| Sthenoteuthis oualaniensis is a kind of Marine cephalopods and widely distributed in the south China sea and abundant in existing resources.At present,the fishery resources in the north of the south China sea are in serious decline,while the ocean fishery resources such as Sthenoteuthis oualaniensis are underutilized.In order to promote the rational utilization of the resources and accurately assess the amount of resources,the acoustic scattering characteristics of Sthenoteuthis oualaniensis were studied in this paper.The research mainly includes two parts:measuring the target strength of living Sthenoteuthis oualaniensis in south China sea by tether control method;the acoustic scattering characteristics of Sthenoteuthis oualaniensis were studied by dB difference technique.In order to investigate target strength(TS)of flying squid Sthenoteuthis oualaniensis,25 squid specimens were collected by jig in the open sea in the northern South China Sea.The TS of 25 specimens were measured respectively one by one using Simrad EY60 split-beam scientific echosounder with 120 kHz and self-made tethering-controling device.In this paper,we primarily explored the impact of different parameters on the single target detection for Sthenoteuthis oualaniensis,and the parameters included pluse length determination level(PLDL,dB),min normalized pluse length(min NPL),max normalized pluse length(max NPL)and maximum standard deviation of minor-axis angles(MIA).Then we analyzed comprehensively the stochastic nature of single TS which was highly variable,and concluded the mathematical regression relation between TS of Sthenoteuthis oualaniensis and mantle length(ML,cm).The results showed:(1)As the PLDL increased,the number of single target of squid detected increased firstly and then decreased,and the number was the largest when PLDL was 6 dB,while the average TS of all single targets increased monotonously.With increase of min NPL,the number of single squid target decreased;in the range of min NPL<0.7,the average TS of all single targets presented increasing obviously.When the max NPL was less than 1.2,the number of single squid target increased as the max NPL increased,but the average TS decreased;when the max NPL fell in between 1.2 and 1.8,the number of single squid target presented increasing slowly,but the average TS remained relatively stable.As the MIA increased,the number of single squid target increased,but the average TS decreased.In this study,the optimum parameters setting of single target detection were as follows:PLDL=6.00 dB,Min NPL=0.7 dB,Max NPL=1.8,MIA=0.8°.(2)The maximum and minimum value of the average single TS of live Sthenoteuthis oualaniensis specimens were respectively-48.6 dB and-63.63 dB,and the corresponding mantle length of Sthenoteuthis oualaniensis were respectively 25.2 cm and 12.4 cm.The mathematical regression relation between TS of Sthenoteuthis oualaniensis and mantle length(ML)were concluded as follows:TS=34.85 1g ML-98.00(N=8,R2=0.881,P<0.01).In order to investigate the scattering characteristics of dB difference,according to the acoustic data was collected by using Simrad EY60 type double frequency(70 kHz and 120 kHz)split beam scientific echosounders in the south-central part of the south China sea in the spring of 2013,and was analyzed combined with the catches data of light-falling net.The dB difference scattering characteristics of flying squid Sthenoteuthis oualaniensis was obtained.The results showed:(1)The Sthenoteuthis oualaniensis gradually rise from the deep water to the shallow water of 55m after 17 o clock and before the first net off the net.The relation of the volume backscattering strength of 5～15m、15～25m、25～3 5m、3 5～45m and 45～55m with the change of illumination time is respectively:SY-0.053x 77.640(N=48,R2=0.789);SV=0.070x 78.725(N=48,R2=0.891);SV＝4.410lnx 20.002(N=48,R2=0.926);SV-3,181.lnx 84.386(N=48,R2=0.807);SV-3,552lnx 86.605(N=48,R2=0.779).(2)The difference of Sv between the two frequencies in the first 20min before the network was calculated,and got a stable linear relationship between the frequency difference △SV and time.Further,the relations between the average weight(m),average mantle length(NL),quantity percentage(n%)and mass percentage(m%)of flying squid Sthenoteuthis oualaniensis fishing samples and dB difference △ SV were fitted as follows::The equation between △SV and m/MLis Y=-475.42X2+627.16X-187.02(R2=0.6795);The equation between △SV and m/(ML-n%)is Y=-586.18X2+784.37X-238.92(R2=0.6988);The equation between △SV and m/(ML*m%)is Y=-373.7X2+512.82X-157.36(R2=0.542)。This TS measurement was the first attempt to investigate the investigate acoustic scattering characteristic and TS of live Sthenoteuthis oualaniensis using tethering controlling method at sea by field work,it is also significant to study the scattering characteristics of the flying squid Sthenoteuthis oualaniensis by using the dB difference characteristics.The results of this paper not only accumulated important information and experience for further investigating the acoustic scattering characteristics and improving the acoustic estimation precision of Sthenoteuthis oualaniensis,reduce the cost of research sampling,also provide the reference for measuring TS of other squid or fish by tethering method at sea and study on dB difference characteristics. |
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