Tidal Influence on the Haul-out Behavior of Harbor Seals (phoca Vitulina) at a Site Available at All Tide Levels

Harbor Seals (Phoca vitulina) are the most abundant and widespread seal species in Washington State. Most seal haul-out sites are unavailable at high tide, hence abundance estimates are conducted at low tide when it is commonly accepted that most seals are hauled-out. On sites unaffected by tides, however, it is less clear whether tide level has an effect on seal abundance. We counted hauled-out Harbor Seals from sunrise to sunset on floating water-breakers at Semiahmoo Marina, Washington, to examine the effect of tides on haul-out behavior. Because haul-out behavior is affected by several factors, we conducted mixed-factor analyses that included tide level, tidal current, time of season, and time of day as fixed factors, and several meteorological variables as random factors. The number of hauled-out Harbor Seals was significantly associated with tide level, time of season, and time of day. Results suggest that seal counts in Semiahmoo Marina should be made late in the pupping season and early in the afternoon at moderately positive tide levels to achieve the highest counts. They also indicate that tide was associated with seal numbers unrelated to site availability because seal numbers were positively related to tide height, a finding opposite to studies at tidal haul-out sites.