SPATIAL STRUCTURE OF COMMERCIAL REEF FISH COMMUNITIES ALONG A TERRESTRIAL RUNOFF GRADIENT IN THE NORTHERN LAGOON OF NEW CALDONIA

The spatial distribution of commercial reef fishes from four areas of the northern lagoon of New Caledonia was studied following a terrestri al runoff gradient ranging from very low on the western barrier reef t o high on the northern hinging reefs. Species richness decreased from eastern reefs (30.4 species transect(-1)) to northern fringing reefs ( 21.4), suggesting a link between terrestrial influence and species ric hness. Density ranged from 0.49 fish m(-2) (western reefs) to 0.69 fis h m(-2) (eastern reefs), but did not show any particular trend. Biomas ses increased from 148 g m(-2) on the northern fringing reefs to 447 g m(-2) on the western reefs. Such high values are Likely due to the ve ry low fishing pressure in the whole area. Numerous significant relati onships were found between fish descriptors (i.e. diversity, density, biomass and mean weight) and terrestrial influences (i.e. distance to coast, visibility, % of carbonates in the sediment, % of mud, granulom etry and color of sediments). Descriptors of total ichthyofauna were a ll positively correlated with decreasing terrestrial influence. The 6 dominant fish families displayed similar trends and were positively co rrelated with decreasing terrestrial influence, except for Siganidae w hich showed the opposite trend. Some of the most common species, such as Scarus sordidus and Siganus doliatus, had no particular distributio n pattern. Size, density and biomass of other species, such as Lethrin us atkinsoni, Cheilinus undulatus, Hipposcarus longiceps, Scarus micro rhinos and Acanthurus xanthopterus, were significantly correlated to p arameters characteristic of offshore reefs. Conversely, Plectropomus l eopardus, Scarus ghobban and Siganus lineatus were closely related to coastal reefs. Some species had significant correlations with paramete rs characteristic of intermediate reefs, such as Scarus schlegeli and Acanthurus nigricauda. Data for Plectropomus leopardus and Scarus ghob ban suggested a possible ontogenic migration of these two species from the northern hinging reefs, where numerous small individuals were obs erved, to offshore barrier reefs, where only large individuals were re corded.