An acoustic approach to study tuna aggregated around fish aggregating devices in French Polynesia: methods and validation

The behaviour and spatial distribution of tuna, aggregated beneath fish agg regating devices (FADs), have been studied through ultrasonic tagging exper iments but, surprisingly, very few studies on FADs have used underwater aco ustic devices. We present techniques, and their limits, incorporating a sci entific echo sounder connected to a split-beam transducer to observe and ch aracterise tuna aggregations around FADs, and propose a general approach fo r future studies. Experiments were conducted in French Polynesia between De cember 1995 and February 1997. Two methods, echo-counting and echo integrat ion, were used. Echo-counting is possible when individual fish are sufficie ntly scattered so that each target can be discerned. On the other hand, ech o integration can be used with both scattered and aggregated fish schools. The knowledge of tuna target strength is useful for separating targets for echo-counting, and essential for obtaining absolute estimates of densities by echo integration. Sonar performances and settings should be considered w hen choosing the most suitable method to determine fish density or assessin g spatial structure of a tuna aggregation. These techniques allow one to st udy an entire tuna aggregation, its behaviour in space and time at very fin e time-space scales (about a nautical mile and over a few hours), and open up a new scientific field to study the spatial structure and behaviour of t una aggregations around anchored or drifting FADs. (C) 1999 Ifremer/Cnrs/In ra/Ird/Cemagref/Editions scientifiques et medicales Elsevier SAS.