Long-term trends in abundance of harbor seals (Phoca vitulina richardsi) and development of monitoring methods in Glacier Bay National Park, Southeast Alaska

Mathews, Elizabeth

Harbor seals in Glacier Bay National Park use more than 20 different land haulouts during summer months, and they are found in large numbers in a tidewater glacial fjord where they are widely dispersed on ice bergs. The use of two very different haulout substrates, and the large area over which seals may be found, have precluded Parkwide counts of seals in the past. In 1992 counts of seals in a glacial fjord from land and counts of seals at land haulouts from aerial photographs were used to census harbor seals throughout Glacier Bay. The mean of daily high counts of seals in the Bay in August, 1992 was 5,982. At least 70% of the seals were found in Johns Hopkins Inlet, making this tidewater glacial system one of the largest documented breeding aggregations of harbor seals remaining in Alaska. Eighteen percent (1,090) were counted at the Spider Island reefs, the second largest haulout area in the Park and the largest land haulout. To assess the relative accuracy of aerial photographs compared to low-level counts, mean counts from land were compared to counts from aerial photographs of seals at Spider Island. Land-based counts from the low-level site were, on average, 51% (44%-61%) of counts from simultaneous photographs. To assess long-term trends in harbor seal abundance in Johns Hopkins Inlet, I compared the mean of daily high counts from the mid-1970s (Streveler 1979), from 1984 (Calambokidis, unpubl. data), and from my team's 1992/93 counts. The mean count from the early 1990s was significantly higher than that from the mid-1970s, however no change was detected between August 1984 and the early 1990s. These results indicate that the increase in seal numbers in Johns Hopkins Inlet occurred between the latter half of the 1970s and 1984. The proportions of mother/pup pairs in Johns Hopkins Inlet and Icy Bay (both glacial fjords) were higher than at the Spider Island land haulout and they appear to be higher than at land haulouts worldwide. Ice habitat generated in tidewater glacial fjords is important and ephemeral habitat for pupping and nursing harbor seals.

Long-term trends in abundance of harbor seals (Phoca vitulina richardsi) and development of monitoring methods in Glacier Bay National Park, Southeast Alaska

Part of