ECOLOGICAL STRUCTURE OF ASSEMBLAGES OF CORAL-REEF FISHES ON ISOLATED PATCH REEFS

A 9-year study of the structure of assemblages of fish on 20 coral pat ch reefs, based on 20 non-manipulative censuses, revealed a total of 1 41 species from 34 families, although 40 species accounted for over 95 % of sightings of fish. The average patch reef was 8.5 m2 in surface a rea, and supported 125 fish of 20 species at a census. All reefs showe d at least a two-fold variation among censuses in total numbers of fis h present, and 12 showed ten-fold variations. There was also substanti al variation in the composition and relative abundances of species pre sent on each patch reef, such that censuses of a single patch reef wer e on average about 50% different from each other in percent similarity of species composition (Czekanowski's index). Species differed substa ntially in the degree to which their numbers varied from census to cen sus, and in the degree to which their dispersion among patch reefs was modified from census to census. We characterize the 40 most common sp ecies with respect to these attributes. The variations in assemblage s tructure cannot be attributed to responses of fish to a changing physi cal structure of patch reefs, nor to the comings and goings of numerou s rare species. Our results support and extend earlier reports on this study, which have stressed the lack of persistant structure for assem blages on these patch reefs. While reef fishes clearly have microhabit at preferences which are expressed at settlement, the variations in mi crohabitat offered by the patch reefs are insufficient to segregate ma ny species of fish by patch reef. Instead, at the scale of single patc h reefs, and, to a degree, at the larger scale of the 20 patch reefs, most of the 141 species of fish are distributed without regard to diff erences in habitat structure among reefs, and patterns of distribution change over time. Implications for general understanding of assemblag e dynamics for fish over more extensive patches of reef habitat are co nsidered.