In situ measurements of fish target strength are selected for use in echo integrator surveys at 38 kHz. The results are expressed through equations in which the mean target strength TS is regressed on the mean fish length I in centimeters. For physoclists, TS = 20 log I --67.4, and for clupeoids, TS = 20 log 1-71.9. These equations are supported by independent measurements on tethered, caged, and freely aggregating fish and by theoretical computations based on the swimbladder form. Causes of data variability are attributed to differences in species, behavior, and, possibly, swimbladder state.

Previous determinations of the swimbladder contribution to the fish backscattering cross section have been hindered by ignorance of the acoustic boundary conditions at the swimbladder wall. The present study circumvents this problem by direct comparison of target strengths of three gadoid species and mackerel-anatomically comparable fusiform fish which respectively possess and lack a swimbladder. The relative swimbladder contribution to both maximum and averaged dorsal aspect backscattering cross sections is shown to be approximately 90% to 95%, which is higher than most other estimates. The new results were established for fish of 29-to 42-cm length and acoustic frequencies of 38 and 120 kHz.

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A new model describes acoustic scattering by swimbladdered fish of lengths from at least 8 to 36 wavelengths. It represents a fish by an ideal pressure-release surface having the exact size and shape as the swimbladder. The backscattering cross section, or target strength, is computed by means of the Kirchhoffapproximation. To test the model, predictions of target strengths based on swimbladder morphometries of 15 gadoids of lengths from 31.5 to 44.5 cm are compared with conventional target strength measurements on the same, surface-adapted fish, anesthetized before acoustic measurement, and shock-frozen immediately afterwards. Details are given of the swimbladder morphometry. In essence, this consists of slicing the frozen fish with a microtome, photographing the exposed swimbladder cross sections, digitizing the contours, and triangulating the surface between pairs of contours on adjacent, parallel planes. Theory and experiment are compared through the dorsal and ventral aspect target strength functions, their averages, and simulated probability density functions.

An operational definition of backscattering cross section is developed for the wideband reception of finite echoes. This is supported by relative measurements on a set of copper spheres by each of four echo sounders operating at frequencies from 38 to 120 kHz. Experiential and theoretical arguments are advanced for the superiority of commercial, electrical-grade copper in the application. An optimization problem for determining the sphere size is then formulated, and solved for the case of calibration of a 38 kHz echo sounder by a sphere of the described material. The solution: that the copper sphere diameter be 60.00 mm, is tested through a variety of measurements. These demonstrate an accuracy of 0.1 dB. The further exercise of theory indicates the feasibility of precision calibration of diverse hydroacoustic equipment by copper spheres over most of the kilohertz frequency range.