Swimbladder shapes of 5 myctophid species captured around northern Japan were measured using an X-ray method. The species were classified into swimbladder and swimbladderless fishes according to the presence of gas. All specimens of Lampanyctus jordani had a well-inflated swimbladder. Notoscopelus japonicus and Stenobrachius nannochir had no swimbladder in any body size range examined. Diaphus theta and Symbolophorus californiensis contained swimbladder and swimbladderless fishes in relation to growth. Based on several morphological parameters of the body or swimbladder, the relationships between body length and target strength at 38 and 120 kHz were estimated using theoretical acoustical backscattering models. The estimated values of the target strength showed 10-dB or more differences between bladder and bladderless fishes, even when the fish were of equivalent size. The target strength values of most fishes did not grow proportionally to the square of the body length at both frequencies because of swimbladder allometric growth or the effect of Rayleigh scattering. Our result suggests the importance of swimbladder observation for target strength study of myctophid fishes, because they have various swimbladder conditions, and the characteristics of acoustic backscattering change dramatically with the swimbladder conditions.

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