Differences in concentrations of plasma cortisol in the trout and the eel following adaptation to black or white backgrounds.

When rainbow trout (Salmo gairdneri) and eels (Anguilla anguilla) were kept in black tanks for 3-4 weeks, their plasma cortisol titres were about fourfold higher tha in fish kept in white tanks. In trout, the difference was apparent only under a long photoperiod of 16 h light: 8 h darkness, but in eels the difference was clear under both a long or short photoperiod (9.5 h light: 14.5 darkness). It is suggested that the increase in plasma cortisol seen in black-adapted fish is dependent on either ACTH or MSH secreted by the pars intermedia melanotrophs. No difference was seen in the total cortisol-binding capacity of the plasma nor in interrenal histology in trout from black or white backgrounds.