Spatial structure of commercial reef fish communities along a terrestrial runoff gradient in the northern lagoon of New Caledonia

The spatial distribution of commercial reef fishes from four areas of the northern lagoon of New Caledonia was studied following a terrestrial runoff gradient ranging from very low on the western barrier reef to high on the northern fringing 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 richness. Density ranged from 0.49 fish m-2 (western reefs) to 0.69 fish m-2 (eastern reefs), but did not show any particular trend. Biomasses 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 very low fishing pressure in the whole area. Numerous significant relationships 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, granulometry and color of sediments). Descriptors of total ichthyofauna were all positively correlated with decreasing terrestrial influence. The 6 dominant fish families displayed similar trends and were positively correlated with decreasing terrestrial influence, except for Siganidae which showed the opposite trend. Some of the most common species, such as Scarus sordidus and Siganus doliatus, had no particular distribution pattern. Size, density and biomass of other species, such as Lethrinus atkinsoni, Cheilinus undulatus, Hipposcarus longiceps, Scarus microrhinos and Acanthurus xanthopterus, were significantly correlated to parameters characteristic of offshore reefs. Conversely, Plectropomus leopardus, Scarus ghobban and Siganus lineatus were closely related to coastal reefs. Some species had significant correlations with parameters characteristic of intermediate reefs, such as Scarus schlegeli and Acanthurus nigricauda. Data for Plectropomus leopardus and Scarus ghobban suggested a possible ontogenic migration of these two species from the northern fringing reefs, where numerous small individuals were observed, to offshore barrier reefs, where only large individuals were recorded.