Structure of patch-reef fish assemblages at St. Croix, US Virgin Islands, and One Tree Reef, Australia

Abstract

The structure and dynamics of reef fish assemblages on small patch reefs in the Caribbean (St. Croix, US Virgin Islands) were compared with those studied previously (Sale et al. 1994) on the Great Barrier Reef (One Tree Reef), Australia, in order to test whether assemblage struc- ture and function are consistent despite overall taxonomic differences. St. Croix reefs were monitored over 5 yr (1991 to 1996), while those at One Tree Reef had been monitored for 10 yr (1977 to 1986). Analyses of species richness and abundances for individual patch reef assemblages revealed con- siderable change in assemblage structure between censuses. Specifically, number of individuals and species varied among summer and winter censuses, with summer censuses generally showing greater numbers. Among years but within a season, there was significant variation in the number of indi- viduals (summer and winter) and species richness (summer only) at St. Croix, while only species richness varied yearly (summer and winter) at One Tree Reef. In addition, when the proportional abundances of species observed on each patch reef were compared among censuses on the same patch reefs but at different times (measured as Czekanowski similarity values), results indicated that faunal similarity declined with increasing length of interval between censuses. The mean, between- census Czekanowski similarity values for individual assemblages were 0.56 and 0.59 for St. Croix and One Tree Reef, respectively. These results suggest that there is substantial temporal change in assem- blage structure at both locations. Species in each location were classified into 12 trophic guilds in order to determine whether the assemblages at St. Croix and One Tree Reef showed similar trophic structure despite different species present. Results indicated that guild structure varied among censuses in both locations and that average guild structure differed substantially between these locations. At St. Croix, diurnal and nocturnal planktivores were proportionately more important, while site-attached herbi- vores, wide-ranging grazing herbivores, wide-ranging browsing herbivores, sedentary carnivores, and demersal planktivores were proportionately more important on the Australian patch reefs. Results confirm that comparable assemblage dynamics occur for patch reef assemblages at St. Croix and One Tree Reef. However, there are also clear differences in structure and dynamics in these 2 locations, despite the very similar physical and hydrographic habitats occupied.