Coral size, health and structural complexity: effects on the ecology of a coral reef damselfish

The structure and dynamics of animal populations are strongly influenced by the physical and biotic characteristics of their habitats. However, in complex habitats such as coral reefs, the particular habitat features that are important and the mechanisms that impact on populations are poorly understood. This study investigates the influence of coral colony size, health and branching structure of the host coral Seriatopora hystrix on the ecology of the resident damselfish Chromis retrofasciata. Field observations indicated C. retrofasciata preferentially occupied larger S. hystrix colonies, but among occupied corals, group size was greatest on colonies of an intermediate size and with wider branch spacing. Healthy corals were observed to support higher abundances. The effects of both coral health (live vs. dead) and coral complexity (fine, medium and coarse branching) on survivorship of stocked juvenile C. retrofasciata were examined in a patch reef experiment. Survival was 20% higher in living colonies compared to dead ones, and fish in medium complexity coral exhibited approximately 10 and 20% greater survival than those in both the low and high complexity categories, respectively. These factors had an additive effect with the highest survival observed on healthy colonies with intermediate complexity (95%) and the lowest on highly complex, dead corals (55%). Body size distributions were also reduced on corals with the narrowest branch spacing. During habitat selection trials, juveniles were observed to preferentially occupy colonies with intermediate branch spacing. Prey may prefer corals of intermediate spacing and survive better as such habitats provide shelter that obstructs predator access. This study provides evidence that suggests small reef fish species will be sensitive to disturbances that result in the loss of live coral and alter preferred structural complexity of habitats.