Potential fitness benefits of the half-pounder life history in Klamath River steelhead

Transactions of the American Fisheries Society
By: , and 

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Abstract

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. The incidence of the half-pounder life history differed among subbasins of origin and smolt ages. 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 alternative (ocean contingent) phenotype. However, our 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 were equal between phenotypes, the 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 phenotypes would require that first-year ocean survival be 21–40% higher among half-pounders in freshwater than among their cohorts at sea. We concluded that continued expression of the half-pounder phenotype is favored by precocious maturation and increased survival relative to that of the ocean contingent phenotype.

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Additional publication details

Publication type Article
Publication Subtype Journal Article
Title Potential fitness benefits of the half-pounder life history in Klamath River steelhead
Series title Transactions of the American Fisheries Society
DOI 10.1080/00028487.2014.892536
Volume 143
Issue 4
Year Published 2014
Language English
Publisher Taylor & Francis
Contributing office(s) Coop Res Unit Seattle
Description 12 p.
First page 864
Last page 875
Country United States
State California