Tuna target-strength related to fish length and swimbladder volume

This paper proposes target-strength (TS) to fork length (FL) and swimbladde r volume relationships, respectively, for bigeye (Thunnus obesus) and yello wfin (T. albacares) tuna, based on previous TS measurements. TS versus FL r elationships were determined using the equations TS=a logFL+b but the resul ts should be regarded as preliminary because of the small, number of observ ations. The TS of bigeye tuna was about 5 dB higher than yellowfin when com paring species at equal size. This result can be linked to the swimbladder volume difference between species. For both species a single trend is obser ved in the relationship between TS and swimbladder volume. This result conf irms that this is the key factor in deciding the TS of physoclists fish. Si nce TS measurements were obtained from as deep as 500 m it would seem that a tuna's swimbladder is hot fully compressed and maintains its cross-sectio n even at great depths. Therefore tuna have the ability to control efficien tly the Volume of their swimbladders. (C) 2000 International Council for th e Exploration of the Sea.