| Summary: | Harbor seal ( Phoca vitulina ) foraging has been well documented during the day, however, research shows that harbor seals frequently feed at night. Our study investigates both the diurnal and nocturnal feeding behavior of harbor seals in the San Juan Islands from August 8-12th, 2023. Diurnal observations near Cattle Pass collected data on dive frequency and surface behavior while nocturnal observations at Friday Harbor Laboratories dock collected data on prey species, foraging duration, as well as strike and success rate. Results from diurnal observations indicated an increase in seal abundance and dive frequency during slack and flood tides which is consistent with the tidal coupling hypothesis. Anecdotal results also showed synchronized group diving, demonstrating evidence for cooperative foraging behaviors. Nocturnal observations identified Pacific sand lance as the primary prey during night feeding. Furthermore, with every additional seal, dive duration decreased, while strike and success rates increased, implying cooperative or opportunistic feeding strategies. Our findings showcase the importance of nocturnal studies and we urge further research to be conducted to better understand the strategies behind nocturnal feeding.
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