Investigating Harbor Seal (Phoca vitulina) haul-out abundance in relation to time of day at San Juan Island haul-out site

Abstract

Harbor seals (Phoca vitulina) are an extremely widespread pinniped in the Pacific Ocean and play a vital role in the Salish Sea ecosystem. Haul-out behavior, where the individuals will temporarily leave the water and move onto rocks, islands, beaches, etc., is essential for most pinniped species in terms of thermoregulatory advantages, rest, reduced risk from predators, rearing pups, molting, etc. Although the behavior of seals at haul-out sites is well studied, significance between abundance and time of day have not been as investigated. This study tested whether harbor seal counts were significantly higher in the afternoon (3:00pm to 4:30pm) than in the morning (9:00am to 11:00am). Results showed that trends between high harbor seal count and morning time frames are significant. Trends from the data below also suggest that low tide events may facilitate high haul out abundance. The significance and trends found in this study imply potential impacts on the Salish Sea ecosystem due to harbor seals being a main prey source for mammal-eating Bigg’s killer whales (Orcinus orca) and predators to Pacific herring (Clupea pallasii), an essential forage fish threatened by climate change. Time of day could also imply higher risks of anthropogenic impacts in areas surrounding San Juan Island, as the channel adjacent to Goose Island is a high vessel-traffic area.