The relationship of target strength (TS), tilt angle and the effect of swimbladder resonance were examined using a theoretical scattering model for Maurolicus japonicus (30.4-46.8 mm standard length (SL)) at 38 kHz and 120 kHz, based on swimbladder measurements. The effect of tilt angle on TS was seen at 120 kHz, but was minimal at 38 kHz. The effect of resonance on TS was clear at 38 kHz, but was minimal at 120 kHz. This was due to the small size (average 11 % of SL) and oval shape with a relatively high aspect ratio (average 0.48) of the swimbladder. Considering the effect of resonance, the TS-SL relationships at 38 kHz were estimated: TS = 10.0 log 10 SL − 61.3 at 200 m depth. Considering the effect of tilt angle, the average TS (TS avg) −SL relationship at 120 kHz was estimated: TS avg = 16.5 log 10 SL − 75.1.
We examined threshold-setting methods to identify Maurolicus japonicus SL (Scattering Layer) in the Sea of Japan. Acoustic survey data were collected in several months (May, June, Aug., Sep., and Oct.
The target strength (TS) patterns of Sardina pilchardus and Sardinella aurita at 38 and 120 kHz were estimated by a prolate-spheroid model, using measurements of swimbladder length and width. The ratio of swimbladder length to total length (TL) was similar in both species, however the ratio of swimbladder width to TL was smaller and more variable for S. aurita. Assuming a normal distribution of fish swimming orientation angle (θfish) with mean ± standard deviation (SD) of 0 ± 10°, the normalized (by TL) average TS (b20) was estimated to be -64.0 dB (38 kHz) and -65.2 dB (120 kHz) for S. pilchardus, and -66.2 dB (38 kHz) and -67.2 dB (120 kHz) for S. aurita. Compared with currently applied b20 values at 38 kHz, our results under four different θfish assumptions (0 ± 10°, 0 ± 15°, -5 ± 10°, and -5 ± 15°) were 6-9 dB higher for S. pilchardus and 5-7 dB higher for S. aurita. This suggests four-to eightfold overestimation risk for S. pilchardus and three-to fivefold overestimation risk for S. aurita when using the currently applied b20 values.Keywords Sardina pilchardus·Sardinella aurita·Target strength·Theoretical approach·Prolate spheroid model ✉Tadanori Fujino
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.
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