Environmental Determinants of Migratory Life History Phenotypes in Juvenile Spring Chinook Salmon

There is considerable variation in downstream-movement life history tactics displayed by Chinook salmon Oncorhynchus tshawytscha in the Upper Willamette River, Oregon, USA.  However, it is unclear if juvenile migration phenotypes are established in response to environmental stimuli or if they are established naturally much earlier in life. We conducted rearing trials to experimentally determine the effects of environmental variables on the developmental trajectories and expression of migration phenotypes in hatchery broodstock juveniles. We found that temperature, density, tank substrate, diet lipid content, and feeding strategy could be manipulated to produce fish that followed trajectories resulting in the expression of specific movement phenotypes, particularly fall and spring smolts. Analyses of body morphology of wild juvenile Chinook salmon in the Upper Willamette River revealed that variation in body shape is correlated with the migratory pathways followed by juveniles. Our results suggest that environmental variables influence developmental trajectories and migration phenotypes of juvenile Chinook salmon.