Visual censuses around drifting fish aggregating devices (FADs): a new approach for assessing the diversity of fish in open-ocean waters

This study provides a baseline measure of pelagic fish species diversity around fish aggregating devices (FADs) in the Western Indian Ocean. Using data from visual censuses made by SCUBA divers around drifting FADs we (1) analysed 11 diversity indices describing 4 main components of pelagic diversity: number of species, evenness, taxonomic diversity and rarity and (2) discussed the viability of using visual censuses around FADs to monitor temporal and spatial changes in diversity patterns of pelagic fishes in the future. Our results not only show that a single index, such as species richness or the Shannon-Wiener index, cannot provide a complete description of pelagic fish diversity around FADs, but also that some components of diversity appear to be multifaceted themselves and require the simultaneous use of several complementary indices in order to be accurately described and monitored. Despite a limited sampling effort, our results suggest that using visual censuses around FADs could provide reproducible 'relative' indices of diversity for a well-delimited assemblage of species (intra- and extranatant species in the epipelagic domain). In this context, we propose a preliminary 'short list' of diversity indices based firstly on their complementarity and secondly on their simplicity. While visual censuses of pelagic fishes at FADs (like any other sampling method) cannot give an exhaustive picture of diversity, they are a fishery-independent survey method that can complement other sampling techniques and provide reproducible information to monitor the diversity of these very difficult-to-access pelagic fish communities.