Comparison of Juvenile Life History Strategies Exhibited by Steelhead within the Potlatch River Drainage, Idaho

In Idaho, juvenile steelhead often leave natal streams to rear downstream.  Evaluation of this life history strategy has been limited by lack of information between traps in natal streams and downstream dams.  We used instream PIT-tag detectors to address this issue.  We observed substantial differences in juvenile steelhead life history strategies exhibited between the eastern and western halves of the Potlatch River drainage.  Age composition of juvenile outmigrants from Big Bear Creek (west side) and East Fork Potlatch River (east side) were much different with age-2 outmigrants (62-67%) dominating Big Bear Creek and age-1 outmigrants (75-86%) dominating East Fork Potlatch River during the 2009 and 2010 spring migration.  The rate of active smolt outmigration from natal rearing areas was also different.  Same year arrival rates to Lower Granite Dam during the 2009 and 2010 outmigration were 38-54% and 5-8% for Big Bear Creek and East Fork Potlatch River respectively.  By examining arrival rates to tributary and mainstem Potlatch River instream PIT-tag detectors we were able to establish migration characteristics and rearing strategy differences between the two drainages during the 2010 outmigration.  We found juvenile fish captured at screw traps were detected on their associated tributary instream array at similar rates, 95% and 91% for Big Bear Creek and East Fork Potlatch River respectively.  There were significant differences however, in arrival rates to the mainstem Potlatch River instream array with 78% and 18% of fish tagged at Big Bear Creek and East Fork Potlatch River screw traps arriving at that array the same year as tagging.  PIT-tag detections from instream arrays have been a valuable tool to identify downstream rearing reaches used by East Fork Potlatch River juvenile steelhead.  We believe that differences in emergence timing and aquatic productivity are influencing life history characteristics exhibited by these groups.