| The DSL(Deep Scattering Layer)is recognized as the horizontal layer being ubiquitous among the deep-water area with multiple creatures inhabiting and greatly scattering the acoustic wave.The South China Sea is located north of Chinese mainland and Taiwan island,east of Philippine islands,south of Kalimantan and Sumatra and west of Indo-China peninsula and Malay peninsula.The great potential of biological resources is embedded in this area.The existing researches have proven that,the DSL exists in the areas of South China Sea.As the fishery resources have been declined progressively along the nearshore area of South China Sea,the resources in the open seas of South China Sea has been attached increasing importance by the Chinese government.The researches of impact of DSL in the central area of South China Sea,the crucial part of marine ecology in the South China Sea,on the marine ecology of South China Sea and on the fishery changes have not been launched.The researches of acoustic characteristics and the spatial and temporal distribution of the DSL in central area of South China Sea are attached great significance to the understanding of ecological change in South China Sea and to the sustainable development of fishery in South China Sea.This paper surveys and researches the DSL in the central area of South China Sea through following the survey modes of sailing and drifting,adopting the underwater acoustics-based oceanographic survey method and the shipborne Simard EK60 acoustical instrument.The Echoview4.9 software is applied to dispose and analyze the acoustic data.The spatial distribution regulation and the variation range of NASC,namely the acoustic characteristic echo integral value,of the DSL are acquired.75.6 thousand square sea miles of sea area is researched,with the longitude and latitude range from 10°N-14°N to 111°E-118°E.Four sections are mainly surveyed following the sailing survey mode.The research conclusion is acquires as follow:1.There are two deep-water ranges of DSL in central area of South China Sea,viz.0-200 m and 300-800 m.The DSL in this area is surveyed as being vertically migrated,with the migration period of 1d.Such migration trend shall be declined at 6:00-7:30 at dawn,and raised at 17:00-18:30 at dusk,accounting for 1.5 h totally,with the averaged elevation of 400 m and in the migration rate of 4.4m/min.The migration shall be less dependent on the variation of latitude as it shall be advanced with the increase of longitude.2.Two peaks of echo integral value,NASC,exist in the water depth from 0 to 800 m,specifically in the depth about 100 m and 450 m.NASC is valued as 225 m2/n mile2 and 50 m2/n mile2 in 13.5°N condition,as 275 m2/n mile2 and 50 m2/n mile2 in 12.5°N,as 190 m2/n mile2?40 m2/n mile2 in 11.5°N condition and as 225 m2/n mile2? 50 m2/n mile2 in 10.5°N condition.The NASC in daytime is smaller valued than nighttime in depth of 0-150 m,and this trend shall be conversed in depth of 300-700 m.As the conclusion indicates,the DSL exists in the central area of South China Sea,and such layer is vertically migrated.The variation trend of NASC is well geared into the scattering layer distribution and the migration phenomenon.The status of DSL in central area of South China Sea is basically anatomized.The direct proportion of NASC value to resource volume shall lay the data foundation for the prospective resource development in South China Sea. |
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