Passage Behaviors of Juvenile Salmonids at Bonneville Dam

The U.S. Army Corps of Engineers, Portland District, continually seeks to improve passage conditions for anadromous fish at dams on the lower Columbia River to meet performance standards mandated by the 2008 Biological Opinion.  Understanding passage behavior of juvenile Pacific salmonids is vital for improving passage efficiency and survival.  Fish behavior and passage varies from dam to dam primarily due to differences in environmental, structural, and operational conditions.  Bonneville Dam (BON) provides a unique setting to study fish behavior and passage because it has two islands that separate the spillway from two powerhouses (B1 and B2).  A total of 3,880 yearling Chinook salmon Oncorhynchus tshawtscha (CH1), 3,885 juvenile steelhead O. mykiss (STH), and 4,449 subyearling Chinook salmon (CH0) were collected from the John Day Dam smolt monitoring facility, implanted with Juvenile Salmon Acoustic Telemetry System (JSATS) tags, and released daily at sites located 41, 73, and 156 km upstream of BON.  Hydrophones that were mounted at two elevations on every pier along the upstream face of the three dam structures detected 87.7% of STH, 86.2% of CH1, and 70.6% of CH0.  For every detected fish, we examined the location of first and last detections at dam structures and used those records to evaluate behavior in the forebay upstream of the dam.  About 10.3% of juvenile STH passed at a dam structure different from where they were first detected, as opposed to <1% of CH1 and CH0.  Results suggest that juvenile STH passage may be deterred by environmental conditions at the spillway (e.g., gate vibration, noise, flow, depth, or some combination thereof).  Of all STH approaching the spillway, 19% subsequently passed at one of the powerhouses by swimming back upstream away from the spillway around the tip of either Bradford Island to B1 or Cascades Island to B2.  Of the 10.3% of STH that passed at alternate routes, 88.9% first approached the spillway before passing at a powerhouse.  Of STH first detected at a powerhouse, only 1.1% were last detected passing at the other powerhouse or spillway.  The spillway was not the safest route of passage for CH1, but survival was significantly higher for STH passing through the spillway (0.951) than it was for STH passing through turbines (0.910-0.923).  Temporary surface flow outlets should be evaluated as a way to reduce the 19% of STH that rejected the spillway as an acceptable passage route.