Prevalence of Aggression as it Relates to Territory Size, Fish Density, and Neighbor Number in the Yellowtail Damselfish, Stegastes arcifrons

Territoriality plays a key role in the survival and reproduction of many species. Damselfish (Family: Pomacentridae), reef-dwelling fish found in tropical locations, are territorial and competitively engage both intra- and inter-specific fish in order to protect their algal lawns. While damselfish territoriality has been examined as it relates to attack distance in con- and hetero-specifics, aggression in damselfish has not yet been related to fish density and proximity. The Galápagos yellowtail damselfish, Stegastes arcifrons, was selected for analysis of the factors that affect its behavioral aggression. Yellowtail damselfish individuals (n=31) were observed in the wild in 5-minute counts for aggressive interactions toward con- and hetero-specifics. The average location of the observed damselfish, including the locations of its nearby neighbors, was noted and dive markers were placed at these locations following the 5-minute count. Photographs of the average locations of the damselfish during the test, as shown by the dive markers, were taken and imaging software was used to determine damselfish density and an approximation of controlled territory through conversion into Voronoi diagrams. Aggression rates toward conspecific encounters, measured in aggressive attacks/minute, were positively correlated with damselfish density and inversely correlated with controlled territory size (p < 0.001 for both correlations, R2 = 0.52 and 0.35 respectively). There were no significant correlations between aggression rates toward heterospecific encounters and any measured variables (p > 0.05 for all correlations). The number of nearby neighbors was also not significantly correlated with aggression (p > 0.05). Damselfish aggression is a direct function of both controlled area and conspecific density, but not of the number of nearby neighbors nor heterospecific individuals.