Trophic relationships of harbor seals in the San Juan Islands

Steven J. Jeffries and Monique Lance
Washington Department of Fish and Wildlife
$34,525

Harbor seals are the most abundant pinniped species in the San Juan Islands (SJIs), and because they are a high trophic level, relatively long-lived species, they have been used as sentinels of marine ecosystem health, providing an integrated indicator of the state of food webs upon which they depend, and the environment in which they live. Harbor seals are primarily piscivorous apex predators that consume seasonally and locally abundant prey, which allows us to investigate changes both temporally and spatially in their foraging ecology by examining diet. Prey selection, both quantity and quality, is essential to assessing the role of harbor seals in marine ecosystems and can be used to detect localized changes in prey availability.

In 2005, with SeaDoc Society funding, we began collecting baseline data on harbor seal diet in the SJIs from haulouts near Marine Protected Areas (MPAs) to determine importance of rockfish, lingcod and other species in their diet. We request a second year of funding to investigate changes in predator prey relationships among years, seasons and regions in the SJIs. We propose an applied ecological study to advance knowledge, as well as improve conservation and management practices by characterizing linkages between harbor seal diet and the SJIs ecosystem.

Preliminary results from 2005 suggest Pacific herring is an important prey species during both spring and summer/fall. Pacific herring is also an important prey species for thousands of seabirds that breed and winter in the SJIs, as well as other marine species. Investigating the relationship between harbor seals and forage fish communities is important in understanding how they integrate into and impact the SJIs marine food web. Seasonal differences were detected with Gadids (codfishes) being important prey during spring and salmon being the dominant prey during summer/fall. When salmon species identification was possible using otoliths, the majority were pink salmon. Examining diet composition during a non-pink salmon year (2006) would provide a more complete picture of the role harbor seals play in the SJIs marine environment.

Harbor seals are truly “samplers” of the marine environment. Scat analysis is a reliable and easy way to investigate relative importance of prey species in the diet of harbor seals both temporally and spatially. We propose to assess seal diet by collecting scats seasonally during March-April (‘spring’), August-September (‘summer/fall’) and December-January (‘winter’). Scats would be collected from haulout sites in six SJI regions including E. side SJIs, N. side SJIs, N. Rosario Strait, S. Rosario Strait, S. San Juan Channel, and N. San Juan Channel. A minimum of 100 scats would be collected in each region, with effort adjusted to ensure statistically valid sample sizes among seasons and regions.

Scats would be processed and prey species identified to characterize seasonal composition by region using standard protocols. Prey size would be estimated using otolith size regressions for selected fish species, and representation of different age classes of fishes in the diet would be analyzed. Dietary preferences would be examined by comparing prey composition in the diet to available fish composition data.