Life History Variation in Klamath River Steelhead

Rainbow Trout from the lower Klamath River basin were found to exhibit partial migration and at least 36 different life history categories at maturity.  Categories differed in duration of freshwater and saltwater rearing, number of spawning runs, and expression of a half-pounder migration.  The half-pounder life history is a steelhead variant characterized by an amphidromous (and less often, anadromous) return to freshwater in the year of initial ocean entry.  Life history trajectories were associated with rates of juvenile growth: fast growers smolted at age 1, an intermediate group remained in freshwater, and slow growers delayed smoltification.  Incidence of the half pounder life history differed among sub-basins and with smolt age.  Incidence of nonanadromy also varied among sub-basins.  Life history models suggested that fecundity-related fitness costs of the half-pounder life history are offset by precocious maturation and increased first-year ocean survival.  Mounting evidence that rainbow trout can produce anadromous offspring has management implications in the Klamath River basin, where proposed dam removal may restore connectivity between the ocean and upper basin.  Following dam removal, inland Redband Trout from the upper basin may contribute toward recovery of the steelhead population.