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Abstract 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.
Yasuma et al. (Wed,) studied this question.