samples identified further significant prey species that collecting scales at the surface often failed to detect.

Pacific halibut, arrowtooth flounder and sablefish occurred regularly in the faecal samples. Halibut represented more than five percent of some samples and appeared across several months.

Western Prince William Sound had the most varied diet, combining salmon with groundfish.

“DNA studies from fecal samples are exciting because they have so much more information than previous techniques,” said Dan Olsen from the North Gulf Oceanic Society.

“This prey diversity is important to understanding the ecosystem, and perhaps future winter samples will show even more variability when times are lean.”

Individual pods favour different fish

Southern Alaska resident killer whales live in family units known as pods. The findings showed that certain pods occurred more frequently at particular hotspots. Some, for instance, were dominant in Kenai Fjords, whereas others were seen more often in Prince William Sound.

One pod, AK2, was recorded in each of the three hotspots. Samples from this pod revealed a clear change from Chinook in early summer to chum later on, before a return to Chinook in western Prince William Sound.

A different pod, AE, displayed a strong association with flatfish, including halibut and arrowtooth flounder. This points to modest differences in feeding behaviour between family groups, potentially passed from one generation to the next.

A flexible diet helps sustain a large population

There are about 1,000 southern Alaska resident killer whales, making them one of the North Pacific's largest resident populations. Their adaptable diet may contribute to supporting such a sizeable population.

The findings also suggest earlier studies may have placed too much emphasis on Chinook salmon, as sampling was frequently carried out in places and during months when Chinook were easiest to identify.

Once location and season were considered, other salmon species and groundfish proved much more significant.

This detailed understanding of their diet is useful for fisheries managers. Several salmon stocks, particularly Chinook, are listed as endangered or are being considered for listing. Identifying the fish that whales rely upon at particular times can support better conservation choices.

The decades-long research demonstrates that even leading ocean predators adapt to shifting food supplies. Rather than depending on one preferred fish, southern Alaska resident killer whales follow seasonal changes and eat a combination of salmon and groundfish.

That adaptability may help explain why this formidable predator continues to flourish in Alaska's cold waters.