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

Abstract

Growth-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.

Received July 3, 2013; accepted September 23, 2013

ACKNOWLEDGMENTS

Funding for this research was provided by the Internal Grants Program of the National Oceanic and Atmospheric Administration, Northwest Fisheries Science Center (NOAA NWFSC) in 2010. This project was conducted in collaboration with the Hood Canal Steelhead Project (NOAA NWFSC). Field collection of steelhead eggs in the Dewatto and Duckabush Rivers was completed with the help of Teresa Sjostrom and Sean Hildebrandt (formally with Hood Canal Salmon Enhancement Group), Joy Lee-Waltermire and Rick Endicott (Long Live the Kings), and Megan Moore (NOAA NWFSC). Steelhead eggs were incubated and hatched at the U.S. Fish and Wildlife Quilcene National Fish Hatchery under the direction of Dan Magnusen and Paul Kaiser. Juvenile steelhead were raised at Long Live the King's Lilliwaup Hatchery for the duration of the experiment with the help of Joy Lee-Waltermire and Rick Endicott. Assistance with tank design and construction was provided by Rob Endicott, Jeff Atkins, and Rick Endicott. Data collection assistance was provided by Megan Moore, Rob Endicott, and Joy Lee-Waltermire. Statistical guidance and advice was provided by Eric Ward (NOAA NWFSC). Markku Karhunen and Otso Ovaskainen (University of Helsinki) gave helpful advice on the Fst and Qst analyses. Megan Moore provided technical assistance in creating Figure 1. Helpful advice was provided by Gary Winans (NOAA NWFSC). Two anonymous reviewers provided helpful comments on the manuscript. The views expressed in this paper are solely those of the authors and are not intended to represent the views of the National Marine Fisheries Service with which the authors are affiliated.