Killer Diets

Flipper tags from 14 Steller sea lions were found in the stomach of a killer whale that died in Prince William Sound in 1992. This prompted considerable interest in understanding the role that killer whales may have played in the decline and lack of recovery of Steller sea lions.

The 14 sea lions eaten by this killer whale were tagged on Marmot Island (near Kodiak) in 1987 (4 pups), 1998 (9 pups) and 1990 (1 pup) as part of a long-term study to monitor sea lions. Two of the tags were consecutively numbered. It is not known when the sea lions were eaten and conversely how long the tags were in the stomachs. If they were consumed shortly before the killer whale died, they would have been about 4 or 5 years old.

Researcher Kathy Heise and colleagues sought to learn more about predation by killer whales on Steller sea lions. They compiled stomach content data from 12 killer whales found stranded from 1990 to 2001. They also distributed a 4-page questionnaire to about 250 researchers, fishermen and tour boat operators in Alaska and British Columbia.

The recovered killer whale stomachs contained either fish or marine mammals. Some of the identified marine mammal bits and pieces were harbor seal claws, whiskers and bones, a Dall’s porpoise dorsal fin and skull, a sub-adult sea lion, some beluga skin and blubber, and a number of river otter bones. Harbor seals were found in all 7 of the stomachs that contained marine mammal remains. Two of these stomachs also had sea lion parts.

Lance Barrett-Lennard photo

About half of the 250 questionnaires were returned from mariners that had spent a median of 14 years on the water (the range was 1 to 58 years). These people reported seeing close to 500 interactions between sea lions and killer whales of which only 10% appeared to be predatory and 6% resulted in a death. Two people reported sea lions harassing killer whales. Time taken to kill and eat a sea lion ranged from 1 to 2 hours, and the highest rate of predation was observed in the Aleutian Islands.

The findings from the stomach contents support the conclusion of others that there is dietary segregation between fish-eating and marine mammal-eating killer whales in Alaska. The most common prey of the marine mammal-eating killer whales were harbor seals. This conclusion is consistent with all other diet studies that show harbor seals are the most common prey throughout the northern hemisphere. Sea lions are part of the killer whale diet, but are not as important as harbor seals.

Further details can be found in:

Examining the evidence for killer whale predation on Steller sea lions in British Columbia and Alaska. Heise, K,. L.G. Barrett-Lennard, E. Saulitis, C. O. Matkin, D. Bain. 2003. Aquatic Mammals 29:325-334.