Does darkening signal submission in territorial contests between juvenile Atlantic salmon, Salmo salar?

Communication by means of visual signals occurs during the competitive, agg ressive and sexual interactions of many animals. Some animals such as fish are able to change their body coloration rapidly, and there is evidence tha t this is used as a means of signalling. However, the precise meaning of su ch signals is rarely understood. We examined whether (1) darkness in juveni le Atlantic salmon is associated with submission, and (2) changing to a dar ker colour could act as a signal to the opponent and hence induce a change in its behaviour. We found that both the sclera of the eye and the overall body coloration tended to darken in fish that were losing territorial encou nters, while victors retained their original coloration. The darkening was rapid and usually occurred during a period of sustained attacks by the oppo nent; however, the aggression level decreased as soon as the losing fish ha d become darker. We suggest that the darkening of the losing fish was assoc iated with submission and may result in a change in the behaviour of its op ponent, so minimizing the potential risk of injury during unnecessarily pro longed fights. (C) 1999 The Association for the study of Animal Behaviour.