PARENT SESSION

HARBOR SEAL MOVEMENTS IN THE ALSEA ESTUARY, OREGON. Bryan E. Wright¹, Robin F. Brown¹, Susan D. Riemer² and Aicha M. Ougzin³. ¹ Oregon Dept. of Fish and Wildlife, 7118 NE Vandenberg Ave, Corvallis, OR; ² Oregon Dept. of Fish and Wildlife, P. O. Box 642, Gold Beach, OR; ³ 820 Canary Drive, Suisun City, CA.

ABSTRACT- The co-occurrence of abundant pinnipeds and depressed salmonid stocks has been identified as a potentially significant problem in the recovery of some salmonid populations. One such problem may occur at the Alsea Estuary, Oregon, where a relatively large population of Pacific harbor seals (Phoca vitulina richardsi) co-occurs with a depressed run of threatened coho salmon (Oncorhynchus kisutch). We investigated this potential problem during fall 2002 using three approaches: (1) surface feeding observations to estimate total diurnal adult salmonid predation by seals; (2) marking and tracking of seals to characterize their movements, habitat use and foraging behavior; and (3) collection of seal fecal samples to describe their diet. We report here on the results from the second approach. We captured and marked 59 seals during August 2002. Seals were outfitted with acoustic transmitters and passively tracked using an array of 15 data-logging hydrophones. Receivers were located from approximately 2 km offshore to 19 km upriver. We logged over 375,000 detections from 56 seals over a 12-week study period. The number of individual seals detected declined with increasing distance upriver. Although 23 seals were detected in the river at least once, just 7 animals accounted for 94% of the total time spent there (5,067 hours). Seals captured on an upper-bay haul-out appeared to forage upriver more than seals captured on a lower-bay haul-out. Seals used the river more at night than expected given the relative availability of day and night. Based on our diurnal surface feeding observations, anecdotal nocturnal observations, and fish survey data, we believe that seals only traveled upriver to prey on salmonids. We speculate that the majority of salmonid predation by seals in this estuary occurred upriver, at night, and by a relatively small proportion of the local seal population.

KEY WORDS: harbor seal, acoustic telemetry, Phoca vitulina, movements