The Effect of Human Exposure on the Anti-predatory Response of Harbor Seals (Phoca vitulina)

Abstract

Increased exposure to anthropogenic activities often results in animals developing higher tolerance to activities that would otherwise disturb them. Although this response is sometimes viewed as a beneficial survival mechanism, habituation to human activities may lead to negative consequences, such as decreased predator aversion. Due to their healthy population status, the numerous and varied locations of their haul-out sites and their anti-predatory response to bald eagles, harbor seals in the Salish Sea are an ideal study system to answer questions concerning the effects of human exposure on wildlife. To examine my hypothesis that increased human exposure reduces anti-predatory response in harbor seals, I used instantaneous scan-sampling techniques to compare the behavioral responses of seals to bald eagles and to humans at six haul-out sites with varying levels of exposure to anthropogenic activities. Sites were classified as high exposure (6.07 ± SE 0.73 boats • h-1, n=2 sites), medium exposure (1.56 ± SE 0.38 boats • h-1, n=2 sites), and low exposure (0.26 ± SE 0.12 boats • h-1, n=2 sites). Based on generalized linear mixed-effects models (GLMM), the presence of harbor seal anti-predatory response to bald eagles was significantly related to human exposure but not to eagle exposure. Seals showed an anti-predatory response to eagles more often at low-exposure sites (77.17%, n=127 events) than at medium- (60.0%, n=15 events) or high- (45.45%, n=33 events) exposure sites. Also based on GLMMs, human exposure significantly influenced harbor seal response to boat traffic. Seals reacted to passing boats more often at low-exposure sites (100%, n=10 events) than at medium- (77.8%, n=72 events) or high- (83.2%, n=208 events) exposure sites. This study provides the first empirical evidence outside of an urban setting to support the hypothesis that increased interactions with humans can lead to a reduction in overall predator aversion. This finding highlights the potential impact that increased human exposure can have on the predation risk of wildlife populations and disputes the traditionally accepted view of habituation as having little-to-no impact on the animals involved.