Cetacean Assessment and Ecology Program
Investigating the Foraging and Diving Behavior of "Transient"-type Killer Whales in the Central and Western Aleutians
The National Marine Mammal Laboratory’s Cetacean Assessment and Ecology Program has a continuing project to study predation on marine mammals by “transient”-type (mammal-eating) killer whales in the Aleutian Islands, initiated in 2001, to investigate the potential role of killer whales in the decline of the western stock of Steller sea lions. Predation on Steller sea lions in the eastern Aleutians has been visually observed to be ~14% of all transient predation events, and stable isotope values of transient killer whales there are consistent with a diet composed of 14% Steller sea lions (Herman et al. 2005, Marine Ecology Progress Series; Krahn et al. 2007, Marine Environmental Research). Acoustic recorders at sea lion rookeries and satellite tagging of killer whales have confirmed foraging movements consistent with some predation on Steller sea lions in the eastern Aleutians. For example, transient killer whales were heard regularly via an acoustic recorder at the Ugamak Island Steller sea lion rookery near Unimak Pass (unpublished data); a kill of a sea lion by a killer whale was observed at that rookery; and satellite tag locations of a transient killer whale showed it to be foraging in areas adjacent to that same rookery (J. Durban and Wade, unpublished data).
The focus of current research has shifted to the western and central Aleutian Islands because of the continuing decline or lack of recovery of Steller sea lions in that area. No observations have been made of predation on Steller sea lions in the western and central Aleutians, but observation effort there has been relatively sparse. The only predation events observed during NMML/NMFS surveys in the central and western Aleutians were of a Dall’s porpoise and a Baird’s beaked whale (unpublished data). Additionally, Estes et al. (1998, Science) described predation on sea otters in the central Aleutians.
During NMML surveys, transient killer whales have been regularly seen in two areas in the central and western Aleutian Islands: 1) the Delarof Islands-Tanaga Island area, and 2) Kiska Island and the Rat Islands, with abundance estimated at approximately 90 whales. A population of killer whales of this size could potentially inhibit the recovery of Steller sea lions in this region if they were a primary prey of transient killer whales there. Killer whales integrate chemical tracers acquired from their prey (e.g., stable isotope ratios of nitrogen and carbon) that reflect both the species consumed and the regions from which the prey were taken (Krahn et al. 2007). Analysis of blubber samples collected from transient killer whales reveals that the nitrogen stable isotope (δ15N) values of transient killer whales in the western Aleutians (Kiska Island and the Rat Islands) are much lower than values of transients in the eastern Aleutians and Bering Sea. However, nitrogen values in the central Aleutians (Delarof Islands-Tanaga Island) show dramatically different patterns: two of the samples group with the western Aleutian samples, and two of the samples group with the eastern Aleutian samples. The low western and central Aleutian samples have values from 12.5 to 14 δ15N, whereas, the majority of the eastern Aleutian samples have values from 16 to 19.5 δ15N. Given a trophic shift of ~3.8, this means that some transient killer whales in the central and western Aleutians are feeding on prey with an average δ15N value of ~8.7-10.2. This appears to be too low a δ15N value to be primarily from marine mammals, as observed values from marine mammals have all been higher: minke whales (central Aleutians, ~12.3 δ15N), Dall’s porpoise (eastern Aleutians, ~12.6 δ15N), sea otters (central Aleutians, ~13.4 and ~15.3 δ15N), and Steller sea lions (central Aleutians, average value of 15.8 δ15N) (Wade et al. 2006, North Pacific Research Board Final Report; unpublished data). One possible explanation for the low killer whale nitrogen values comes from a single observation of transient killer whales near Kiska Island feeding on squid in 2006 (J. Durban, unpublished data). Some species of squid have δ15N values in the range of ~8.7-10.2, suggesting that predation on squid may explain the low nitrogen values observed in killer whales in the western Aleutians and in some killer whales in the central Aleutians. (continued)