Independence of the pituitary-interrenal axis and melanotroph activity in the brown trout, Salmo trutta L., under conditions of environmental stress

Adaptation of brown trout to a black or white background had no statistically significant influence on the activity of the pituitary-interrenal axis in unstressed fish, in fish under conditions of mild chronic stress, or in fish under conditions of severe, acute stress. This finding is in contrast to several other studies on salmonid fish in which background color adaptation has been shown to modulate the pituitary-interrenal response to environmental stress. It is argued that these differences in results may be related to the prior history of interrenal activation in the experimental fish and that in previously unstressed brown trout the activity of the pituitary-interrenal axis is independent of melanotroph activity in the pars intermedia when fish are exposed to subsequent stress.