Ultrasonic Telemetry Reveals Seasonal Variation in Depth Distribution and Diel Vertical Migrations of Sub-Adult Chinook and Coho Salmon in Puget Sound

Seasonal and diel vertical movement (DVM) patterns in aquatic systems have been well documented for many aquatic organisms but little is known about salmon distributions in marine waters. In this study, we examined the vertical distributions of sub-adult coho and Chinook salmon in Puget Sound using acoustic telemetry to: 1) compare the depths occupied by the two species, 2) determine whether either species displayed DVM, and if these patterns differed between the two species, and 3) ascertain if changes in depth distributions changed seasonally. To evaluate these objectives, we tagged coho and Chinook salmon in Puget Sound from 2005-2012 with ultrasonic transmitters that positioned the fish in three dimensions in the water column. Detections were compiled from a set of receivers located throughout Puget Sound. We modeled data on individual fish depths as a function of species, time of day, and time of year, considering both the proximity of the fish to the surface and to the bottom. Results indicated that the salmon species differed in average depth and also in the seasonal and diel patterns. The mean depth of Chinook (40.6 m) was deeper than coho (28.3 m). Coho exhibited diel movements during the spring with fish deeper at night (often to 100 m) and closer to the surface (6.6 -21.8 m) during the day (a pattern opposite of DVM reported for other organisms), but this pattern disappeared during other times of the year. Chinook salmon showed less pronounced diel movement, and were shallowest in June (23.6 m), moving deeper each month until a maximum in October (64.7 m). Depth distributions of salmon were complex and changed dramatically between seasons and may be a result of several physical and biological factors. Combined with data on environmental conditions in the water column throughout the year, these results provide new insights into how and why salmon are vertically distributed within Puget Sound