Passage Behavior 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.  Understanding passage behavior of juvenile Pacific salmonids is vital for improving passage efficiency and survival.  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).  During 2010 and 2011 over 23,000 yearling Chinook salmon Oncorhynchus tshawtscha (CH1) and juvenile steelhead O. mykiss (STH) combined, were collected, implanted with Juvenile Salmon Acoustic Telemetry System (JSATS) tags, and released daily at sites located 41, 73, and 156 km upstream of BON.  Hydrophones mounted on the upstream face of BON detected >86% of STH and CH1 during 2010 and >72% during 2011.  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.  In 2010 and 2011, 10.3% and 2.8% of juvenile STH passed at a dam structure different from where they were first detected, as opposed to <0.3% of CH1.  The majority of STH (>85%) that passed at alternate routes first approached the spillway before passing at a powerhouse.  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).  During 2010, STH passage survival increased for fish that approached the spillway but passed at B2 indicating that spillway modifications such as temporary surface flow outlets should be evaluated to increase survival rates.