Haul Out Site Use and Substrate Preference in Harbor Seals (Phoca vitulina) on Yellow Island in the San Juan Islands, Washington

Abstract

Harbor seals (Phoca vitulina) are highly abundant marine mammals in the Salish Sea and require various substrates for haul-outs. The purpose of this study is to identify the potential differences in haul-out substrate preferences, see how use changes with different conditions, and how different demographics use the haul-out. We used land-based, visual surveys on Yellow Island in the San Juan Islands to assess haul-out abundance and use. We found that tide height increases abundance and increases the proportion of lone adults using the haul-out. Increasing tide height also reduces the availability of the preferred rocky substrate. We found that increasing air temperature increases abundance and increases the proportion of mother/pup pairs using the haul-out. Substrate use showed a preference for rocky substrate available at low tide and compulsory gravel use as the tide rose. Our trends in tidal effects on haul out use are well supported, but further study is needed to decouple the time of day and air temperature variables. Understanding haul-out use and dynamics helps agencies establish protections around essential territories for marine mammals.