Potential Fitness Benefits of the Half‐Pounder Life History in Klamath River Steelhead

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. Received October 23, 2013; accepted February 4, 2014

Use of Morphological and Physiological Indices to Characterize Life History Diversity in Juvenile Hatchery Winter‐Run Steelhead

“…It is unclear whether some of these fish may forgo migration to the ocean and remain in freshwater essentially as a Rainbow Trout. In native steelhead populations from rivers of northern California, southern Oregon, and eastern Russia, there exists a unique life history variant referred to as the "half-pounder," a name descriptive of their relatively small size (Snyder 1925;Kesner and Barnhart 1972;Savvaitova et al 2005;Hodge et al 2014). Half-pounders return to freshwater after only 2-4 months in seawater, can be both male and female, and are predominantly immature (Kesner and Barnhart;Everest 1973;Hodge et al 2014).…”

“…In native steelhead populations from rivers of northern California, southern Oregon, and eastern Russia, there exists a unique life history variant referred to as the "half-pounder," a name descriptive of their relatively small size (Snyder 1925;Kesner and Barnhart 1972;Savvaitova et al 2005;Hodge et al 2014). Half-pounders return to freshwater after only 2-4 months in seawater, can be both male and female, and are predominantly immature (Kesner and Barnhart;Everest 1973;Hodge et al 2014). However, several investigators have reported that some half-pounders return from the ocean and precociously mature in freshwater at age 2 (Kesner and Barnhart 1972;Savvaitova et al 2005;Hodge et al 2014).…”

“…Half-pounders return to freshwater after only 2-4 months in seawater, can be both male and female, and are predominantly immature (Kesner and Barnhart;Everest 1973;Hodge et al 2014). However, several investigators have reported that some half-pounders return from the ocean and precociously mature in freshwater at age 2 (Kesner and Barnhart 1972;Savvaitova et al 2005;Hodge et al 2014). Hodge et al (2014) determined that approximately 8% of the wild half-pounders (both male and female) returning to the Klamath River, California, underwent precocious maturation.…”

“…However, several investigators have reported that some half-pounders return from the ocean and precociously mature in freshwater at age 2 (Kesner and Barnhart 1972;Savvaitova et al 2005;Hodge et al 2014). Hodge et al (2014) determined that approximately 8% of the wild half-pounders (both male and female) returning to the Klamath River, California, underwent precocious maturation. Precociously mature half-pounders tend to be the largest fish during their first winter in freshwater (Hodge et al 2014).…”

“…Hodge et al (2014) determined that approximately 8% of the wild half-pounders (both male and female) returning to the Klamath River, California, underwent precocious maturation. Precociously mature half-pounders tend to be the largest fish during their first winter in freshwater (Hodge et al 2014). Larger smolt size has been positively correlated with increased jack production in Coho Salmon O. kisutch and Chinook Salmon (Appleby et al 2003;Vøllstad et al 2004).…”