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National Marine Mammal Laboratory (NMML)

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


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