In situ acoustic target-strength measurement of bigeye (Thunnus obesus) and yellowfin tuna (Thunnus albacares) by coupling split-beam echosounder observations and sonic tracking

A research programme was carried out in French Polynesia to study tuna beha viour using acoustics and fishing experiments. Acoustics are of great impor tance for the study of tuna behaviour and estimation of abundance but estim ates of individual target strength that are available are particularly inac curate. In this study, four yellowfin tuna (Thunnus albacares) and two bige ye tuna (Thunnus obesus) of weight 4-50 kg were individually caught, identi fied, and equipped with ultrasonic tags for telemetry experiments. While tr acking the fish, simultaneous underwater acoustic data were recorded with a split-beam echosounder in order to estimate their in situ acoustic target strength. It was observed that target strength was stronger when fish were diving than when they were ascending toward the surface. This can be explai ned by the tilt angle orientation of the swimbladder. A target strength bia s according to depth was also observed. (C) 1999 International Council for the Exploration of the Sea.