16. Differing Levels of Aggression in Relation to Group Size of Sergeant Major Damselfish (Abudefduf saxatilis) in Calabash Caye Reef, Belize

Abstract

Abudefduf saxatilis (the Sergeant Major) is one of many species of damselfish living among coral reefs of the Caribbean. Juveniles will most often live in groups with other juveniles. Upon maturation, adults join an adult community or live alone. Adult Sergeant Majors behave territorially and display aggression towards approaching fish. In contrast, territoriality is not commonly observed among juveniles. The purpose of our study was to compare differences in group size and territorial behavior between juveniles and adults at two reef sites near Calabash Caye, Belize: Shallow Patch Reef (SPR) and Calabash Caye Reef (CCR).

At each reef, we randomly selected starting points from which we surveyed transects 10 meters in length. We sampled 17 transects at SPR and 11 at CCR. At each transect, we recorded the number of juveniles and adults, territorial behavior, water depth, and habitat characteristics.

The mean depth of SPR was shallower than CCR. We observed more juveniles at SPR than at CCR. There were almost no adults at SPR. Most adults displayed territorial behavior. In contrast, juveniles darted into protected crevices when approached.

Our results are consistent with findings from previous studies on Sergeant Major damselfish. Shallow patch reefs may provide a common nursery for many species of juvenile fish. By schooling together, juveniles may deter potential predators from attacking, thereby increasing their chances of survival. As damselfish mature and grow, their defense may switch from safety in numbers to aggressive behavior, which allows adults to transition to deeper patch reefs and establish territory.