A southern Alaska resident killer whale catches a salmon at the surface. Credit: Photo by Eva Saulitis/ North Gulf Oceanic Society.
Fish-eating killer whales in southern Alaska have a diverse, seasonally changing diet featuring salmon and groundfish.
The types of fish consumed also differ greatly across foraging hotspots in the region.
Scientists have been observing killer whales in Prince William Sound and Kenai Fjords as part of a long-term monitoring program launched in 1984 by the North Gulf Oceanic Society.
Working from May to September, researchers have gathered a total of roughly 400 remnants of prey fish and scat to develop a picture of the whales’ diets.
The samples highlighted how primary prey shifted between Chinook, chum and coho salmon, depending on where and when killer whales were hunting.
Their diets also included smaller amounts of Pacific halibut, arrowtooth flounder and sablefish. For some pods, or family groups, such groundfish were especially important.
Historically, killer whale diet research relied on surface sampling of prey fragments, usually scales, which allowed researchers to primarily detect salmon species.
Recent techniques that analyse DNA in scat have revealed the full breadth of the whales’ diets.
Hannah Myers, an assistant professor at the University of Alaska Fairbanks College of Fisheries and Ocean Sciences, said the change in primary prey across foraging hotspots — even those that are very close together — was notable.
Myers, lead author of the paper, said: “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 emphasises Chinook salmon as their primary prey.”
Sampling bias
The study also highlights the importance of accounting for sampling bias in diet studies.
Myers noted that, in this study, Chinook salmon samples were collected most often but were also the easiest to gather.
Separating diet by season and location revealed the importance of other prey species.
Researchers collected prey samples by following whales as they foraged, looking for tight turns and movement at the surface that are characteristic of fish chases.
Moving in carefully afterward, they scooped up scales or pieces of flesh with a pool net.
Faecal samples were collected by following behind the animals at a distance, looking for scat floating up in the upwellings of water from their flukes as they dove.
In the North Pacific, killer whales that exclusively eat fish (often called ‘residents’) are a distinct subspecies from two other types — one that eats only marine mammals and one that eats mostly sharks.
Fish-eating killer whales are the most common type in the North Pacific; about 1,000 fish-eating killer whales live from Southeast Alaska to Kodiak Island.
They live in family groups, with a female killer whale and her offspring together for life.
Diverse diets could allow the region’s killer whale population to be more resilient when fish populations change.
Fisheries management also relies on estimates of natural mortality, so more details about how species are affected by killer whale predation could ultimately influence management decisions about their prey.
Dan Olsen, a biologist with the North Gulf Oceanic Society and a co-author of the paper, said: “DNA studies from faecal samples are exciting because they have so much more information than previous techniques.
“This prey diversity is important to understanding the ecosystem, and perhaps future winter samples will show even more variability when times are lean.”
The study is published in the journal Ecosphere.
