Declines in Harbor Seal Numbers in Glacier Bay National Park, Alaska, 1992–2002

Glacier Bay National Park had one of the largest breeding aggregations of harbor seals in Alaska. It is functionally the only marine reserve for harbor seals in Alaska, yet numbers of seals in the Bay are declining rapidly. Understanding why seals in Glacier Bay are declining may clarify their minimal habitat needs. We estimated population trends using models that controlled for environmental and observer-related factors. In 1992, 6,200 seals were counted on icebergs in a tidewater glacial fjord and at terrestrial sites. By 2002, only 2,550 seals were counted at the same haul outs. Numbers of non-pups in the glacial fjord declined by 6.6 percent per year (−39 percent over 8 years) in June and by 9.6 percent per year (−63 percent over 11 years) in August. At all other haul outs numbers of non-pups declined by 14.5 percent per year (−75 percent over 10 years) during August. In the glacial fjord the number of pups remained steady from 1994 to 1999. Pups made up an increasing proportion of seals counted (5.4 percent per year) in the glacial fjord, and the proportion of pups peaked at 34–36 percent. The rapid declines do not appear to be due to changes in seal behavior or redistribution. The declines reinforce genetic evidence that harbor seals in Glacier Bay are demographically isolated from other populations and indicate that current management stocks need to be redefined. Changes in Glacier Bay’s ecosystem and population demographic data from the glacial fjord suggest that interspecific competition and predation are likely factors in the declines.

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Elizabeth A. Mathews and Grey W. Pendleton. Published in Marine Mammal Science, 22(1): 167–189 (January 2006).