Effects of Vessel Disturbance on Harbor Seal (Phoca vitulina) Abundance and Behavior at Yellow Island and Goose Island, WA, USA

Abstract

Harbor seals (Phoca vitulina) are pinnipeds inhabiting northern temperate coastal regions, with one of their most dense populations found in the Salish Sea of the Pacific Northwest. They use haul-out sites for activities such as rest, nursing, and protection from predators. Vessel traffic around haul-out sites has previously been shown to cause behavioral disturbance. We conducted a behavioral study on two harbor seal haul-out sites in the San Juan Islands, WA, USA: Yellow Island and Goose Island. By conducting behavioral focal and scan sampling with and without vessel presence, we quantified the impact of vessel disturbance on behavior. We observed a higher baseline vigilance rate at Goose Island. Moreover, we found that vessel presence increased vigilance at Yellow Island and decreased abundance on land at Goose Island. We believe that the varying behavior and vessel responses observed at each location are likely due to the ecological and topographical differences between the two haul-out sites. Increased vigilance and flushing incur higher levels of stress and increased energy expenditures, causing these seals to spend less time on important behaviors like rest and reproduction. While harbor seal populations are not currently at risk or endangered, vessel traffic in the Salish Sea is expected to increase by at least 25% in the near future as more vessel projects are approved every year. As this anticipated rise occurs, the observed effects of vessel disturbance on harbor seal behavior observed in this study are likely to worsen over time. This could potentially threaten the future health and abundance of harbor seal populations in the Salish Sea.