Age,growth And Quantity Distribution Of Nemipterus Virgatus In The Northern Coast Of The South China Sea

Nemipterus virgatus(N.virgatus)is a warm-temperature near-bottom fish and mainly distributed in the western Pacific Sea area.In China,it mainly distributed in the South China Sea,followed by the East China Sea.It is one of the main catches of the bottom trawl in the northerm part of South China Sea.The fish has a solid,and delicious meat with high economic value.Also,it is one of the important economic fish species in the northern part of the South China Sea.N.virgatus has the characteristics of early sexual maturity and batch spawning,and can withstand certain fishing pressure.With the increase of fishing intensity of offshore fishery resources in the northern part of the South China Sea,N.virgatus resources are significantly lower than those in the 60s of the last century,and the declining trend of N.virgatus resource has been shown.In the present study,we carried out relevant research based on the samples and survey data of N.virgatus collected in the northern part of South China Sea resource Bottom trawl survey during July,2014 to May,2015,aiming to keep abreast of resource changes in N.virgatus and provide basic data for its rational use and management.The main research contents are as follows:1)Analyzing the morphological characteristics of the otolith by measuring the length,width,perimeter and other morphological parameters of otolith.2)Establishing body length and body weight's growth equation and the relationship between body length and body weight of N.virgatus according to the age from otolith reading,and analyzing its growth characteristics.3)By taking the Beibu Gulf as an example,analyzing the quantitative distribution characteristics of N.virgatus,and preliminarily examining the resource changes and causes of gold-line fish in combination with historical data.The main findings are described as follows:(1)The morphological characteristics of the otolith of N.virgatus.Results showed that there was no significant difference in those otolith morphological characteristics between female and male(P>0.05).There were significant differences in the otolith width,area and the ratio of length-width between left and right otolith(P<0.05),and no significant difference in the otolith length,maximum radius,minimum radius,perimeter,area of minimum circumscribed rectangle and the ratio of radius between left otolith and right otolith(P>0.05).Based on principal component analysis,it is clear that otolith length,width,maximum radius,minimum radius,perimeter,area,area of minimum circumscribed rectangle and the ratio of radius were the main morphological indexes of N.virgatus otolith.The relationships between main otolith morphological characteristics and body length and body weight were fitted optimally by quadratic polynomial function and the correlation coefficient of the length fitting equation was higher than that of the body weight.(2)Age and growth of N.virgatus.The results showed that the relationship between body length and body weight of the N.virgatus samples was W=5*10-5L2.8668(R2=0.93 89),and the t test indicated that there was a significant difference between b value and 3,which could be regarded as negative growth.The daily age range of N.virgatus samples was from 104d to 476d,and the hatching peak months in 2014 were February,March and May.The optimum growth equation for body length was the Von Bertalanffy model,the relational formula was:L=285*[1-e-0.410(t+0.888)],and the optimum growth equation of body weight was:W=547.85*[1-e-0.410(t+0.888)]2.8668.It is calculated that the growth performance index ?' was 2.522.The growth rate of body length was decreased with the increase of age,and the acceleration of body length was increased with the increase of age,both of which were approaching 0.The growth rate of body weight was increased with age,the growth rate reached the maximum when it reached the inflection point,and then was decreased with the increase of age.The inflection point was 614 days,the corresponding length was 186 mm,and the body weight was 157 g.The acceleration of body weight was decreased with the increase of age.(3)Quantity distribution and dynamics of N.virgatus in the Beibu Gulf.The results showed that the N.virgatus in the Beibu Gulf mainly appeared in the 40?100 m water depth,among which the frequency of 80?100 m water depth was higher than that of the other water layers.There were both seasonal and spatial changes in the quantity distribution of N.virgatus in the Beibu Gulf.The stock density in autumn was lower than those in the other three seasons(Spring:1.33 kg/km2;Summer:1.34 kg/km2;Autumn:0.72 kg/km2;and Winter:2.32 kg/km2),and the high value area of stock density mainly appeared in the middle of the Gulf,which was related to its spawning habits.Stepwise regression analysis revealed that water depth,bottom temperature and bottom salinity were the main causes affecting the distribution of N.virgatus stock density,but there were seasonal differences in influencing factors.Compared with the historical survey data,it was found that the stock density of N.virgatus was fluctuating greatly over 50 years,and the overall trend has been decreasing.Thus,it is urgent to strengthen the protection measures.