Spatial structure of coral reef fish communities in the Ryukyu Islands, southern Japan

Coral reef fishes often exhibit a high degree of structure in their distribution and abundance, but the factors that influence their spatial arrangement are poorly understood. This study sought to explain the spatial structure of coral reef fish communities in the Ryukyu Archipelago, southern Japan. Visual surveys of coral reef fishes were conducted at multiple stations across the fringing reef (from the inner reef flat to the outer slope) at each of three different islands; Aka, Ishigaki and Sesoko. Spatial variation in fish communities (within and among reefs) was then compared to changes in environmental variables (water depth, distance from the shoreline, reef zone, coral abundance and coral richness). Despite being small islands, the total species richness of coral reef fishes in the Ryukyu Islands was very high (>87 species per reef). Mean abundance of reef fishes was also very high (x = 2.26 ± 0.20 S.E. fishes per m²), but more than 80% of all fishes were from the family Pomacentridae. The structure of fish communities was very consistent among reefs, but varied greatly among zones within each reef. On the reef flat, the fish community was dominated by the pomacentrid Chrysiptera cyanea and the acanthurid Acanthurus nigrofuscus. In contrast, the reef slope community was characterised by high densities of the pomacentrids Pomacentrus lepidogenys and P. richardsoni, as well as high densities of the acanthurid Ctenochaetus striatus. Canonical correspondence analysis showed that the structure of the coral reef fish communities varied in relation to reef zone, water depth, and distance from the shoreline, but these changes were not related to spatial variation in either coral abundance or coral richness.