Acoustic survey of the Symplectoteuthis oualaniensis resources in the South China Sea based on light falling-net

Symplectoteuthis oualaniensis is a pelagic squid species widely distributed in the Indo-Pacific region,especially in the South China Sea,S.oualaniensis is the dominant species and therein there exists abundant biomass in the central and southern deep sea.It is necessary to carry out the exploitation and assessment of S.oualaniensis resources to promote the development of the fishery resources of the South China Sea.Due to the low population densities in natural conditions,resource survey by using squid jigging machine or pelagic trawl is ineffective.Although fisheries acoustics is a faster and more efficient way to assess the resources,because of insufficient data on biology and acoustic characteristics,it(when used alone)is not a perfect method.Therefore,the aim of this study is to explore a reasonable and effective assessment method and provide theoretic basis for reasonable utilization.This study proposes a model which combines light falling-net and acoustic survey by using a split-beam echo-sounder(Simrad EY60,120 kHz).Based on the acoustic data and biological data obtained from light falling-net conducted in the South China Sea from April to May in 2011,S.oualaniensis distribution,population structures,as well as method of acoustic survey were analyzed.The results showed that the effects of light trapping squids were obvious and the squid was mainly distributed at depths of 0-100 m.The average mantle length range was 10.4 to 14.2 cm,and the corresponding range of average target strength was-58 to-60.7 dB.Based on an analysis of collected data,the relationship TS=21.23logML-82.48 was obtained for S.oualaniensis at 120 kHz,with TS in dB and ML in centimeters.Our results indicated that this method is feasible and could provide important theoretical support for the squid resource assessment.