Evidence of between-population variation in morphology and thermal plasticity of agonistic behavior in two genetically distinct populations of steelhead ( Oncorhynchus mykiss )

Morphological and behavioral traits affect an individual’s fitness and can reflect both evolutionary adaptations and phenotypic responses to environmental conditions. We conducted a reciprocal transplant ‘common garden’ experiment at two temperature regimes to test for phenotypic plasticity in morphological and behavioral traits between and within two populations of steelhead Oncorhynchus mykiss from Hood Canal, WA, USA. The Dewatto River and Duckabush River populations exhibited asymmetric changes in body morphology in response to the two temperature regimes, suggesting both between- and within-population variation in morphological plasticity. In most cases, within population variation in body shapes was less than between temperature regimes. Most notably the populations differed in dorso-ventral and caudal regions, body depth, and head shape, with some differences on the anterior-posterior placement of the dorsal and ventral fins. The warm temperature regime caused more exploratory behavior, more charging behavior, and higher fin erosion, and population effects included slight differences in feeding aggression frequency. Morphology appeared to vary more between populations than between temperature regimes, and behavioral traits varied more between temperature regimes than between populations. Morphological variation may reflect adaptations to variation in freshwater habitat conditions, and both populations show behavioral plasticity in response to temperature. This study sheds new light on the role of genetic and environmental influence on morphology and behavior in juvenile steelhead.