Photos: Kathy Frost and Vladimir Burkanov
Harbor seals (Phoca vitulina) have the broadest distribution and inhabit the widest range of habitats of any other pinniped (Burns, 2002). Denizens of the coastal and continental shelf waters of the north Atlantic and Pacific oceans, harbor seals haul out on land or glacial ice floes to rest, breed, molt and nurse their young. Breeding and molting colonies can number in the thousands. These medium-sized seals are members of the family Phocidae, or true seals, and can grow to 186 cm in length and up to 170 kg in weight, although typically are much smaller. Adult males are larger than adult females, sexual maturity is reached at 4-5 years of age and adult females typically produce a single pup each year. Despite occasional long-distance movements in excess of 200km, harbor seals in both the Pacific and Atlantic oceans rarely forage more than 50km away from haul-out sites (e.g., Lowry et al., 2001). Although they have a near continuous distribution in both ocean basins, harbor seals exhibit geographic variation in body size, coat color, annual timing of births, and cranial morphology.
Harbor seals in the Pacific Ocean are distributed along 16,00km of coastline across the North Pacific rim from Hokkaido, Japan northward to the Kamchatka peninsula, eastward across the Aleutian Islands, southern Bering Sea and Gulf of Alaska, and southward to Baja California, Mexico.Once considered abundant throughout their Alaska range, harbor seal numbers have declined dramatically in some areas over the past few decades while abundance has increased or remained stable in other areas over similar time periods. These declines and differences in trend highlight the need for the definition of management units where abundance, population trend and other aspects of harbor seal biology can be monitored in relation to both direct and indirect-human caused mortality at spatial scales where effective measures can be taken to fulfil the management objectives of preventing depletion, promoting recovery and maintaining the species range.
The National Marine Fisheries Service (NMFS) currently recognizes three separate stocks of harbor seal in Alaska, identified primarily on the basis of regional differences in trend. At the time of their designation, however, it was recognized that large gaps existed in our knowledge of dispersal and movement patterns and population structure, and it was recommended that more information on these aspects of seal biology were required to define more meaningful management units.
Over the past decade we have conducted a number of molecular genetic studies on the evolutionary history, population structure and dispersal patterns of harbor seals in the north Pacific. By examining variation within the mitochondrial genome (mtDNA), we discovered extensive macrogeographic subdivision among primary centers of abundance from southeast Alaska to Japan. Genetic differentiation was correlated with geographic distance, suggesting that dispersal, when it occurs, is among neighboring subpopulations (Westlake and O'Corry-Crowe, 2002). A more detailed study within Alaska required the development of new techniques to assess population structure in continuously distributed species. This study revealed substantial population structure on scales of 150-540km, depending on the region and indicated that current stocks were too broadly defined to meet the management objectives of the Marine Mammal Protection Act (MMPA) of maintaining population stocks as functioning elements of their ecosystem. Furthermore, the genetic findings provided a framework for the definition of more meaningful stocks (O'Corry-Crowe et al., 2003).
Currently, we are analyzing variation in a number of other genetic markers and working on ways to integrate our genetic results with recent findings from studies on harbor seal distribution, movement and foraging behavior, trends in abundance, and diet in order to better resolve the spatial pattern of population structure of this species in Alaska.
Burns, J.J. (2002) Harbor and Spotted Seal. pp. 552-560 In Encyclopedia of Marine Mammals. (W.F. Perrin, B. WÃ¼rsig and J.G.M. Thewissen, eds.), Academic Press. San Diego.
Lowry, L.F., Frost, K.J., Ver Hoef, J.M. and DeLong, R.A. (2001) Movements of satellite-tagged subadult and adult harbor seals in Prince William Sound, Alaska. Marine Mammal Science, 17: 835-861.
O'Corry-Crowe, G.M., Martien, K.K. and Taylor, B.L. (2003). The analysis of population genetic structure in Alaska harbor seals, Phoca vitulina, as a framework for the identification of management stocks. National Marine Fisheries Service, Southwest Fisheries Science Center Administrative report LJ-03-08. 54p
Westlake, R.L. and O'Corry-Crowe, G.M. (2002). Macrogeographic structure and patterns of genetic diversity in harbor seals (Phoca vitulina) from Alaska to Japan. Journal of Mammalogy, 83(4): 1111-1126.