Tidal influence on harbor seal foraging activity in the San Juan Islands, WA: Indirect insight on salmon in Cattle Pass

Abstract

Due to the generalist nature of its diet, the harbor seal (Phoca vitulina richardsi) has been thought to be an indicator of change in ecosystems. By utilizing expected patterns in salmon presence, this study sought to assess the capacity of harbor seal foraging to act as an accurate proxy for salmon presence and activity in Cattle Pass, Washington. Salmon were expected to be present in the channel beginning mid-summer, at which point it should be easier for individual fish to migrate northward through Cattle Pass during northward-flowing flooding tides. During the summers of 2010 and 2011, harbor seal presence and foraging was observed with respect to tidal phase and current velocity. Of all salmon captures observed during 2010, 79.2% occurred during flooding tides and 65.81% occurred during the month of August. During 2010, harbor seal foraging reflected expected seasonal and tidal patterns of salmon presence in the pass. Results support the proposal that with additional data collection, harbor seal foraging has the potential to act as successful indicators of salmon presence and activity in Cattle Pass.