Spatial and temporal variation in a soft-bottom fish assemblage in St Vincent Bay,New Caledonia

Monthly trawl surveys were performed in 1989 in North Bay and South Bay of St Vincent (New Caledonia) with both a shrimp trawl and fish trawl to produce a reference standard of the natural variability of an unexploited tropical soft-bottom fish assemblage. A total of 230 species belonging to 62 families were recorded. The mean density and biomass were 0.18 fishes m^-2 and 4.31 g m^-2, respectively. The major variations were explained by spatial factors. Species richness, density and biomass were greater in South Bay (204 species, 0.26 fishes m^-2 and 5.90 g m^-2) than in North Bay (105 species, 0.10 fishes m^-2 and 2.71 g m^-2), 34% of the species being present in both areas. The North Bay assemblage was characterized by four abundant benthic species (Saurida undosquamis, Gerres ovatus, Secutor ruconius and Upeneus moluccensis) and by numerous pelagic species (Carangidae, Sphyraenidae and Scombridae). The South Bay assemblage was characterized by several Mullidae, Bothidae and Balistidae, and by some rare species usually found on coral reefs (Pomacentridae and Chaetodontidae). These differences were induced by the physical and benthic characteristics of the two bays. North Bay was an homogeneous, confined, deposit area with few benthic organisms, whereas the substrate was more heterogeneous and the benthic organisms more diversified and abundant in South Bay, which was connected to the adjacent reef lagoon. Species richness remained stable in time, except in January when a hurricane disturbed the environment. Seasonal tendencies in species composition were evidenced in North Bay, with an autumn-winter structure opposed to a spring-summer structure, and characterized by the relative importance of the major species. No seasonal tendencies were observed in the organization of the South Bay assemblage. Nevertheless, mean density and biomass were at a minimum in summer in both bays; maxima occurred in winter. Biomass was negatively correlated to both temperature and rainfall, and reflected the population variations of the main species, particularly their reproductive migrations. Thus, the soft-bottom fish assemblages were strongly organized spatially in New Caledonia, but remained relatively stable over time.