Harbor seal diet in Hood Canal, South Puget Sound and the San Juan Island archipelago

Harbor seals are the most abundant resident pinniped species in Puget Sound. 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 river mouths in Hood Canal, sandy haulouts in South Puget Sound and rocky island and reef haul outs in the San Juan Islands to describe diet seasonally and regionally and to examine potential changes in diet over time.

In Hood Canal during fall (1998-2001 and 2003-2005), harbor seals fed mainly on Pacific hake (occurring in 79% of samples), Pacific herring (30%) and adult salmonids (26%). Diet shifted during spring (1999 and 2005) with Pacific hake (85%) and Pacific herring (26%) remaining important, but Northern anchovy (35%) increasing significantly and adult salmonids (8%) decreasing significantly.

South Puget Sound diet was dominated by gadids (99%), both Pacific tomcod and Pacific hake. Clupeid species (69%) also composed a large proportion of the diet, but unlike the other regions, plainfin midshipman (47%) and flat fish (33%) were primarily primary prey for harbor seals. Minor seasonal fluctuations in occurrence were observed for cephalopods, shiner surfperch, juvenile salmonids and rockfish. Prey assemblages in south Puget Sound seal diet are likely due to differences in habitat and the predominance of relatively shallow and calm inlets and bays. In the San Juan Islands, Pacific herring was the most important prey species overall (57%). Gadid species (winter and spring), adult salmon (summer/fall), Pacific sand lance (winter and spring), and Northern anchovy (winter) were important more seasonally. The San Juan Islands are influenced by the Strait of Juan de Fuca and Strait of Georgia and seal diet reflects this dynamic region.

In comparing these three very different regions, clupeids (primarily herring) were a significant component of seal diet in each region. Adult salmonids are important seasonally in Hood Canal and the San Juan Islands and a gadid species and Pacific herring dominate the diet year round. As adult salmon migrate into the region and become abundant, harbor seals forage primarily on salmon (range 25â€"67% frequency of occurrence). For gadids, seals preyed primarily on Pacific hake in Hood Canal, Pacific tomcod and Pacific hake in south Puget Sound, and walleye pollock in North Puget Sound/San Juan Islands. Northern anchovy were important in Hood Canal and the San Juan Islands during winter, but not in South Puget Sound. High proportions of midshipman and flat fish in seal diet were unique to South Puget Sound. Many of the other important prey species were similar among regions including shiner surfperch, cephalopods, sand lance and sculpins. These data correlate well with species composition in each region and suggest that harbor seal diet provides a good indicator of fish availability and can be used to detect localized changes in prey availability.