Changes in plasma cortisol during stress and smoltification in coho salmon, Oncorhynchus kisutch

The cortisol stress response in juvenile coho salmon, Oncorhynchus kisutch, considered as the difference between resting and peak poststress cortisol levels, increased from 80 ng/ml in March to 166 ng/ml in July, the period when smoltification normally occurs. Resting plasma cortisol levels also continually increased from 4 ng/ml in March to a maximum of 39 ng/ml in May, but then declined again to 3 ng/ml in July. The results indicate that there is an increased interrenal responsiveness to stress during the period of smoltification in coho salmon.