Alaska’s orcas don’t eat what you might think: a study reveals a surprising diet

A decades-long study challenges long-held assumptions

For many years, it was widely believed that orcas in the cold waters of southern Alaska fed almost exclusively on Chinook salmon. However, a detailed, long-term study has overturned this assumption, revealing a far richer and more complex reality.

This extensive research project was conducted jointly by scientists from the University of Alaska Fairbanks and the North Gulf Oceanic Society. By tracking resident orcas in southern Alaska for decades, the team was able to accumulate an impressive amount of data on their behavior.

To reach these conclusions, the researchers combined traditional observational methods with modern DNA analysis tools. This hybrid approach revealed clear changes in the whales’ diet, influenced by both the seasons and their geographic location.

A Detailed Route Through Three Geographic Areas

The study focused on three main foraging areas frequented during the summer. These areas include the Kenai Fjords, the eastern part of Prince William Sound, and the western part of Prince William Sound. The scientists observed that each area corresponded to a specific time window during which the majority of foraging took place.

In the Kenai Fjords, the majority of samples were collected between mid-May and mid-June. The peak feeding period then shifted to the eastern part of Prince William Sound, where activity was most intense from mid-June through July. Finally, in the western part of Prince William Sound, most feeding activity took place from July through September.

This temporal pattern demonstrates that the whales move from one hotspot to another as the summer progresses. Their food choices therefore evolve in tandem with these changes in geographic location.

The Salmon Dance: A Constant Adaptation

To analyze the cetaceans’ diet, researchers collected 255 prey remains between 1991 and 2021. They also analyzed fecal samples collected between 2016 and 2021. The results obtained through these two methods revealed an identical and marked trend: orcas are shifting the species of salmon they target over time.

In the Kenai Fjords, chinook salmon accounted for approximately 77% of the prey remains found. The situation changed dramatically in the eastern part of Prince William Sound, where chum salmon dominated, accounting for about 62% of the prey samples. Finally, in the western part of the sound, coho salmon became the primary prey, making up about 77% of the remains.

This gradual shift from sockeye to chum, and then to coho throughout the summer suggests that the whales follow the seasonal availability of fish. When one species of salmon becomes less available, another takes its place and becomes more important for the group’s survival.

DNA Reveals Unexpected Consumption of Bottom-Dwelling Fish

The study did not stop at salmon alone. DNA analysis of fecal samples revealed the presence of other important prey that surface scale collection often missed. Pacific halibut, large-mouthed flounder, and black cod (sablefish) appeared regularly in the samples. In some cases, halibut accounted for more than five percent of certain samples and was found over the course of several months.

The western part of Prince William Sound showed the most diverse diet, with a mix of salmon and groundfish. Hannah Myers, a co-author of the study, highlights this finding: “Switching between different salmon species—with significant contributions from groundfish—tells a different story than what we usually hear about the diet of piscivorous North Pacific orcas, which emphasizes chinook salmon as the primary prey.”

Dan Olsen of the North Gulf Oceanic Society adds a technical and ecological perspective: “DNA studies using fecal samples are exciting because they contain much more information than previous techniques.” He also notes: “This diversity of prey is important for understanding the ecosystem, and perhaps future winter samples will show even greater variability when food is scarce.”

Family-Specific Feeding Traditions

Resident orcas in southern Alaska live in family groups called pods. The study revealed that certain pods appeared more frequently at specific hotspots than others. For example, some groups were dominant in the Kenai Fjords, while others were more common in Prince William Sound.

One specific pod, named AK2, was observed in all three main areas. Samples from this group showed a clear shift: they consumed chinook salmon in early summer, switched to chum salmon later on, and then returned to chinook salmon in the western part of Prince William Sound. This demonstrates a dynamic adaptive strategy within a single family.

Another pod, named AE, showed a strong association with flatfish such as halibut and American plaice. This suggests that different family groups may have slightly different feeding habits, possibly passed down from generation to generation within the pods.

Flexibility and Conservation: The Keys to Survival

The resident killer whales of southern Alaska number about 1,000 animals and form one of the largest resident populations in the North Pacific. A flexible diet could help sustain this important population. The study, published in the journal Ecosphere, also shows that previous research may have focused too heavily on king salmon because sampling often took place in areas and during months when this fish was easiest to detect.

When scientists took location and season into account, other salmon species and bottom-dwelling fish became much more significant. Understanding this complex diet helps fisheries managers. Some salmon stocks, particularly chinook, are listed or considered endangered. Knowing exactly which fish the whales depend on at different times can guide better conservation decisions.

This long-term study demonstrates that even marine predators at the top of the food chain adapt to changes in food resources. Instead of relying on a single favorite fish, resident orcas in southern Alaska follow seasonal changes and feed on a mix of salmon and groundfish. This flexibility may be one of the reasons why this powerful predator continues to thrive in the cold waters of Alaska.

Source: earth.com

Alaska’s orcas don’t eat what you might think: a study reveals a surprising diet