Abstract
Mobile acoustic surveys of 24-h duration were undertaken at Steveston Bend between 1991 and 1993, close to the time of maximum adult Early Stuart sockeye salmon (Oncorhynchus nerka) abundance in the Fraser River estuary. During each survey, extreme variations in the abundance and directional orientation of fish targets appeared to be related to local flow direction and velocities. Acoustic Doppler Current Profiler (ADCP) measurements of water currents adjacent to the fish monitoring site showed that the water column stratified into two vertical layers under certain tidal conditions, with an upper outflowing layer over a lower inflowing layer. Under strong flood or ebb conditions, most of the water column was, respectively, inflowing or outflowing. Fish abundance in the water column appeared to be related primarily to tidal current variations. Maximum fish abundance and upstream orientation occurred during flood tidal periods with upstream current flows. We conclude that migrations of Early Stuart sockeye salmon are synchronized with the tidal cycle for energetically efficient net upstream displacement through the Fraser River estuary.
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