Monitoring fish communities at drifting FADs: an autonomous system for data collection in an ecosystems approach
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An increasing proportion of landings by tuna purse seine fishing vessels are taken around drifting Fish Aggregating Devices (FADs). Although these FADs and their use by the fishing industry to capture tropical tuna have been well documented, operative tools to collect data around them are now required. Acoustic, video, photographic and visual data were collected on fish aggregations around drifting FADs in offshore waters of the western Indian Ocean. Multibeam sonars, multifrequency echosounders, pole-mounted digital video camera and an automated 360° rotating digital photographic camera were deployed from a vessel in the vicinity of FADs, and their observation capability evaluated with underwater visual census made by divers. Two prototypes of instrumented buoys equipped with scanning sonar were tested providing positive results on their feasibility and operability as pelagic observatory. Acoustics methods combined with digital underwater video represent interesting possibilities to remotely study the composition and behaviour of these fish aggregations. The acoustic methods allowed the accurate description of the spatial organisation and dynamics of individual fishes, schools and biotic scattering layers around the FAD, but species identification was difficult. In situ visual, photographic and video observations systems permitted species identification within a range of 0 to ~ 25 m. However, scuba divers observations were more efficient compared to the photographic and video cameras at detecting the presence of certain fish species around FADs. Obviously both methods are complementary, since the acoustic methods could not identify most fish species and could not detect the presence of small fishes found less than 5 meters under the FAD. These fishes represent a small part of the overall biomass of fish aggregations but they are part of the biodiversity of pelagic ecosystems and may play a major role in ecological processes associated with FADs. The opportunity to incorporate observation tools into the development of future autonomous instrumented drifting buoys for remotely monitoring fish diversity and abundance in the pelagic ecosystems is presented. The perspective of autonomously collecting large amounts of basic information useful for ecological and fisheries studies in an ecosystemic approach for open sea or coastal pelagic environment is emphasized. Keywords: Visual and video Fish census / Acoustics / Buoy system / Pelagic species / Monitoring / Observatory.