Fig. 2. Blood plasma levels of (A) cortisol, (B) adrenocorticotropic hormone (ACTH) and (C) L-tryptophan (TRP) in isolated rainbow trout fed for 3, 7 or 28 days with a feed supplemented with L-tryptophan (TRP feed) to a level eight times the TRP content of the non-supplemented control feed (CTRL). Stressed fish were exposed to a standardised stressor, a lowering of the water level, whereas non-stressed fish were kept undisturbed. Results (F- and P-values) from a 2-way ANOVA with treatment (stress or no stress) and feed (CTRL or TRP feed for 3, 7 or 28 days) as class variables are given in the figure. Different letters indicate significant differences at the P<0.05 level (LSD post-hoc test) between the groups. Different letters above brackets indicate significant differences (P<0.05, LSD post-hoc test) between fish fed control feed and TRP-supplemented feed for 3, 7 and 28 days (stressed and non-stressed fish taken together). Values are means ± S.E.M.; for fish fed CTRL feed, N=24; for fish fed TRP feed for 3, 7 and 28 days, N=8.