Migration of Late Fall-Run Chinook Salmon and Steelhead Smolts Relative to Dredge Removal and Placement Sites in the San Francisco Bay Estuary

500 Late Fall Chinook salmon smolts and 500 steelhead smolts, each fitted with internal ultrasonic tags, were released in February 2009 and 2010 at Elkhorn Landing on the Sacramento River. A watershed-wide array of underwater receivers placed at strategic sites across bridges and at dredging and dredge placement sites was used to study the downstream migration of the smolts and their residency at these sites. The overall survival of the smolts from the release site to the start of the study area (Benicia Brdige) was 48% (steelhead) and 62% (Chinook) in 2009. Of these, approximately one third (Chinook) and one quarter (steelhead) survived to the Golden Gate Bridge in 2009. In 2010 the overall survival of steelhead to the Benicia Bridge was 21% (steelhead) and 41% (Chinook).  In 2010 two thirds of Chinook and one half of the steelhead survived to the Golden Gate Bridge.  The difference between the two years implies an inversion of location of mortality.  In Both years each species transited the study area over a median period of 2 days.  Fish tended to move on peak flows for both flood and ebb tides. Some individuals displayed repeated upstream and downstream movements, which we related to the tidal state. The instantaneous rate of transit through the San Pablo receiver arrays was higher than the overall rate for that river reach. Exposure of smolts at marinas and dredged sites near shoals was low (only a few minutes for up to 5 fish), whereas a higher proportion of fish were detected at the channel sites. Analysis of time elapsed between detections at the different Arrays in San Pablo Bay gave conservative estimates of up to 200 minutes exposure time through these sites. Several fish from both species were detected at the Bay Bridge (potentially entering the South Bay), yet this did not appear to affect their survival rate to the Golden Gate.