Unusual Open Water Grouping Behavior in Salish Sea Harbor Seals (Phoca vitulina richardii)

• Main Author: Derie, Trevor
• Format: Text
• Language: English
• Published: Western CEDAR 2022
• Subjects: Canada; British Columbia; Phoca vitulina
• Online Access: https://cedar.wwu.edu/ssec/2022ssec/allsessions/182; https://cedar.wwu.edu/context/ssec/article/3344/viewcontent/202_e382b4845c90431d81d0627a04359842.pdf

Most pinniped species are relatively solitary when in water, but some species, most notably the otariids, will form large groupings (referred to as rafts) in open water for thermoregulation or rest, as well as participating in group foraging behaviors. Alternatively, individuals of many species may concentrate in one area, forming foraging aggregations when prey are in high abundance. Open water grouping behavior that is distanced from haulout sites is less common in phocid species, and in particular has not been documented in the literature for harbor seals (Phoca vitulina richardii). In the Salish Sea, the inland waters of Washington, United States and British Columbia, Canada, harbor seals are the most abundant pinniped species. Recent observations in two locations in the south and central Salish Sea have documented large groupings ranging from 6 to 50 individuals (x=23.9) not located near haulout sites (more than at least 1 mile from known large group haulout locations). These observations occurred only in April/May 2019-2020 off Fidalgo Island (n=14) and in February 2017 and January 2019 in southern Puget Sound (n=2). These groups consisted of only adults/juveniles floating together within 1-2 body lengths of each other, unlike larger aggregations where individuals are in the same area, but not necessarily as a group. Some observations included systematic diving where individuals took turns periodically diving, appearing to be foraging, while others remained at the surface. This behavior appears to differ from the rafting behavior observed in otariids. Though harbor seals are known to habitually haul out together on beaches or islands, grouping behavior while in the water, like that observed here, has not been previously described. The purpose of such large groupings is unknown and continued monitoring of these occurrences and further analysis of behavior is needed.