Determinants of social organization in a coral reef fish, the blue tang, Acanthurus coeruleus

We examined the abundance of blue tang surgeonfish, Acanthurus coeruleus, in each of three social modes (schooling, territorial, and wandering) in relation to size class, ecological variables, population density and time of day to discern potential determinants of social organization. We found individuals from all three social modes in all four fringing reef habitats (back reef, flat, crest and spur and groove zones) at our main site. Territorial tang density was highest in the flat zone, lowest in the spur and groove zone and negatively related to adult damselfish density. A higher proportion of tangs formed schools in reef zones with the highest densities of territorial damselfishes (spur and groove, crest). In the back reef and flat zones, where damselfish densities were very low, tangs rarely formed schools. Tangs in the wandering mode were most abundant in the back reef. The density of territorial tangs did not change with time of day, but non-territorial tangs tended to wander more in the morning and to form schools more at midday. Small yellow-phase juveniles were always territorial, and the incidence of territoriality decreased in the larger size classes, while schooling and wandering increased. Among similar fringing reefs, the incidence of territoriality increased with increasing population density. These data suggest that life history stage, damselfish density, and conspecific population density are important determinants of blue tang social organization.