Ontogenetic dietary changes of coral reef fishes in the mangrove-seagrass-reef continuum : Stable isotopes and gut-content analysis

Juveniles of a number of reef fish species develop in shallow-water 'nursery' habitats such as mangroves and seagrass beds, and then migrate to the coral reef. This implies that some reef fish species are distributed over the mangrove-seagrass-reef continuum in subpopulations with different size distributions that are spatially separated for considerable periods of time. To test this assumption, and to determine ontogenetic dietary changes (which may drive fish migrations from nursery habitats to the coral reef), we selected 9 herbivorous and carnivorous reef fish species whose juveniles are highly abundant in nearby nursery habitats. 13C:12C and 15N:14N ratios were measured in fish muscle tissues and in potential food items collected from each of the 3 habitats, and fish gut contents were identified. All δ13C signatures of fishes sampled from the coral reef were close to the carbon isotope signatures of food items on the reef, and were significantly depleted compared to those of fishes from the nursery habitats (with the exception of Scarus iserti). Gut-content analysis of herbivorous fishes entailed problems with identification of the ingested food items, but there was no change in the δ13C or δ15N signatures of the muscle tissue as a function of fish size. Regression analysis showed significant positive relationships between δ15N content and fish size in all carnivorous fish species; this was correlated to the decreasing dietary importance of small crustaceans and the increasing consumption of decapod crabs or prey fishes with increasing size. The combined study of stomach contents and stable isotopes showed that the juveniles and adults of these reef fish species are separated ecologically and spatially for a considerable period of time, and that herbivorous fishes do not change their trophic status with increasing size, whereas carnivorous fishes feed on increasingly larger prey at increasingly higher trophic levels prior to their migration from the nursery habitat to the coral reef.