Shifts in a fish's resource holding power during a contact paired interaction: The influence of a copper-contaminated diet in rainbow trout

The influence of sublethal chronic dietary copper (Cu) exposure on the dominant-subordinate relationship between pairs of juvenile rainbow trout (Oncorhynchus mykiss) was examined. Fish were fed either a normal (11 mg Cu kg(-1) food) or Cu-contaminated (721 mg Cu kg(-1) food) diet for 8 wk. Paired interactions were observed-control versus control, Cu-exposed versus Cu-exposed, and control versus Cu-exposed fish-using a computer-aided video tracking system to measure duration of interactions, total distance moved, and the number of encounters during each contest. In concurrence with game theory, each interaction became escalated with a lesser size disparity between contestants. However, in Cu-exposed versus Cu-exposed fish interactions, the dominant-subordinate relationship was decided sooner and with less aggression than a control versus control fish interaction with fish of a similar relative body mass disparity. During control versus Cu-exposed fish interactions, control fish would normally dominate interactions (12 out of 16 bouts) unless the Cu-exposed fish had a 15% body mass advantage. Muscle glycogen and lactate levels after each contest reflected the duration of bouts and winners of the contests, irrespective of Cu exposure. We conclude that Cu-contaminated fish are less able to compete and have lower resource holding power than controls and will withdraw from a contest at a lower level of interaction, unless a size advantage in the Cu-exposed fish increases the probability of winning.