Behavioral response of Harbor seals (Phoca vitulina) to vessel disturbances on Yellow Island, WA, USA

Abstract

In the Salish Sea, harbor seals use gravel beaches and rocky shores to haul out for rest and raising pups. While their populations have rebounded after the implementation of the Marine Mammal Protection Act (MMPA), increases in vessel traffic pose energetic and behavioral threats such as increased vigilance, flushing, and raised heart rates. Loud noises as well as human presence greatly contribute to these trends through high levels of disturbance. To understand the extent of these effects, we used opportunistic and interval surveys at Yellow Island, WA, USA to observe the types and speeds of boats that entered within 400 meters of seal haulouts and caused disturbances. We also observed harbor seal behavioral responses and categorized them by adults, mother-pup pairs, and lone pups. Our results were then compared to a prior study on vessel disturbance to analyze trends over time. We found that fast moving motor boats were the most common vessel, though kayaks and other slow vessels caused the highest levels of disturbance, notably flushing. The highest percentage of vessels was within 300 meters of the seals, though the closest vessels caused the most disturbance, especially when they entered the buffer zone (91 meters). Adults reacted the most to boat presence, with lone pups reacting the least. Vigilance also decreased from 2021 to 2025, indicating signs of habituation to consistent vessel noise. These trends indicate that regulations should be updated to account for increased habituation and stress levels to protect harbor seals. Education on the buffer zone should also be expanded. Keywords: