Description:
<jats:p>In situ visual and photographic observations of organisms made during a dive of the bathyscaph Trieste in waters off San Diego, California, have been compared with acoustic volume-scattering measurements made simultaneously from a nearby surface ship. The dominant organisms observed in zones of peak reverberation were the physonectid siphonophores, Nanomia bijuga. These colonial coelenterates consist of a number of different individuals, one of which is modified into a gas-filled flotation sac. Acoustic-scattering coefficients, which were measured at 12 kc/sec during the course of the dive, vary from −48 to −58 dB over the most densely populated portion of the layer and show a variation with depth similar to that indicated by visual estimates of population density. Although the filamentous structure of a colony and its waterlike consistency reduce the effective scattering cross section, the gas floats borne by the siphonophores are near resonant size for this frequency and depth and they alone can account for the observed scattering levels. Because of the high scattering coefficients obtained and because these organisms have a wide geographic distribution, we believe that siphonophores should be considered seriously as perhaps the major cause of the deep scattering layer throughout the oceans of the world.</jats:p>