Ex situ target strength of yellow croaker (Larimichthys polyactis) in a seawater tank

Abstract

We conducted a TS study of yellow croaker (Larimichthys polyactis), a major commercial fish in Asia that has not been studied before. Ours is the first study that examines the relationship between target strength (TS) and fish length using 38, 70, and 120 kHz split-beam echosounders. We measured TS in five size groups of yellow croaker (Larimichthys polyactis) based on length (8.2–26.5 cm total length) in a seawater tank and calculated the Kirchhoff-ray mode (KRM) model. When the slope of the least-squares regression line was forced to 20 in the TS equation, the resulting values for the constant term (b20) at each frequency (38, 70, and 120 kHz) using the caged method were –67.69, –67.52, and −68.83 dB, respectively. The swimming angle (mean ± S.D.) in the net cage was − 4.2 ± 21.9°. The b20 values for the KRM model were –65.01, –64.74, and −66.27 dB at 38, 70, and 120 kHz, respectively. In both methods, the KRM model’s b20 values were relatively higher than those of the caged method by 2.68, 2.78, and 2.56 dB at 38, 70, and 120 kHz, respectively. Such TS ex situ measurements can be applied to improve the yellow croaker’s acoustic abundance estimates.