Fig. 1. The number of aggressive acts performed during each daily observation session (A) and the total number of aggressive acts (B) performed during four daily 5 min observation sessions by the socially dominant fish during social interaction in size-matched pairs of Arctic charr interacting on white or black backgrounds. Values are means ± S.E.M., N=7 pairs on a white and N=7 pairs on a black background. Different letters indicate a significant difference (P<0.05; Mann—Whitney U-test).