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 di...