Differential Scanning between Sex and Age Classes of Pacific Harbor Seals (Phoca vitulina richardii) at Yerba Buena Island

摘要

Previous studies focusing on haul out behavior of harbor seals (Phoca vitulina spp.) have shown conflicting results in the frequency and the reasons for scanning. These studies indicate that scanning (movements increasing a seal's visual field) is used differently by one sex or by certain age classes of seals over others at different haul out sites and in different contexts. This study, which compares differences in the number of scans per unit time (scanning bouts) between seals of different ages and sexes, was conducted between October 1998 and September 1999 at Yerba Buena Island (YBI). YBI is one of the largest haul out sites in the urbanized estuary of San Francisco Bay, California. The haul out site, located along the southeast shoreline of YBI, is primarily utilized by adult male seals at all times of the year. During each survey, up to three seals at different locations of the site were selected. The behavior of the focal seals was recorded for 15 seconds per minute per seal for up to eight ten-minute observations during 45 four-hour surveys. Analysis of the number of scanning bouts indicates that adult females and immature seals scanned significantly more often than adult males. This difference was not significant between adult females and immature seals, nor was it significant between the three groups in times of disturbance. This suggests that other factors, such as the high proportion of adult males at the site, might be a factor in the increased scanning rates of adult females and immature seals. Adult females and immature seals using the site may scan more frequently in order to avoid aggressive encounters between male seals.