Relative Influence of Release Age, Migration Behavior, Length, Weight, and Condition Factor on Downstream Migration Survival of Juvenile Hatchery Steelhead

One strategy to reduce negative genetic effects of hatchery steelhead on natural populations is switching from non-local broodstock to locally-sourced, natural origin broodstock.  This can be problematic in hatcheries with cold water sources and late seasonal natural broodstock maturity because these conditions preclude juvenile steelhead from growing to release size as one year smolts.  To address this problem the Winthrop National Fish Hatchery developed a method to raise two year old smolts that is more closely aligned with a natural life cycle.  Since production of two year old smolts is uncommon and poorly understood, we modeled individual downstream survival of steelhead smolts from the “traditional” non-local one year and the “new” local two year programs in relation to migration behavior, length, weight, and condition factor.  In three release years, we found survival was highly influenced by migration behavior and length. Release age was also important but at a much smaller level.   For both rearing programs individual fish that were 190mm-210mm in length at release and volitionally migrated out of the hatchery (vs being forced to migrate) had the highest downstream survival.  These data support the use of a two year rearing cycle.