Abstract
In two year classes of Willamette River spring chinook salmon, reared at the Willamette Hatchery, and two groups of Yakima River spring chinook salmon, one sampled from the Yakima River and the other reared in a hatchery, fish which had relatively high growth rates in the summer–autumn period smolted in the autumn as measured by increases in gill Na + K + AT Pase activity. In contrast, groups with relatively low growth rate did not smolt in the autumn. Plasma levels of insulin‐like growth factor‐I (IGF‐I) showed discrete differences between groups, with high levels associated with increased gill Na + K + AT Pase activities. These results demonstrate that smolting is plastic in spring chinook salmon, occurring in the autumn or the spring. In addition, smolting appeared to be related to growth rate; however, the relationships shown were correlational and causal mechanisms were not elucidated. Yet, the results do indicate a relationship between growth, an endocrine growth factor and smolting, suggesting a mechanistic link between developmental plasticity and the environment mediated by the endocrine system.