Demersal fish distribution in Northeast Brazil from multifrequency active acoustics and video observation

Abstract

Tropical waters hold major biodiversity hotspots and are priority areas for protection in the context of over-exploration and climate change. Suitable management of marine resources requires comprehensive, spatially explicit, high-quality, and up-to-date data. In that regard, multi-frequency active acoustic is a powerful tool that can provide relevant solutions to the increasing need for comprehensive data. In that framework, this thesis aims at filling some gaps in fish distribution knowledge for two tropical ecosystems of the South Western Tropical Atlantic, an oceanic archipelago, and the neritic coastal ecosystem of Northeast Brazil by using acoustic active methods combined with other approaches. In the first two chapters, we used an innovative integrated method combining acoustic and video data to study fish distribution around the oceanic Archipelago of Fernando de Noronha (FNA). FNA encompasses a marine protected area, preventing extractive biological sampling. Simultaneous scrutinizing of video and echogram allowed discriminating between similar echotypes leading to a classification of all the echoes into ten fish assemblages and two triggerfish species. The most abundant species observed by acoustics was the black triggerfish (Melichthys niger). We estimated the target strength of this species, a prerequisite for acoustic biomass estimation that we estimated to be 700 tonnes using geostatistics. To provide a detailed comprehensive description of fish distribution and their environment around FNA we analyzed the distribution of fish assemblages in relation to habitat characteristics obtained from acoustic data and video (e.g., depth and sediment type). This comprehensive description allowed highlighting fish hotspots’ location and in particular the importance of the shelf-break on the windward side of FNA, which includes well-known fishing hotspots. In contradiction with the expected result, FNA exhibits an asymmetrical fish distribution with a higher concentration on the windward side of the main island as opposed to the higher primary production on the leeward side. These results led us to a revision of the island mass effect for high trophic levels. In the last part of this thesis, we applied the methodology developed above to acoustic data collected continuously along the diel cycle during two surveys performed in different seasons on the neritic environment of the Northeast Brazilian coast. The main objective was to describe the demersal and pelagic fish spatial distribution. For that, we classified fish echoes into eight fish assemblages. The patterns of the distribution of fish total biomass and fish assemblages varied according to the season. The acoustic scape varied along the diel cycle with a higher acoustic response at night due to different gregarious patterns. Many demersal and pelagic fish exhibit diel migrations. This behavior change is known to affect the acoustic data since fish were more scattered over space and observable at night. Still, the acoustic data highlighted the presence of hotspots along the coast of North-east Brazil in particular a hotspot in the Pernambuco plateau and another in the Rio Grande do Norte shelf. The spatial distribution of fish biomass varied according to season, depth, and distance to the shelf-break. At the end of the manuscript, a discussion is initiated on the perspectives offered by the dataset developed during this thesis, especially for Marine Spatial Planning.