In situ acoustic target strength measurements of tuna associated with a fish aggregating device

In situ measurements of target strength (TS) at 38 kHz have been taken from tuna gathering around a fish aggregating device (FAD) during two cruises c arried out at an interval of a year. Mean TS of - 31.9 dB and - 32.8 dB wer e measured on tuna aggregations composed mainly of 50 cm mean fork length T hunnus obesus. Mean TS were very similar between the different surveys of t he same cruise. Between cruises a small difference in average TS was observ ed. It may be explained by the Variation of the species composition of the aggregation. A greater proportion of Thunnus albacares and Katsuwonus pelam is was observed in the second year. In situ TS measurements on tuna associa ted with a FAD provide references of value in estimating aggregation's biom ass. Furthermore TS data obtained on aggregated tuna may be useful for beha vioural studies of the aggregation such as school behaviour and cohesivenes s, fish length distribution, and vessel avoidance. (C) 2000 International C ouncil for the Exploration of the Sea.