| Summary: | There is increasing evidence that predation by harbour seals (Phoca vitulina) on salmon smolts out-migrating from rivers may be a significant source of mortality for coho (Oncorhynchus kisutch) and Chinook (O. tshawytscha) salmon populations in British Columbia. Studies supporting this have focused on documenting what and how much seals eat—and the potential impact this has on salmon populations. However, little attention has been given to understanding where, when and how this predation occurs, and the extent to which it might be opportunistic or specialist feeding behaviour by a few or many individuals. I documented the spatiotemporal foraging behaviour of harbour seals in the Salish Sea by equipping 17 seals with biologgers—and tracking them before and after the release of thousands of coho and Chinook smolts from the Big Qualicum Hatchery. Reconstructing the high-resolution movements of the seals―and quantifying feeding using counts of prey chasing events (PCEs) detected by accelerometry—revealed that the Big Qualicum estuary was a feeding hotspot for 47.0% of the seals, but was relatively small (accounting for 3% of PCEs) compared to the largest feeding area outside the estuary (26% of PCEs). Comparing the foraging behaviours of smolt specialists with non-specialist seals revealed 4 different foraging strategies. One consisted of seals (17.6%) that fed on coho smolts and ignored Chinook in the river mouth, while a second group of seals (17.6%) appeared to target larger fish that preyed on Chinook smolts near the estuary. The two other seal groups did not feed in the estuary in association with the concentrated numbers of smolts, but either remained resident (52.9%) and fed near their main haulouts, or were transient (11.8%) and left the study area. My results suggest a high degree of individual foraging specialization—and show that a small number of seals were specialized in consuming coho smolts, but did not appear to respond to the large pulse of smaller bodied Chinook smolts during the outmigration. Such information concerning the fine-scale foraging behaviour of harbour seals in relation to pulses of out-migrating smolts can be used to design mitigation strategies to enhance coho and Chinook populations. Science, Faculty of Zoology, Department of Graduate
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