Temporal and spatial variability of harbor seal diet in the San Juan Island archipelago

Harbor seals are the most abundant resident pinniped species in the San Juan Islands of Washington. They forage high on the food chain, are a relatively long-lived species, and have been used as sentinels of marine ecosystem health. Harbor seals are primarily piscivorous and consume seasonally and locally abundant prey, which allows us to investigate changes in their prey base, both on a temporal and spatial basis by examining diet. We use percent frequency of occurrence of prey species in fecal samples (scats) collected from rocky island and reef haul outs in the San Juan Islands to describe diet seasonally and regionally in 2006-2007 and to examine changes in diet between 2005-06 and 2006-07.

Harbor seals fed mainly on Pacific herring (occurring in 57% of samples), adult salmonids (19%), and Walleye pollock (15%). Diet differed among seasons with Pacific herring and Northern anchovy important during spring, adult salmonids and Pacific herring important during summer/fall and Pacific herring, walleye pollock, shiner perch, rockfish species, and sculpins important during winter. Mean number of different prey species differed among seasons with winter diet the most diverse (2.79 prey species) and summer/fall and spring slightly less diverse with (2.20 and 1.98 prey species respectively). In general, species composition in harbor seal diet was similar to fish abundance based on bottom trawl data and the timing and abundance of salmon return through the San Juan Islands. Pacific herring was the most important prey species for both study periods. In 2006-07, gadid species, adult salmon, Pacific sand lance, Northern anchovy, spiny dogfish and flatfish species decreased in occurrence and shiner perch, rockfish species, plainfin midshipman and eelpout species increased in occurrence. We also collected Steller sea lions scats and found dogfish and skate occurred most frequently in the diet. Our results suggest that harbor seal diet provides a good indicator of fish availability and consequently, ecosystem health and can be used to detect localized changes in prey availability.