Spatial trophic plasticity of two dominant seagrass-associated fishes in Toliara lagoon (SW Madagascar)

In the seagrass zone of Toliara (South West of Madagascar), the local fishermen deployed seine nets with small mesh size that likely affect the structure of seagrass meadow systems and associated fishes. Nevertheless, the responses of fish communities living in this ecosystem to these threats are currently unknown for the WIO region. This present study aims to evaluate the trophic diversity of fishes living in seagrass beds and to assess their trophic plasticity. More specifically, along this work, we compare the diet of two dominant species (the rabbitfish Siganus sutor and the goby Oplopomus oplopomus) found in the catches of local fishermen on seagrass beds and we evaluate a potential variation in their trophic ecology. The sampling of fishes was achieved by collecting the catches of traditional fishermen using bottom seine nets. Fish sampling occurred in front of the Ankilibe fisherman village in December 2017 and 2018, and in front of the village of Sarodrano (December 2018). This sampling strategy allowed us to make spatial and temporal comparisons. In order to assess the diet of these two fish species, stomach content analyses was performed on 40 specimens/species/village/year. Using a binocular microscope, animal preys were identified to the level of class and assigned to the planktonic or the benthic compartment. Plant items are classified as either phytoplankton, fragments of algae, or seagrasses. The trophic niche of each fish populations was quantified in two ways: as a percentage of occurrence and as a mean percent composition of each item in the gut content. The rabbitfish has gut filled by 50% of detritus, 40% of fragments of seagrass, and 10% of phytoplankton. The diet of the goby was more diversified and composed of 40% of detritus, 25% of copepods, 20% of operculum, 10% of gastropods and 5% of ostracods. There was no significant difference between the two sites along the two years of campaign. An isotopic analyses will be effectuated soon to know the trophic position of the studied fishes.