Pelagic assemblages composition in the Pelagie Archipelago: effects of seafloor geomorphology and comparison of video-based sampling methods

Pelagic community species play a key role in the marine ecosystem, but gathering information on their abundance and distribution is often challenging and requires significant sampling efforts mostly coming from fishery-dependent methods. This thesis aimed to assess the composition of the pelagic community in the Pelagie Archipelago, through two video-based sampling methods: BRUVs (Baited Remote Underwater Video systems), and LMC (Longline Mounted Camera), an innovative way to assess pelagic assemblages interacting with the longline. The research was conducted in areas characterized by different seabed morphologies creating structural discontinuities: shoals and slopes. The hypothesis was that these seabed structures and the sampling methods influenced the observed composition of pelagic communities. Through video analysis, proxies of species abundance (NMax and sightings) were recorded and analysed using multivariate approaches to evaluate community differences. Time of first arrival (TFA) in the field of view was also recorded for each species. The results revealed no clear segregation of pelagic assemblages between the two geomorphological structures, suggesting a high connection between habitats. The LMC method detected more species and shorter TFAs than BRUVs, while BRUVs were more efficient in estimating species abundance with less sampling effort but produced more variable data in species composition and TFA. Both methods detected rare and threatened species, including Carcharhinus plumbeus and Isurus oxyrinchus. Relying solely on catch data would underestimate both diversity and abundance of species in the sampled areas. Overall, non-extractive video-based sampling methods provide valuable additional information on pelagic communities. The Pelagie archipelago is therefore confirmed as an area of ecological interest for testing innovative methodologies to expand knowledge of the pelagic ecosystem and associated communities.