| Small yellow croaker(Larimichthys polyactis)is an important economic fish species in China's coastal waters.Since the 1970 s,due to overfishing and water quality and environmental pollution,there has been a significant decline in resources.Under the conservation of the fishing moratorium policy,the resource quantity has recovered since the 1990 s.Compared with the long-term decline of the resources of large yellow croaker,the biological characteristics of small yellow croaker have a stronger ability to adapt to the habitat environment,which ensures the relatively high resource yield of small yellow croaker.In addition,threatened by intensive fishing activities,the resources of small yellow croaker are still in a precarious situation.Therefore,it is of great practical significance to study the biological characteristics and resource status of yellow croaker.Based on this study in May and August,November 2017 and March 2018 in offshore waters of southern Zhejiang(120.93 ° E ? 122.95 ° E,27.21 ° N ? 28.97 ° N)at the bottom trawling resources survey,mainly for the preliminary study of growth,death and feeding,trophic level characteristics of small yellow croaker,and based on catches unit transshipment model to evaluate the population resource condition of small yellow croaker,mainly use in the study of body length frequency characteristics,preliminary data for the growth of small yellow croaker and death and based on the evaluation of resource utilization;Fatty acid analysis method and stable isotope technique were used to analyze the characteristics of feeding sources and nutrient levels of the yellow croaker and the changes with the development of individuals,so as to explore the internal biological characteristics of the yellow croaker.The research contents and results of this paper are mainly as follows:(1)According to the biological data of 1,404 small yellow croaker collected in the offshore waters of southern zhejiang during four voyages from May 2017 to March 2018,the research results are as follows: the body length and weight range of small yellow croaker are 31mm-312 mm and 0.8g-182.2g respectively,the ratio between males and females is 1:1.29,and the body length and weight relationship is: W = 0.0214 L 2.8707(R2 = 0.9019).By ELEFAN ? method to estimate the small yellow croaker up growth parameters L,K,t0 346.5 mm,0.22 and 0.651 respectively.A,weight growth inflection point age through calculation of 4.14 a,inflection point of the body length of 225.67 mm;The total mortality coefficient was estimated to be 3.09,the natural mortality coefficient was 0.28,and the fishing mortality coefficient was 2.81.The development rate was 0.91,indicating that the yellow croaker was suffering from high fishing intensity.The average body length and dominant body length of the small yellow croaker in the offshore waters of southern zhejiang decreased,the size of the catch became smaller and the age of the fish became younger.(2)Fatty acid labeling method was used to study the differences in fatty acid composition and dietary habits among different body length groups of small yellow croaker in the offshore waters of southern Zhejiang.The results showed that there were 30 kinds of fatty acids in the muscle of small yellow croaker,including 10 kinds of saturated fatty acids,9 kinds of monounsaturated fatty acids and 11 kinds of polyunsaturated fatty acids,accounting for 39.80%,33.80% and 26.40% of the total fatty acids,respectively.The content of SFA was the highest in the < 100 mm body length group,the content of MUFA was the highest in the 140-159 mm body length group,and the content of PUFA was the highest in the < 100 mm body length group.The content of n-3 series fatty acids in each body length group was higher than that in the n-6 series,and all of them showed a trend of decreasing with the increase of body length.The contents of fatty acids in spring,summer and autumn were all SFA > MUFA > PUFA,and the content of SFA in winter was still the highest,but the content of MUFA was lower than that of PUFA.The content of n-3 series of fatty acids was the lowest in winter,while the content of n-6 series was the highest in winter.No significant seasonal difference was observed between the two series(P >0.05).The content of characteristic fatty acids was generally higher in the body length group of 140-159 mm in autumn,and lower in the body length group of > 180 mm in winter.According to the analysis of biomarker fatty acids,the yellow croaker should mainly feed on zooplankton,benthic animals and swimming animals as a mixed feeding species,among which the zooplankton may be copepods.(3)Based on the analysis of the trophic level characteristics and the variation trend with individual development of small yellow crock in the offshore waters of southern zhejiang,the results showed that the carbon stable isotope variation range of small yellow crock was-19.04 ‰ ?-14.93 ‰,with an average value of-(16.76 ± 0.98)‰,and the nitrogen stable isotope variation range was 6.94 ‰ ? 12.06 ‰,with an average value of(10.53 ± 0.89)‰.With the increase of body length,the carbon stable isotope value did not change significantly,while the nitrogen stable isotope value showed an increasing trend,and the nutrient level also increased with the increase of body length,showing a significant positive correlation with body length(P < 0.05).The average nutrient level of the yellow croaker was 2.62,the highest in winter was 2.75,and the lowest in autumn was 2.48.When the body length increased from 89 mm to 120 mm,the feeding habits of the small yellow croaker in the offshore waters of southern zhejiang changed.The feeding habits did not change significantly when the body length was larger than 121 mm. |
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