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Cetacean Assessment & Ecology Program

Genetic Evidence for Population Structure in Northern North Pacific Killer Whales

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Jan-Feb-Mar 2013
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In accordance with the guidelines of the U.S. Marine Mammal Protection Act (MMPA), the National Marine Fisheries Service (NOAA Fisheries) consults annually with Scientific Review Groups to prepare assessment reports for marine mammal stocks in U.S. waters. These reports include a description of the stock’s geographic range, estimates of population size and trends, and the current stock status. Stock Assessment Reports (SARs) are used to determine allowable levels of “takes” of marine mammals incidental to various anthropogenic activities and are also used to determine the scope and scale of necessary conservation measures. One critical element necessary to meet the objectives of the SARs is an accurate characterization of the stock being assessed, and for highly mobile cetaceans, such as killer whales, Orcinus orca, (Fig. 1) this can often be a difficult task.

killer whale  

Figure 1. Adult male Bigg’s killer whale photographed in the western Aleutians during the 2010 CAEP survey. Photograph by Dave Ellifrit.

 

To address data needs for killer whales in Alaska waters, the National Marine Mammal Laboratory’s Cetacean Assessment and Ecology Program (CAEP) conducted annual vessel surveys throughout the Aleutian Islands and western Gulf of Alaska between 2001 and 2010. These surveys comprised part of CAEP’s ongoing research to determine the distribution, abundance, stock structure, and diet of killer whales throughout western Alaska and the Aleutian Islands. Two of the primary objectives of the vessel surveys were to collect identification photographs and small tissue biopsy samples of individual killer whales encountered throughout the survey area. To maximize the geographic extent of the survey area and include killer whales from neighboring regions, samples collected by CAEP surveys were supplemented with biopsies contributed by other NOAA Fisheries and non-governmental research organizations within both Russian and Alaskan waters. Geospatially-referenced photographic data and genetic data acquired from tissue samples can then be incorporated in analyses designed to look for geographical or ecological boundaries that define killer whale subpopulations or stocks. The scope of this large collaborative project provides us with the opportunity to explore patterns of genetic subdivision among killer whales sampled across the northern North Pacific.

Despite killer whales’ relatively ubiquitous distribution, data from photographic resightings, analysis of social associations among individual whales (Durban et al. 2010, Marine Biology; Fearnbach 2012, Ph.D. Thesis), and satellite telemetry data (J. Durban, SWFSC, unpublished data; Matkin et al. 2012, Fishery Bulletin) suggest that some individual killer whales and pods (matrilineally-related stable social groups) exhibit a high degree of site fidelity. However, estimates of gene flow and a quantitative assessment of genetic structure are lacking for killer whales in the northern North Pacific, and documented movements of individual whales between regions suggest a certain degree of connectedness. Reflecting the uncertainty surrounding population structuring and a lack of data for the westernmost reaches of the northern North Pacific, current stock designations encompass very broad areas. Currently, “resident” (fish-eating) killer whales are recognized as a single stock from Southeast Alaska through the Aleutian Islands and Bering Sea (Allen and Angliss 2011, NOAA Tech. Memo.). The U.S. MMPA stock designation for Bigg’s (aka “transient” or mammal-eating) killer whales recognizes two stocks with overlapping geographic distributions, comprising the Gulf of Alaska, Aleutian Islands, and Bering Sea stock and the much smaller community of AT1 killer whales, whose range appears to be largely restricted to Prince William Sound and the Kenai Fjords (Allen and Angliss 2011; Matkin et al. 1999, Fishery Bulletin). (continued)


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