Growth‐Mediated Life History Traits of Steelhead Reveal Phenotypic Divergence and Plastic Response to Temperature

Abstract

AbstractGrowth‐mediated early life history traits affect an individual's fitness and reflect both evolutionary adaptations and phenotypic responses to environmental conditions. We tested for phenotypic plasticity of growth‐mediated life history traits between and within two depressed populations of steelhead Oncorhynchus mykiss from Hood Canal, Washington. We conducted a reciprocal transplant “common garden” experiment at two temperature regimes and measured individual growth rate, condition factor, proportion of age‐1 smolts and proportion of age‐1 mature males. We found phenotypic plasticity in growth rate, condition factor, and proportion of age‐1 smolts in both populations, demonstrating that genotype–temperature interaction plays an important role in determining phenotypic expression of growth and development. Growth rates were highest in the warm temperature treatment for both populations. More Dewatto River individuals smolted in their first year than Duckabush River individuals, which is consistent with data from the natural populations and provides evidence for phenotypic divergence in this life history trait. However, direct tests of neutrality provided no evidence that this divergence had resulted from diversifying selection, suggesting instead that the divergence may be largely plastic. All age‐1 mature males were observed in the warm temperature treatments for both populations, indicating that temperature plays a large role in determining age‐1 male maturation under these conditions. Broad‐sense heritability estimates for growth rate, condition factor, and smolts at age‐1 were generally high, revealing the potential opportunity for selection to act on these traits in both populations. Understanding the effect of temperature on life history differences between populations is important for management decisions and conservation, including anticipating responses to changing environmental conditions.