Early Self-Sorting Behavior in Chinook Salmon (Oncorhynchus tshawytscha) Is Correlated with Subsequent Variation in Growth, Behavior and Morphology

Juvenile Chinook salmon (Oncorhynchus tshawytscha) exhibit an array of life history tactics in Oregon’s Willamette River Basin, yet we do not know to what extent it is driven by phenotypic plasticity or whether it is predetermined and how the rearing environment may affect phenotype expression.  Hatchery-origin fish sort into distinct surface- and bottom- oriented groups at exogenous feeding and this separation persisted for each group. At 2 months post-swim-up, surface and bottom phenotypes differed morphologically.  The surface fish had shorter heads and deeper bodies than the bottom phenotype.  The two phenotypes differed in their willingness to interact with their mirror image in a novel environment.  Surface fish were larger than bottom fish after 8 months, regardless of feeding location.  Surface and bottom phenotypes grew at the same rate at 12°C but surface fish were larger than bottom fish after just 3 months of rearing at 7°C and the initial morphological differences in head shape persisted.  These differences in morphology between the surface and bottom groups are similar to differences between wild subyearling and yearling migrants in the basin.  These early phenotypic differences may have potential for predicting subsequent juvenile life history.