Harbor Seals in Hood Canal: Predators and Prey
A dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the University of Washington, School of Aquatic and Fishery Sciences. Seattle, Washington, USA. 2006.
The foraging ecology and population dynamics of harbor seals (Phoca vitulina richardsi) were studied in Hood Canal, Washington from 1998 to 2005. Initial work was conducted in response to concerns over the potential impact seals may have on recovering populations of summer chum salmon. Direct observation of harbor seals consuming salmon within the inter-tidal regions of four rivers in Hood Canal were conducted from 1998-2001 and 2003. Seals were observed feeding on chinook, coho, pink, summer chum and fall chum salmon. Estimates of summer chum consumption by seals at each of the observation sites averaged 8.0% of returning adults across all sites and all years. The maximum percentage of returning chum consumed was 27.7% and the lowest was 0.84%. The number of seals observed foraging in the river for salmon averaged from two to seven seals. Summer chum populations in the study streams have increased over the time of the study to near historical highs. Because of this increase, the levels of predation observed are not believed to significantly impact the recovery of summer chum in Hood Canal. A protocol for extraction of DNA and identification of seal sex from scats was developed to examine differential diets between male and female harbor seals. Scats from both sexes contained similar levels of Pacific hake, but male scats contained more salmon and female scats contained more Pacific herring. In 2003 and 2005, mammal-eating killer whales foraged exclusively within Hood Canal for 59 and 172 days respectively. Bio-energetic models and boat based observations were used to estimate harbor seal consumption by killer whales and, in both years, the predicted consumption was approximately 950 seals. However, aerial surveys conducted following the two foraging events have not detected a significant decline in the harbor seal population.
