Swimbladder-depth relationships of fish causing volume reverberation in the Cariaco Trench

Oceanic volume reverberation in the frequency range of 1 to 20 kHz is caused primarily by scattering from swimbladders of small midwater fish. As these fish undergo daily vertical migrations, the scattering characteristics of their swimbladders change, due to changes in ambient pressure. Two simple swimbladder-depth relationships have been postulated. The first is that fish work to keep their swimbladders at constant volume in order to remain neutrally buoyant at all times. The second is that fish conserve energy by passively allowing their swimbladders to remain at constant mass, or PV = constant. In order to investigate swimbladder-depth relationships, we have collected acoustic and biological volume reverberation data in the Cariaco Trench, where only two species of midwater fish were found. Acoustic results were reported previously [J. Acoust. Soc. Am. Suppl. 1 73, S97 (1983)]. Measured scattering strengths were compared to values obtained by applying a swimbladder scattering model to fish caught in discrete-depth biological trawls. Results of this comparison show that swimbladder-depth relationships in the Cariaco Trench are more complex than the constant volume or constant mass postulates. [Work supported by NORDA.]