This study examines the trophic status and relationships of coral reef fish
in the lagoon of New Caledonia, southwest pacific. The feeding habits of 34
fish species collected at three contrasted sites were first described using
a compilation of gut contents observations and data from the literature. The
carbon and nitrogen isotope signatures of these fish and of some of their
potential ultimate food sources were also determined at each site. Despite
some spatial variations in the isotopic signatures of most food sources and
fish trophic groups, the overall trophic structures of fish assemblages were
similar at the three sites. Stable isotope data were then used to re-assign
fish species to trophic groups based on the δ15N signatures of
fish and their food sources. Herbivorous fish species were clearly
distinguished from the other trophic groups by their lower δ15N
signatures, consistent with an estimated trophic position of ~2 for
all species examined. Scaridae were however characterized by relatively
higher δ13C and lower δ15N, which is probably
linked with the role of detritus in their diet. The estimated trophic
positions of planktivorous fish species were consistent with their gut
contents-based classification. Conversely, the isotopic signatures of
carnivorous and piscivorous fish species largely overlapped, and their
estimated trophic positions were much lower than expected. This suggests
that these species feed over a broader range of trophic levels and food
sources than implied by the gut contents observations, and indicates that
their diet is partly omnivorous. Finally, the relationships between body
mass and the isotopic signatures of four fish species were significant for
at least one isotopic ratio for each species. Since ontogenetic variations
and omnivorous diets are difficult to assess with gut contents data only,
stable isotopes revealed essential in estimating the actual trophic status
and relationships characterizing the fish species under study.