Estimating importance of rockfish, lingcod and other bottomfish in the diet of harbor seals in the San Juan Islands

Harbor seals are the most abundant pinniped species in the inland waters of Washington east of Cape Flattery, numbering nearly 15,000 animals and are important upper trophic level predators. In the San Juan Islands, harbor seals are the most commonly seen pinniped and use over 150 sites as haulout locations. This study uses percent frequency of occurrence of prey species in fecal samples (scats) collected from rocky habitat haul outs in the San Juan Islands to describe overall diet and determine importance of rockfish, lingcod and other bottom fish in the diet of harbor seals, a primarily piscivorous apex predator. Harbor seals fed mainly on Pacific herring (occurring in 57% of samples), adult salmonids (31%), Gadid species (24%), Pacific sand lance (20%) and Northern anchovy (19%). Diet differed among seasons with Gadids, Pacific herring and Pacific sand lance important during spring, adult salmonids and Pacific herring during summer/fall and Pacific herring, Northern anchovy, Gadids, Pacific sand lance and Spiny dogfish during winter. Overall, Rockfish and Hexagrammid (lingcod and greenling) remains were found in only 2.3% and 1.0% of samples collected, respectively. Mean number of different prey species differed among seasons with winter diet the most diverse, 3.59 different prey species and summer/fall and spring less so with 1.72 and 2.14 different prey species, respectively. In general, species composition in harbor seal diet was comparable to fish abundance based on bottom trawl data and salmon return abundance timing in the San Juan Islands. We suggest these data are an important component for assessing how predator populations, specifically harbor seals, may impact fish populations and overall biodiversity in the San Juan Islands.