Variation of the trophic ecology in two teleost species associated to seagrass beds at Toliara Reef (Madagascar)

Small scale fishermen in Toliara (South West of Madagascar) extensively use small mesh seine nets in the seagrass zones and this practice may affect the structure of seagrass-associated fishes by catching small fish, juvenile and even post-larvae. The general objective of the present study is to evaluate the trophic ecology of two dominant teleost species (the rabbitfish Siganus sutor and the goby Oplopomus oplopomus) from catches of local fishermen using bottom seine nets. More specifically, this study aims to evaluate the temporal and spatial variation of the trophic ecology of those species by using stable isotopes and stomach contents. The sampling was carried out in seagrass beds in front of two villages: Ankilibe on December 2017 and 2018, and in Sarodrano on December 2018. To assess variation in diet, stomach content analyses were performed on 40 specimens/species/village/year. Preys were identified to the level of class and then assigned to different food groups. Stable isotope analyses were performed on muscles to complete information on variation in feeding habits. The combination of these methods revealed that the diet of species varies according to the year and the site of collection. This probably results from plasticity and accommodation to the availability of food sources and changes in environmental factors. In both species, the type of food consumed remains the same but the difference is observed in the frequency and the abundance of prey. Trophic variation is also observed over relatively long period of time as it is confirmed by stable isotope analyses. This study clearly demonstrates the trophic ecology of these two seagrass-associated fishes vary across time and space but additional works are now needed to understand which factors drive this variation.