Influence of Sex and Body Mass on Harbor Seal (Phoca vitulina) Diving Behavior

• Main Author: McFarland, Alexandra Sasha
• Format: Text
• Language: unknown
• Published: Western Washington University 2013
• Subjects: Bird Rocks; San Juan; harbor seal; Phoca vitulina
• Online Access: https://dx.doi.org/10.25710/4vkn-n967; https://cedar.wwu.edu/wwuet/288

The oceans are under increasing stress, both anthropogenic and natural. In the inland waters of Washington State, several fish species are showing depressed numbers. The harbor seal (Phoca vitulina), a piscivore, is the only year-round resident pinniped in the San Juan Islands. The numbers of this species have increased dramatically since the end of the bounty hunt in Washington State in 1960; however, the potential impacts of this predator on fish stocks are unknown. To predict population trends in prey fish stocks, we first must understand how they interact with their predators. Predators have energetic needs that vary with sex and body mass, potentially leading to different prey choice and consumption rates and, hence, to declines of targeted fish stocks. Harbor seals are an abundant, monomorphic predator, and their diving behavior can help us determine how sex, body mass, or both might use their habitat, and thus influence prey populations. I examined the diving behavior of adult harbor seals captured from the San Juan Islands, Padilla Bay, and Hood Canal, Washington, by comparing five variables summed or averaged per week: average maximal dive depth, total dive duration, total number of dives, total number of diving bouts, and Dive-profile Index (DPI; this variable is derived from the dive profile using an image analysis tool, and represents dive depth and dive duration). All five variables were tested using linear mixed-effects models to determine differences in the effects of body mass (small, medium or large) and/or sex (pregnant females, non-pregnant females, and males) while accounting for the random effects of haul-out site, date (week number) and individual seal. Comparisons were conducted during the pre- (April 5-July 5) and the post- (July 5-October 3) pupping seasons of 2002 and 2005 in Hood Canal, and 2007 and 2008 at Bird Rocks and Padilla Bay. Body mass influenced several dive variables: larger seals dived significantly deeper than smaller seals during the pre-pupping season and for significantly longer durations during the post-pupping; they also had a significantly higher DPI than smaller seals during both seasons. Sex however had no effect on any dive variable. Deeper dives, longer dive durations, and higher DPI suggest that larger seals may have a greater negative impact on their targeted prey stocks than do smaller ones. When coupled with data on size distribution and bioenergetics models for harbor seals in the study area, the results of this study can help in assessing the impacts of this species on fish stocks.