Subordinate brown trout exaggerate social signalling in turbid conditions

Visual displays allow animals to communicate information about social status and to minimize costs of dangerous fighting. However, in aquatic animals, visual signals may be seriously affected by increased turbidity. In juvenile salmonids, subordinates signal defeat through a darkening in coloration and in doing so reduce further attacks from dominant individuals. We examined the behaviour and physiology of socially interacting brown trout, Salmo trutta, dyads in clear water, low turbidity and high turbidity. Overall, the characteristic aggression associated with socially competing salmonids was reduced in turbid conditions and visual displays of subordinates were exaggerated. It has been suggested that darkening of subordinate salmonids is primarily a result of increased stress, and acts secondarily in communication. However, although plasma cortisol was highest in subordinates, turbidity did not affect cortisol concentrations. In conclusion, exaggeration of subordinate visual signals in turbid conditions appears to be a response to alterations in environment, rather than a secondary stress effect.