Daily variations in plasma cortisol levels of individual female rainbow trout Salmo gairdneri: Evidence for a post-feeding peak in well-adapted fish

Blood sampling through a catheter caused a significant and variable increase in the plasma cortisol concentration of most tested female rainbow trout. In well-adapted catheterized individuals sampled at 24- or 28-hr intervals, baseline cortisol levels were less than 8 ng/ml. A peak (15–50 ng/ml) repeatedly appeared shortly after feeding and was still present when the fish could not see the experimenter at feeding time. Other cortisol increases (12–24 ng/ml) occasionally occurred but were not reproducible. In non-feeding chronically disturbed trout, cortisol concentrations fluctuated widely (2–100 ng/ml), with no indication of periodicity.