Effects of Mixed-species Foraging Groups on the Feeding and Aggression of Juvenile Parrotfishes

Many parrotfishes (Scaridae) co-occur in mixed-species aggregations as juveniles, but diverge in resource use and social structure as adults. Focal observations of three juvenile parrotfishes (Scarus coeruleus, Sparisoma aurofrenatum, and Sparisoma viride) were conducted on inshore patch reefs in the Florida Keys to determine how feeding and aggressive interactions vary with group participation. All three species spent more time in groups than alone, most often in groups of less than ten individuals. Feeding rates were significantly higher for S. viride when foraging in groups than when alone. All species fed most often from Halimeda, and overall diet composition was similar for fish whether feeding in groups or alone. The frequency of aggressive interactions varied with group participation. Focal S. aurofrenatum were more aggressive when in groups than when alone, and both S. aurofrenatum and S. viride were attacked more often by damselfishes when they were alone than when in groups. In contrast, feeding rates, diet breadth, and aggressive interactions of S. coeruleus were less affected by group participation. Small mixed-species aggregations of coral reef fishes may be large enough for individuals to assume some of the benefits of group participation while at the same time avoiding the costs of competition realized in larger groups.