Spatial and Temporal Patterns of Predation By Harbor Seals on Migrating Steelhead Smolts Revealed By Acoustic Telemetry

We investigated interactions between harbor seals outfitted with acoustic transceivers and steelhead trout tagged with acoustic transmitters to estimate the degree of association between them and evidence for predation by harbor seals on steelhead smolts in Puget Sound. Detections of steelhead tags by harbor seals occurred both during and after the smolt outmigration period.  A total of 6,846 tag detections from 44 different steelhead trout smolts (243 were released into two rivers) were recorded by the 11 seal-mounted transceivers.  The 4 seals monitored in Central Puget Sound detected 37 of the 44 smolts (84%), and the 7 seals monitored in Admiralty Inlet detected 7 (16%) tagged smolts Central Puget Sound seals detected a greater proportion of available smolts (29 of 50; 58%) than Admiralty Inlet seals (7 of 50; 14%; P < 0.001).  At least 9 steelhead smolts detected by harbor seal transceivers were likely consumed by other (non-tagged) harbor seals based on stationary tag locations at haulouts and several additional indicators.  Steelhead smolts tagged with silent tags (for 10 d) had no greater survival than those tagged with continuously pinging tags, indicating no effect of tag noise on the survival of steelhead in Puget Sound.