Black porgy are hatched and released in marine ranching areas in Japan, Korea and Taiwan to maintain fisheries resources in coastal areas. If acoustic methods are used for field surveys, target strength ( TS ) information on the species is needed to convert acoustic data to biomass density. Target strength of black porgy ( Acanthopagrus schlegeli ) 15.5-32.9 cm fork length (nine individuals) was measured at 38 and 120 kHz (split beam) by ex situ measurements (cage and fishhook methods) of living fish, a tethered method with shock-frozen fish, and an acoustic scattering model, the Kirchhoff ray mode (KRM) model based on fish morphology. The ex situ TS -to-fork length ( FL , cm) relationships were TS 38kHz = 20log 10 FL − 64.6 ( r = 0.77) and TS 120kHz = 20log 10 FL − 65.2 ( r = 0.70). The values for the KRM model were TS 38kHz = 20log 10 FL − 64.8 ( r = 0.88) and TS 120kHz = 20log 10 FL − 65.4 ( r = 0.80). The agreement between the ex situ measurements and the KRM model for the mean TS was very good. In contrast, the results for the tethered method ( TS 38kHz = 20log 10 FL − 69.3 [ r = 0.42]) differed markedly from the other two methods, perhaps because of the effects of the shock-freezing procedure in air. The results of the ex situ measurements and scattering model provide basic information for studying TS and conducting acoustic surveys of black porgy, even though the tethered method must be modified.KEY WORDS: black porgy, ex situ measurements, Kirchhoff ray mode model, target strength, tethered method.
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.
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