Diel and seasonal changes in resting plasma cortisol levels in juvenile Atlantic salmon, Salmo salar L.

Plasma cortisol levels in juvenile Atlantic salmon, Salmo salar L., were higher at night than during the day from September to May, and were higher in the morning than at night from June to August. Mean nocturnal levels increased slightly from August (less than 1.4 ng/ml) to January (18.2 ng/ml), sharply to a peak in March (46.0 ng/ml), and then declined sharply again reaching minimal values in July (less than 1.4 ng/ml). With the exception of a February peak (7.7 ng/ml) mean diurnal levels also increased slightly from a September minimum of (less than 1.4 ng/ml) to April (3.3 ng/ml), then more sharply to plateau from May to July (8.2-11.0 ng/ml), and declined again to September. The data support the hypothesis that rising cortisol levels in spring represent a generalized stress response to behavioral and physiological maladaptation at smolting.