Seasonal and diel changes in plasma cortisol levels of the brown trout, Salmo trutta L.

A marked diel variation in plasma cortisol concentration was demonstrated in the brown trout, Salmo trutta L. For most of the year this variation took the form of an elevation of cortisol levels during the hours of darkness. There was some evidence of a phase shift from a peak at 2400 hr during the spring and early summer to a peak at 0400 hr during the late summer and early autumn. During the winter months a nocturnal elevation of plasma cortisol was not evident. A shorter, episodic elevation of plasma cortisol was associated with feeding in the brown trout. These findings are discussed in relation to the effects of stress and intensive cultivation on teleost fish.