Cooperative Fish and Wildlife Research Units Program: California
Education, Research and Technical Assistance for Managing Our Natural Resources

Steelhead Oncorhynchus mykiss from several of the world’s rivers display the half-pounder life history: a variant characterized by an amphidromous, and less often anadromous, return to freshwater in the year of initial ocean entry. We evaluated factors related to expression of the half-pounder life history in wild steelhead from the lower Klamath River basin, California. We also evaluated fitness consequences of the half-pounder phenotype using a simple life history model that was parameterized with our empirical data and outputs from a regional survival equation. Incidence of the half-pounder life history differed among sub-basins of origin and with smolt age. Precocious maturation occurred in approximately 8% of half-pounders and was best predicted by individual length in freshwater preceding ocean entry. Adult steelhead of the half-pounder phenotype were smaller and less fecund at age than adult steelhead of the alternate (ocean contingent) phenotype. However, data suggest that fish of the half-pounder phenotype are more likely to spawn repeatedly than are fish of the ocean contingent phenotype. Models predicted that if lifetime survivorship was equal between alternate phenotypes, fitness of the half-pounder phenotype would be 17 – 28 % lower than that of the ocean contingent phenotype. To meet the condition of equal fitness between alternate phenotypes would require that first-year ocean survival is 21 - 40% higher among half-pounders in freshwater than among their cohorts at sea. We conclude that continued expression of the half-pounder phenotype is favored by precocious maturation and increased survival relative to that of the ocean contingent phenotype.