| Abstrakt: |
Explosive volume-scattering measurements made in widely separated areas in the Pacific and Indian Oceans exhibit a similarity in frequency pattern over the range 1-20 kHz. Typically, column scattering strength (for columns extending from the water surface to a depth of 1000 m) increases sharply in the range from 1 kHz to about 6 kHz and is fairly constant or decreases slowly thereafter. The similarity, from area to area, in the shape of this pattern suggests a mechanism common to all these areas even though the individual scatterers vary, from area to area, in size and depth distribution. We believe that this characteristic pattern is produced by resonant scattering from an assemblage of several species of mid-water fishes, principally myctophids, which vary in size within the rather narrow limits characteristic of these mid-water fishes. (Myctophids, for example, have swim-bladder radii falling within the limits 0.5 to 5 mm.) Tests of this hypothesis using a fictitious but plausible assemblage of fishes have produced patterns similar to those measured acoustically. Further comparisons in which column strength measurements in a given area are compared with column strengths which were calculated from net-haul data giving the sizes and depth distribution of the mid-water fishes in that area also give considerable support to our hypothesis. Refinement of the calculations, provided by a better knowledge of net-haul efficiency and more exact information on target characteristics (resonance frequency, Q, and target strength), is expected to provide further support for this hypothesis. [ABSTRACT FROM AUTHOR] |
