Preliminary results of acoustic target strength measurements of bigeye (Thunnus obesus) and yellowfin tuna (Thunnus albacares)

Acoustics is a common method for stock assessment and to observe pelagic fish schools or individuals in their environment, but it was never applied on tuna. Knowledge about individual target strength (TS) is nevertheless the primary condition for quantitative studies and this is particularly sparse for tuna species. In this study, an original method was used to measure in situ acoustic target strength of 4 yellowfin tuna (Thunnus albacares) (4 to 30 kg) and 2 bigeye tuna (Thunnus obesus) (30 and 50 kg). These fish were individually caught, identified and measured, then equipped with ultrasonic tags for telemetry experiments. During the tracking operation, simultaneous underwater acoustic data were recorded with a split beam echo-sounder when the fish was inside the acoustic beam of the research vessel's echo-sounder. This experiment has shown that this method provides coherent results even if a great variability is observed. Further extensive use of this method, particularly in tuna studies, may be undertaken in the future.