Comparison of survey methods for estimating abundance of harbor seals (Phoca vitulina) in glacial fjords

• Main Authors: Bengtson, John L., Phillips, Alana V., Mathews, Elizabeth A., Simpkins, Michael A.
• Format: Article in Journal/Newspaper
• Language: English
• Published: 2007
• Subjects: Biology; Ecology; Fisheries; Glacier Bay; Hopkins Inlet; glacier; Phoca vitulina; Alaska
• Online Access: http://hdl.handle.net/1834/25520

The importance of glacial ice habitats to harbor seals (Phoca vitulina) in Alaska has become increasinglyapparent. 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-basedcounting method to a large-format aerial photography method to estimate seal abundance. During each aerial survey, shore-based observers simultaneously counted seals from anobservation 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 mobileseals from shadows or glacial debris of similar size. Aerial photography provided a permanent record of eachsurvey, allowing both a reconciliation of counts in overlapping zones and the documentation of the spatialdistribution of seals and ice within the fjord.