Interactions between patch size and predation risk affect competitive aggression and size variation in juvenile convict cichlids

Abstract

We tested the predictions that the frequency of aggression, degree of resource monopolization and individual size variation within groups would decrease as patch size and predation risk increased. In a preliminary experiment, groups of 10 convict cichlids, Archocentrus nigrofasciatus, were allowed to compete for food presented in either small (20.7 cm2) or large (208.7 cm2) patches. As predicted, the frequency of aggression and size variation within groups decreased and the number of fish feeding at any particular time increased as patch size increased. In small, defensible patches, the increased aggression by dominant individuals apparently excluded some fish from the foraging patch, causing an increase in size variation within groups. In a second experiment, we manipulated patch size and perceived predation risk (addition of conspecific skin extract versus distilled water) in a 2 × 2 factorial design. There were significant interactions between the two main effects for all three dependent variables. In small patches, increasing the perceived predation risk caused a decrease in aggression and size variation within groups and an increase in the number of fish foraging at a time, whereas in large patches, increasing the perceived predation risk had no significant effect on aggression, but decreased the number of fish feeding, resulting in an increase in size variation within groups. In defensible patches, increasing perceived predation risk had the same effect as reducing the economic defensibility of the patch.