Comparison of Survey Methods for Estimating Abundance of Harbor Seals in Glacial Fjords
A study to compare the relative effectiveness of shore-based and aerial survey methods to estimate harbor seal abundance.
The importance of glacial ice habitats to harbor seals (Phoca vitulina) in Alaska has become increasingly apparent. However, enumerating harbor seals hauled out on ice in glacial fjords has been difficult. At Johns Hopkins Inlet in Glacier Bay, Alaska, we compared a shore-based counting method to a large-format aerial photography method to estimate seal abundance. During each aerial survey, shore-based observers simultaneously counted seals from an observation post. Both survey methods incurred errors in double-counting and missing seals, especially when ice movements caused seals to drift between survey zones. Advantages of shore-based counts included the ability to obtain multiple counts for relatively little cost, distinguish pups from adults, and to distinguish mobile seals from shadows or glacial debris of similar size. Aerial photography provided a permanent record of each survey, allowing both a reconciliation of counts in overlapping zones and the documentation of the spatial distribution of seals and ice within the fjord.
John L. Bengtson, Alana V. Phillips, Elizabeth A. Mathews, and Michael A. Simpkins. Published in the Fishery Bulletin 105(3), July 2007.