Fig. 2. Plasma levels of cortisol in isolated juvenile rainbow trout fed feeds supplemented with L-tryptophan to a level corresponding to two (2x), four (4x) and eight (8x) times the L-tryptophan content of the non-supplemented control feed (1x). Stressed fish (filled squares) were exposed to a standardized stressor (lowering the water level), whereas non-stressed fish were kept undisturbed (open triangles). Different letters indicate a significant difference at the P<0.05 level (LSD post-hoc test). Values are means ± S.E.M. from 5-7 individuals.