多鳞鱚目标强度的模型法研究

鱼类目标强度测量是渔业水声学工作的核心内容之一。本研究采用基尔霍夫近似模型对19尾多鳞鱚(Sillago sihama)样品的理论目标强度进行近似计算,模型计算所需鱼类形态学参数由中国水产科学研究院南海水产研究所所属X光机(SOFTEX M-100)拍摄X光影像获得。通过编程计算获得多鳞鱚不同频率下目标强度随姿态倾角的变化图案,并通过数据拟合方法建立了不同频率和倾角分布函数下多鳞鱚目标强度随体长变化的经验公式,并与常规的b20表式进行对比。结果表明:在70 kHz、120 kHz和200 kHz 3种频率下多鳞鱚目标强度的倾角变化图案呈多峰状特征分布,且频率越高目标强度对倾角变化越敏感,波峰数增加,目标强度最大值对应的倾角增大。70 k Hz下多鳞鱚目标强度最大值出现在–15°~5°,120 k Hz和200 k Hz下目标强度最大值则出现在–10°~0°,且各频率下目标强度最大值出现的位置各不相同。不同频率及倾角分布函数下多鳞鱚目标强度随体长变化特性各不相同,其中在角度函数为(–5°,15°)、频率为120 kHz,以及角度函数为(0°,10°)、频率为200 k Hz时目标强度对体长经验公式和常规的b_(20)表式曲线基本重合,拟合度较高,可将常规的b20表式直接用于多鳞鱚的资源评估;而其他情形下2种表式存在一定偏差,采用直接拟合的参数方程更为恰当。研究表明,基尔霍夫近似模型能够很好地反映多鳞鱚的目标强度特性,可为中国南海近岸鱼类目标强度研究提供有益借鉴,为提高渔业资源水声学评估的准确度和可信度提供科学依据。

Modeling study on the target strength of Sillago sihama

Measurement of fish target strength is one of the core components of fisheries hydroacoustics. Research focused on offshore fish in the South China Sea has not met the needs of fisheries-resource estimations. As a typical offshore fish, little is known about the target-strength characteristics of Sillago sihama. To understand the target-strength characteristics of S. sihama, a Kirchoff ray-mode approximation was used to calculate the theoretical target strength of 19 S. sihama samples and the morphological parameters, calculated using a model of the fish, were obtained using an X-ray machine (SOFTEX M-100) belonging to the South China Sea Fisheries Research Institute, Chinese Academy of Fishery Sciences. The changes in patterns of target strength of S. sihama with tilt angle were obtained by calculating programmatically at different frequencies. The empirical formulas of target strength of S. sihama with body length were established for different frequencies and the distribution function of the tilt angle was produced using a data-fitting method, and compared with the conventional b₂₀ expression. The results show that the various patterns of target strength of S. sihama, with tilt angle at 70 kHz, 120 kHz, and 200 kHz, show multiple-peak-like distributions, and the variation of target strength with the tilt angle is more sensitive at higher frequencies. As the number of crests increased, the maximum target strength corresponding to the tilt angle increased. Maximum target strength of S. sihama occurs between -15° and 5° at 70 kHz, and the maximum target strength appears between -10° to 0° at 120 kHz and 200 kHz. The position of maximum target strength occurred differently at each frequency. The varying characteristics of target strength of S. sihama with body length changed under different frequencies and distribution functions of the tilt angle. When the angle function is (-5°, 15°), frequency is 120 kHz, and when the angle function is (0°, 10°), frequency is 200 kHz, the empirical formula and the conventional b₂₀ expression curve of target strength with body length coincide, the degree of fit is higher, and the conventional b₂₀ formula can be used directly for resource assessment of S. sihama. In other cases, there are some deviations in the two formulas, and the direct fitting parameter equation is more appropriate. The study confirms that the Kirchoff ray-mode approximation can reflect the target-strength characteristics of S. sihama, providing a useful reference for the study of target strength of inshore fish in the South China Sea and a scientific basis for improving the accuracy and credibility of hydroacoustic assessment of fisheries resources.