Target strength of a mesopelagic fish Maurolicus japonicus at 38 and 120 kHz

Abstract

Target strength (TS) of Maurolicus japonicus was examined at 38 and 120 kHz using experimental measurements (at 38 kHz) and theoretical swimbladder scattering models. Experimental measurements showed that the average TS for dominant size (32.1 to 34.8 mm standard length) were −60.5 to −56.7 dB, and the effect of the fish orientation on TS was slight. These results coincided with the model estimation, proving that swimbladder models are valid to estimate the TS of M. japonicus. The slight effect of the fish orientation on TS was presumed due to the oval shaped swimbladder with high aspect ratio, which is relatively little to the wavelength. This is a merit because tilt angle is usually a major source of TS variation. However, a little swimbladder would cause acoustic resonance at low frequencies, which leads to insatiability on TS according to depth change. Using the resonant model, relationships of the TS and standard length were estimated for depth 50, 100, 150, 200, and 250 m at 38 and 120 kHz, respectively. While TS at 120 kHz showed little change according to depth, TS at 38 kHz changed with depth and showed larger TS than 120 kHz for each size.