Cascades Island Lamprey Passage Structure: Evaluating Passage and Migration Following Structure Modifications

Pacific lamprey (Entosphenus tridentatus) (Columbia River Basin, U.S.A) has experienced decreases in returns to spawning territories in recent decades.  To address passage issues at hydropower dams, lamprey specific passage structures have been designed and constructed.  The Cascades Island Lamprey Passage Structure (LPS) at Bonneville Dam is the longest and steepest of its type; a 2012 exit pipe addition allowed lampreys to travel from tailrace to forebay.  This study, carried out during the 2013 migration season, utilized three treatment groups, released on five dates (n=75).  Two groups (n=50), with differing tagging methods, released directly into the LPS assessed passage efficiency, travel time, and tagging effect.  The third group (n=25) released into the forebay tested whether the structure impedes upstream migration.  Fish were monitored via an existing network of receiver arrays on the LPS and at Columbia River Dams.  Passage efficiency in the LPS was 74% (n=37/50), mean travel time was 12 h, and there was no statistically significant effect of tagging on either metric.  The groups that used the LPS performed better than the group that bypassed regarding migration upstream to subsequent dams; the difference was not statistically significant.  Such findings have important implications for lamprey management in the region.