Conditions for precise measurement of in situ fish target strength (TS) are empirically studied and two indexes are introduced for this purpose. One is the number of fish in the effective reverberation volume which contributes echo tormation at a certain instant and the other is the percentage of the multiple echoes which is derived from a residual of the single echo extraction. With the decrease of both indexes measured target strength approach a certain asymptotic value which is admitted as reliable from the past study. This shows the existence of some threshold values and below these threshold values TS measurement will be successful. The effectiveness of both indexes is confirmed by the data set obtained from one large same fish school in the eastern shelf of Bering sea during the intership calibration between Japanese and U.S. vessels on 15 and 16 August 1991.

Analytical and numerical scattering models with accompanying digital representations are used increasingly to predict acoustic backscatter by fish and zooplankton in research and ecosystem monitoring applications. Ten such models were applied to targets with simple geometric shapes and parameterized (e.g., size and material properties) to represent biological organisms such as zooplankton and fish, and their predictions of acoustic backscatter were compared to those from exact or approximate analytical models, i.e., benchmarks. These comparisons were made for a sphere, spherical shell, prolate spheroid, and finite cylinder, each with homogeneous composition. For each shape, four target boundary conditions were considered: rigid-fixed, pressure-release, gas-filled, and weakly scattering. Target strength (dB re 1 m(2)) was calculated as a function of insonifying frequency (f = 12 to 400 kHz) and angle of incidence (θ = 0° to 90°). In general, the numerical models (i.e., boundary- and finite-element) matched the benchmarks over the full range of simulation parameters. While inherent errors associated with the approximate analytical models were illustrated, so were the advantages as they are computationally efficient and in certain cases, outperformed the numerical models under conditions where the numerical models did not converge.

This article reports theoretical values of target strength (TS) for mesopelagic lanternfishes based on morphological measurements of their swimbladders. Three species of lanternfishes, Diaphus theta (26.9–77.4 mm standard length (SL)), Symbolophorus californiensis (85.0–108.4 mm SL), and Notoscopelus japonicus (126.0–133.2 mm SL), were examined. After external morphological measurement of the fish body, a specialized “soft X-ray” imaging system was used to map the swimbladders and obtain their morphological parameters. The swimbladder was inflated in D. theta, uninflated in S. californiensis, and was absent in N. japonicus. For D. theta, the swimbladder length does not increase in proportion to the body length, suggesting that the contribution of the swimbladder to acoustic reflection is reduced with growth in this fish. Based on the morphological measurements, the theoretical TS of the fish at 38 kHz was calculated using the approximate deformed-cylinder model (DCM) and the general prolate-spheroid model (PSM). For all three species, the calculations showed about 3 dB difference between the TS indicated by the DCM and PSM. Given that the description of body shape is poor in PSM, the DCM results were adopted for fish without a swimbladder or an empty one. The intercept b20 in the standard formula TS = 20 log SL + b20 was −85.7 dB (DCM) for S. californiensis and −86.7 dB (DCM) for N. japonicus. On the other hand, the PSM model was adopted for D. theta since its swimbladder has too small an aspect ratio to apply the DCM. For D. theta, the relationship between SL and TS is best expressed by TS = 11.8 log SL − 63.5, which implies that its scattering cross-section is not proportional to the square of the body length.

This paper reports a laboratory target strength (TS) measurement and theoretical modelling of walleye pollock and Pacific hake. The measurements are performed in a tank as a function of fish tilt an gle, which is controlled in one-degree steps. A precision echo sounder is used and sphere calibrations are conducted before or after the measurement. A specialized soft X-ray imaging system is used to map the fish swimbladder. The X-ray images are digitized to obtain the fish morphological parameters. Based on these parameters, theoretical TS of the fish is calculated using the approximate deformed cylin der model and the general prolate spheroid model. An advantage of using these models lies in their sim plicity and the fact that few parameters are required in the model computation. Calculations indicate a negligible contribution of the fish body to the total scattering in the wide range of tilt angle. The theoret ical TS values are compared with the measured values. Agreement between measured and theoretical TS is reasonably good for near normal incidence, but deteriorates for larger tilt angles. The level of agreement is discussed. Obtained maximum and average TS values are nearly equal to or a little bit smaller than the published data. Using the deformed cylinder model, broad band TS characteristics are generated. Our results highlight the usefulness of the simple method to predict fish TS using the deformed cylinder model associated with X-ray imaging.

Yasuma, H., Sawada, K., Takao, Y., Miyashita, K., and Aoki, I. 2010. Swimbladder condition and target strength of myctophid fish in the temperate zone of the Northwest Pacific. – ICES Journal of Marine Science, 67: 135–144. We report theoretical values of the target strength (TS) of four myctophid fish (Ceratoscopelus warmingii, Myctophum asperum, Diaphus garmani, and Diaphus chrysorhynchus) based on morphometry of the swimbladder. None of the D. chrysorhynchus had an inflated swimbladder, but the other species had both inflated and non-inflated swimbladders, depending on body size. The relationships between swimbladder and body length showed that once gas production started, the swimbladders grew faster than the rest of the body (positive allometric growth). However, M. asperum showed regression of the swimbladder after positive allometric growth, so larger specimens had non-inflated swimbladders. Based on the measurements of swimbladder and body length, the theoretical TS values at 38 and 120 kHz were calculated using existing sound-scattering models. In fish with inflated swimbladders, TS values were relatively low (less than −67 dB, reduced TScm) at both frequencies. Regression slopes on TS–body length (log) plots were >20, suggesting that their scattering cross sections were not proportional to the square of the body length. In contrast, the TS values of M. asperum decreased with growth in large fish (60–80 mm long) through swimbladder regression. Scattering cross sections of fish without swimbladders were not proportional to the square of the body length over the whole size range.