A Comprehensive Approach for Estimating Harbor Seal Abundance in Alaska

Estimating the statewide abundance of harbor seals (Phoca vitulina) in Alaska has always been challenging. Harbor seals range from southeastern Alaska through the Aleutian Island chain into Bristol Bay. Past eorts have involved dividing the state into ve regions and conducting aerial surveys from small aircraft in one of those regions annually. An estimate is only possible once every ve years, and trends were determined from surveys conducted by the Alaska Department of Fish and Game (ADFG) at a small subset of sites. We present a comprehensive, new approach that overhauls the design and techniques for aerial surveys of harbor seals in coastal regions and tidewater glacier habitats. The new approach adopted by the NOAA National Marine Mammal Laboratory (NMML) and ADFG allows surveys to be conducted annually across the entire range. This is accomplished by dividing the coastline into distinct survey units and focusing survey eort such that units with more seals are surveyed more frequently. This was rst implemented for the statewide surveys in August 2008. Seven planes were distributed throughout most of the range and new technologies were used to improve navigation and data collection eciency during the surveys. We estimate approximately 70 percent or more of the seals hauled out during our survey were photographed from the air. This approach also incorporates a new model of haul-out behavior based on the deployment of over one hundred satellite tags. The model accounts for eects of time of day, day of year and tidal state. Seals are more likely to haul-out near mid-day within 15 minutes of the low tide during the August molt period. Not only is this critical for estimating the number of seals in the water at the time of a survey, it also provides additional exibility in the timing and implementation of the survey. In collaboration with ADFG, we hope this new approach will result in annual estimates of harbor seal abundance and trends in Alaska.