Population length structure analysis of Oratosquilla oratoria in the marine area of Shandong based on finite mixture model

Individual length frequency is an important structural feature of fishery resource population. Generally, analytical indicators such as average individual body length or average body weight are used in studies, which is difficult to fully reflect the population composition differences with multiple age structures and different individual growth rate. These study analyzed the temporal and spatial variation of individual size composition of Oratosquilla oratoria, according to the bottom trawl survey data collected in October 2016 and January, May and August 2017 in the marine area of Shandong, using the Finite mixture model to analyzed the length frequency distribution characteristics of individual size of O. oratoria with different time and depths. The research shows that, the O. oratoria in the marine area of Shandong can be distinguished as high and low age cohorts except autumn, and there is a certain difference in the distribution of male and female body length. The proportion of female O. oratoria in the low age cohorts decreased first and then increased from winter to summer, while the proportion of male O. oratoria decreased gradually. In terms of different depths, the individuals in the low age cohorts are mainly concentrated in the offshore areas, and the proportion decreases gradually from offshore to the open sea. The distribution of age cohorts showed seasonal differences. In spring, the proportion of O. oratoria in high age cohort was the largest in the offshore area, and the proportion gradually decreased from offshore to open sea. The percentage of O. oratoria in the high age cohort in summer was the highest in water depth of 20-30 meters. The percentage of O. oratoria in the high age cohort in winter increased gradually from offshore to open sea. This paper preliminarily analyzed the temporal and spatial distribution of O. oratoria in different age cohorts in the marine area of Shandong, and the research results are of great significance for in-depth understanding of population distribution dynamics and scientific fishery management.Previous studies on the size composition of O. oratoria were only grouped according to the size of body length, ignoring the differences in the growth rate among individuals of different O. oratoria, and there might be overlap of body length frequency among different age groups. Therefore, a Finite Mixture Model was used in this study to divide the body length frequency of O. oratoria into subgroups of different sources (different age groups) according to the statistical test results. This is a new exploration of the distribution of O. oratoria resources. In future studies, the proportion of different age cohort obtained by the Finite Mixture Model can be used to infer the spawning area of O. oratoria. Meanwhile, the distribution of different age cohorts of O. oratoria can be studied in depth by combining Lipofuscin analysis. It is of great significance to fully understand the temporal and spatial changes of O. oratoria distribution, and is helpful for the scientific development and management of O. oratoria resources.