Cetacean Assessment and Ecology Program
Southeast Alaska Offshore Killer Whales
Since 1991, Southeast Alaska killer whales (Orcinus orcas) have been the focus of long-term investigations by
NMML staff. Two to three trips are conducted annually aboard the NOAA ship John N. Cobb to survey throughout the
inland waterways of Southeast Alaska. Killer whale abundance, trends, and short- and long-range movements have been
documented through photo-identification methodology. Stock structure has been determined through biopsy sampling and
genetic analysis. We have established that three, distinct ecotypes of killer whales inhabit the study area termed:
resident, transient, and offshore. Although considerable data exist on the resident and transient form of killer whale,
little is known about the offshore type.
Offshore killer whales were first seen in Southeast Alaska in 1989. It was assumed these whales entered inland waters
from pelagic areas, thus the name “offshores.” As more sightings of offshore killer whales became available, the
following characteristics appeared to be consistent among this ecotype. It was clear that morphologically, offshore
whales more closely resembled resident whales than transient whales. However, the overall size of offshore whales appeared
to be smaller than that of resident and transient whales. Given the smaller overall size of an adult offshore male, less
sexual dimorphism was observed between adult males and females.
In the offshore form, the tip of the dorsal fin is rounded, similar to those of resident whales, however, unlike the
resident form, the offshore dorsal fin is round over the entire tip with many whales having multiple nicks on the
trailing edge. Nicks are obtained at a rapid rate; the cause of this is unknown. Saddle shape of offshore whales is
similar in size to that of resident whales and is typically closed (i.e., no intrusion of black pigmentation into the
gray saddle); however, saddle shape does vary. Offshore group size can range from several members up to 200 whales.
Although the ranges of the three ecotypes overlap, offshore whales have not been seen to intermix with resident or transient
ecotypes. Unlike the movements of residents and transients, the movements of offshore whales appear to be long-range.
Many of the whales we see in Southeast Alaska have also been seen in the Bering Sea and off Washington State and central
California. (Photographic matches among regions are based on cooperative studies with several independent researchers).
The high percentage of resightings of the same individual whales in various regions indicates that the overall population
size may be relatively small. Considerable group mixing exists within the offshore ecotype, suggesting a more fluid
association than that described for residents or transients.
The preferred diets of the three different ecotypes vary considerably. Resident whales are known fish eaters, whereas
transient whales primarily prey on other marine mammals. Recently feeding ecology of killer whales has received much
attention, with transient killer whale predation being linked to population declines of pinnipeds and sea otters in western
Alaska. Little is known about the diet of offshore whales, but it is assumed they target fish as their primary prey.
To verify food preferences of offshore whales, skin and blubber samples have been analyzed for both contaminant levels
and stable isotope/fatty acid profiles by our colleagues at the Northwest Fisheries Science Center (Montlake Laboratory;
contact M. Krahn). Results of these analyses suggest that offshore killer whales are consuming prey species that are
distinctly different from the resident or transient ecotype.
By Marilyn E. Dahlheim
OND2005 quarterly sidebar
Research Reports Oct-Dec 2005