Fig. 3. Plasma thyroxine (T₄, ng ml^-1; A), triiodothyronine (T₃, ng ml^-1; B) and cortisol (ng ml^-1; C) levels in juvenile Atlantic salmon subjected to different temperature and photoperiod treatments (see Fig. 1). Values are means ± S.E.M. (N=12). Temperature increases in the advanced temperature group began on February 14 (arrowhead). There were significant effects of photoperiod, temperature and time on plasma T₄ and T₃ levels (P<0.05, three-way ANOVA). There was a significant effect of time on plasma cortisol and a significant interaction between time, photoperiod and temperature (P<0.05, three-way ANOVA). Vertical lines unconnected to other lines indicate a significant difference from other groups at that time; points without vertical lines are not significantly different from one another (P=0.05, Kruskal-Wallis test).