Biological Characteristics And Resource Assessment Of Sardinops Melanostictus In The Northwest Pacific Ocean

The Northwest Pacific Ocean is very rich in fishery resources due to the convergence of the Thousand Islands Cold Current and the Kuroshio Current,which provides good habitat and bait organisms for marine organisms（especially fish）.Japanese sardine is one of the main economic fish species caught in purse seine and trawl fisheries in countries around the Northwest Pacific Ocean,and its resources are very abundant.However,due to the large fluctuation of its stock resources and intermittent related fisheries,relatively few studies on the biological characteristics of the fishery,fishery characteristics and resource status of this species have been reported at home and abroad,and studies on the assessment and prediction of its resource status are even rarer.Japanese sardine has been listed as one of the priority species for assessment by The North Pacific Fisheries Commission（NPFC）in 2022.Therefore,it is necessary to understand the basic biological characteristics of Japanese sardine in the Northwest Pacific Ocean in time,establish its age identification method,make a reasonable assessment of its stock status and suggest corresponding management recommendations.In this paper,we conducted a preliminary study on population structure,gonadal maturity,initial sexual maturity fork length,feeding class and fatness,age identification methods,stock status,and resource management of Japanese sardine in the Northwest Pacific based on samples collected in the high seas purse seine fishery in April-December 2020,April-November 2021,and April-November 2022（a total of4,264 fish）and historical production data from 1990 to 2022,with the following results:1.Biological characteristics of the population:the fork length of the Japanese sardine population from 2020 to 2022 ranged from 85 to 240 mm,the dominant fork length group was 160 to 200 mm,and the fork length of individuals of different sexes did not differ significantly（P>0.05）;body mass ranged from 5.6 to 144.6 g;the dominant body mass group was 35 to 75 g;the body mass of individuals of different sexes did not differ significantly（P>0.05）;age composition ranged from 0⁺to 6⁺;the dominant age group was 1⁺to 3⁺.The relationship between fork length and body mass was in accordance with the power function exponential equation,and the growth parameter b=3.1538,which was consistent with positive anisotropic growth.Sexual maturity was dominated by stage Ⅱ（more than 90%）,and the difference in gonadal maturity between 95～104 mm and 105～114 mm in the fork length group was highly significant（P<0.01）,and the difference in sexual maturity between 205～224 mm and225～245 mm in the fork length group was significant（P<0.05）.feeding intensity was dominated by 0～2 The feeding intensity was mainly 0～2,and the difference in feeding intensity between fork length groups was significant（P<0.05）.The range of fertility coefficient was from 0.82 to 1.42,with fertility increasing and then decreasing with fork length,and the highest fertility coefficient was found in the fork length group of135～164 mm;the fertility coefficient varied among different months,and the highest value of 1.25 was reached in October.2.Age identification method construction:Firstly,the biological data such as fork length,body weight and otolith weight of 1448 Japanese sardine samples were measured,then their sagittal otoliths were embedded,ground and photographed,and their exact ages were read by whorl observation method,and three age identification methods based on one-dimensional linear regression equation,multiple linear regression equation and deep learning model were constructed by combining the biological data,and the process of age identification method construction and accuracy improvement of Japanese sardine was elaborated.Analysis of the R² coefficients of the one-dimensional linear regression equations fitted by individual biological characteristics showed that the otolith weight had the highest correlation coefficient（R²=0.8235）with the age results obtained by the whorl method of observation.The accuracy of age identification by otolith weight fitted to the one-dimensional linear regression equation and the multiple regression linear equation with the three deep learning models was 43.6%,54.0%and71.6%,respectively.Therefore,the age identification by the deep learning model was identified as a new method for age identification of Janpanese sardines.This method will significantly improve the accuracy and efficiency of age identification of Japanese sardine.3.Stock resource assessment model and management recommendations:The Catch-MSY model and the length-based Bayesian biomass estimation method（LBB）were used to assess the maximum sustainable yield（MSY）and the status of the stock,and the stock management strategy was also analyzed.The results showed that（1）the Catch-MSY model assessed its MSY as 170×10⁴t with a confidence interval of 134.1×10⁴t～211.4×10⁴t;（2）the LBB model assessed the results of biological parameters for three years（2020,2021,2022）as L_c/L_copt（0.76～0.95）,B/B_MSY（0.79～0.84）,and B/B₀（1.9～2.3）,indicating that the Japanese sardine resource is in good condition and is in the recovery period of the resource;（3）Combining the results of the two models with the KobeⅡmanagement strategy matrix for future（3）Combining the results of both models with the Kobe Ⅱ management strategy matrix to predict the probability value of the resource status,for the medium-term phase（5-10 years）its management target range should be 60%TAC to MSY value i.e.102×10⁴ t～170×10⁴t as TAC,and in the long-term,the TAC value can be gradually increased to between 120%TAC to MSY value i.e.122.40×10⁴t～170.00×10⁴t.This management strategy can be used to sustainably utilize the Japanese sardine fishery resources in a longer time scale.This study enriches the information on the biological characteristics of Japanese sardine populations in the Northwest Pacific Ocean,provides new methods and ideas for age identification of pelagic fish,and may provide reasonable suggestions for stock resource management of Japanese sardine in the Northwest Pacific Ocean.