Abstract :
[en] Coral reef-associated teleost fishes have complex life cycles, often involving the use of several habitats and changes in diet during their ontogeny. Stable isotope ratios of carbon and nitrogen are potential tracers of these ontogenetic changes. In this study, we use the crystalline lens of six species of coral reef fish to trace the variation of their habitat and diet during their ontogeny. The crystalline lens of adult fish is made up of successive layers, each formed during a different part of the individual's life. Once formed, these layers retain their initial isotopic composition, making it possible to trace the isotopic ontogeny of the individual and, potentially, to interpret this in terms of habitat use and diet. Aiming to study their ecological ontogeny, we selected six fish species (n = 5 specimens per species, 5 - 36 layers per individual, n = 600 isotopic measurements) from different families with contrasting trophic ecologies (herbivore, invertivore, piscivore) and habitat uses (e.g. external reef slope, reef flat, water column, seagrass ecosystems) at adult stage. These species are all targeted by artisanal fishing on the Great Reef of Toliara (SW Madagascar). The results not only enabled us to compare these species showing very contrasted ecological strategies during their ontogeny but also enabled us to reveal intra-specific variation in the isotopic trajectories. Diversity of individual trajectory differed among species. For example, some species (e.g. Strongylura incisa) displayed very low intra-specific variability with individuals having the same isotopic trajectory, indicating that individuals exploited the same habitat and the same food sources during their ontogeny. By contrast, other species (e.g. Lethrinus harak) showed a high intra-specific variability, indicating more inter-individual variability in habitat/food sources use during their life. These data have the potential to guide conservation decisions as all these species are potentially overfished on the Great Reef of Toliara but do not have the same ecological ontogeny.
