| Sillago sihamais mainly distributed in the tropical waters of the Indian Ocean to the western Pacific Ocean and more concentrated in tropical beach,coastal bay and estuarine sandbank waters.It is distributed in coastal areas of China,especially in the majority of the South China Sea,and it's an important economic edible fish in the South China Sea.It belongs to carnivorous fish,and staple food includes zooplankton,polychaetes worms and shrimp and so on.Otolithes ruber is mainly distributed in India-the western Pacific Ocean,and it's mainly distributed in the South China Sea and the East China Sea in our country.It's a middle and lower inshore warm water fish,which likes to live in 10 m to 40 m high turbidity waters.It can sound in swimbladder,of ferocity,and it's a carnivorous fish,often with small fish and small benthic invertebrates for food.It belongs to coastal economic edible fish,and often captured by gill net.In recent years,due to the destruction of the ecological environment of the South China Sea and the development of the marine fishing industry,inshore fisheries resources are on the verge of exhaustion,how to effectively monitor and rationally manage the fisheries resources to ensure the sustainable development of the fishery economy becomes an urgent problem.The assessment of fisheries resources is the basis of fish resources development,the hydroacoustic assessment method of fisheries resources has been designated by developed fishery countries as the main technical means which can be used to estimate the biomass of aquatic resources with its fast and efficient,large scale investigation of space and non-contact that can't be harmful to biological resources.Hydroacoustic assessment of fisheries resources is based on the size of acoustic integral value(nautical area scattering coefficient,NASC,m2/nmile2)to measure the amount of biomass in the investigated area,while target strength is the key factor both converted.In order to improve the accuracy and reliability of the hydroacoustic assessment of fisheries resources,in this paper,the relationship which target strength of Sillago sihama and Otolithes ruber is changed with tilt angle and body length is measured and studied based on the kirchoff ray-mode approximation,the results of this study are as follows.1.The tilt angle variation pattern in both fish's target strength at 70,120 and 200 k Hz showed the feature distribution of multiple peaks,and the higher frequency the target strength to tilt angle variations is more sensitive.The maximum target strength corresponding to the tilt angle increases with the number of peaks increasing.At 70 k Hz,the maximum target strength of Sillago sihama and Otolithes ruber appears at the tilt angle of-5°~15°,where the Sillago sihama concentrate around-10°,and the Otolithes ruber concentrate near-11°.Maximum target strength of Sillago sihama is between-10° and 0°,concentrated in-9° at 120 k Hz,and the maximum target strength of Otolithes ruber appears at the tilt angle of-15°~10° at the same frequency.Maximum target strength of Sillago sihama occurs between-10°~0° and corresponding tilt angle is uncertain,and the tilt angle characteristic of the maximum target strength of Otolithes ruber for similar frequency is not obvious.2.When the function of tilt angle is(-5°,15°),for Sillago sihama,the relationship between target strength and body length is TS=362.29-lg L.18,and the formula is LTS-=37.64lg20 using b20 form at 70 k Hz;the relationship is TS=75.1762.33-lg L and the formula is LTS-=07.65lg20 using b20 form at 120 k Hz;the relationship is TS=84.1965.12-lg L and the formula is LTS-=32.65lg20 using b20 form at 200 k H.When the function of tilt angle is(0°,10°),for Sillago sihama,the relationship between target strength and body length is LTS-=68.65lg54.21,and the formula is LTS-=8.63lg20 using b20 for form at 70 k Hz;the relationship is LTS-=58.64lg15.20 and the formula is LTS-=4.64lg20 using b20 form at 120 k Hz;the relationship is LTS-=6.66lg63.21 and the formula is LTS-=62.64lg20.When the function of tilt angle is(-5°,15°),for Otolithes ruber,the relationship between target strength and body length is TS=64.1355.49-lg L,and the formula is LTS-=69.63lg20 using b20 form at 70 k Hz;the relationship is TS=4757.5-lg L5.1 and the formula is LTS-=26.64lg20 using b20 form at 120 k Hz;the relationship is TS=063.29-lg L7.19 and the formula is LTS-=49.64lg20 using b20 form at 200 k H.When the function of tilt angle is(0°,10°),for Otolithes ruber,the relationship between target strength and body length is TS=860.02-lg L4.16,and the formula is LTS-=09.64lg20 using b20 form at 70 k Hz;the relationship is TS=28.1963.71-lg L and the formula is LTS-=63.64lg20 using b20 form at 120 k Hz;the relationship is TS=97.2370.12-lg L and the formula is LTS-=65lg20 using b20 form at 200 k H.In this paper,based on the Kirchoff ray-mode approximation,the relationship of target strength with the tilt angle and body length at different frequencies for Sillago sihama and Otolithes ruber is obtained in order to provide useful reference for model study of target strength of coastal fish in the South China Sea,and providing a scientific basis for improving the accuracy and credibility of hydroacoustic assessment of fisheries resources. |
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