Scattering Characteristics Of Chub Mackerel(Scomber Japonicus) In The Northwest Pacific Ocean And The Use In Stock Assessment

The Northwest Pacific Ocean,where has abundant fishery resources,is the most productive marine fishery area in the world.As a warm-water pelagic fish,chub mackerel（Scomber japonicus）widely distributed in the East China Sea,the Yellow Sea,the Sea of Japan and the Far East of the Northwest Pacific Ocean,and the most northerly reaches the Sea of Okhotsk.And as one of the most economically important fish species in the Northwest Pacific Ocean,S.japonicus plays an important role in marine fisheries of China,Japan and South Korea,and it is recognized as one of the priority species of the North Pacific Fisheries Commission（NPFC）.After a sharp decline at the end of the last century,its stocks have recovered in recent years.In order to formulate reasonable fishery management measures and utilize the S.japonicus resources sustainably,it is indispensable to accurately assess its stocks.Fishery acoustics has now become an important method of stock assessment.In order to accurately assess S.japonicus stocks in the Northwest Pacific Ocean,it is important to understand the basic scattering conditions in this area and the target strength characteristics of S.japonicus.Data was obtained by the fishery resources survey vessel"Songhang"of Shanghai Ocean University in August 2019 and June 2021 in the Northwest Pacific Ocean.First,the scattering strength range of zooplankton sound scattering layer（SSL）in the northwest Pacific Ocean was obtained using fishery acoustic technique.Second,the Kirchhoff-ray mode（KRM）model was used to calculate theof S.japonicus,and to analyze variation inwith fish body and swimbladder shape at different frequencies.Four common frequencies used in acoustic fishery surveys were selected to obtainbody length equations.And a broadbandspectrum was produced by simulating the variation ofwith frequency in the range of 0 250 k Hz.Finally,the nautical area scattering coefficient（）and acoustic density of S.japonicus were preliminarily calculated using the echo integration method.The main results are as follows:（1）The analysis of the acoustic data collected in the Kuroshio-Oyashio confluence region of the Northwest Pacific Ocean in 2019 shows that the scattering strength of the SSL in the water depth range of 0-200 m is between-71.02 d B and-57.36 d B,and the main sound scattering originate from the zooplankton.Theof each layer increases with the rise of the transducer frequency.The magnitude ofwas closely synchronized with the solar altitude angle,which gets smaller when the angle is positive,then larger when the angle is negative.It means that the SSL has a diel vertical migration（DVM）behavior with the variation of solar height.Meanwhile,scattering strength was positively correlated with temperature in the vertical direction and showed a maximum of-54.31d B at 20 40 m under the influence of the thermocline.The Kuroshio and Oyashio currents had an obvious influence on the scattering strengths in this study,indicating a low value when next to the Oyashio side and a high value on the Kuroshio side.The scattering strength near the warm vortex center was higher than that at the vortex edge.（2）The results of KRM model were revealed thatvalues varied with pitch angle shifts,with the impact on fish scattering strength being greater at higher frequency.Therefore,it is better to use low transducer frequency when using acoustic methods to estimate S.japonicus biomass,so as to reduce the error resulting from pitch angle shifts.At frequencies of 38 k Hz,70 k Hz,120 k Hz and 200 k Hz,was greatest at a pitch angle range of-10°to 0°,which was related to the angle of the swimbladder tilt.There were almost no differences betweenestimated using the measured pitch angle distributions and using the universal distribution.When the measured pitch angle was N[-3°,4°],the averageof S.japonicus with body length of 12.04 22.17 cm at four frequencies was-48.88 d B,-49.14 d B,-49.75 d B and-48.55 d B,respectively.The standardbody length equations of four frequencies were₃₈=20log₁₀-73.27,₇₀=20log₁₀-73.56,₁₂₀=20log₁₀-74.18 and₂₀₀=20log₁₀-73.46,respectively.Variation inrange at 0 300 m depth was about 10 d B.The broadband target strength spectrum shows resonance frequency decreases as swimbladder length（）increases.of individuals with differentshowed two trends after 150 k Hz.of fish with>3（88）increased as frequency increasing,while theof fish with<3（88）maintained stability.（3）A total catch of 4442.83 kg was obtained during the marine survey in 2021,and the proportion of S.japonicus accounted for 24.90%.The results of acoustic assessment show that the total（Nautical area scattering coefficient,m²/nmi²）was 1962756.12m²/nmi² and theof S.japonicus was 488647.13 m²/nmi².The S.japonicus density was 0 5 t/nmi² in most areas of the investigated sea area,and the average was 20.07t/nmi².The highest density reached 674.89 t/nmi²,which located near 148°E,39°N,and its correspondingvalue was 2881.77 m²/nmi².S.japonicusdecreased with the increase of water depth,and the maximum value was 46.48 m²/nmi² in the 10 50 m water layer,indicating that the S.japonicus mostly gathered in the shallow layer above100 m.