The Composition Of Nekton And Biological Characteristic Of Main Dominant Species In Northwestern Pacific Ocean

Based on the data from light-purse seine nekton resource survey with two cruises?2015-2016?in Northwestern Pacific Ocean,and combined with related temperature data,the species composition,biomass and abundance distribution,dominant species composition and diversity characteristics of pelagic nekton of Northwestern Pacific Ocean were described;In addition,the distribution,biological characteristics of main dominant species and economic species were also discussed,and the relationship between the catch of Scomber japonicus and surface temperature as well as the vertical temperature structure was analyzed emphatically.The main research results are as followings:1?155 nekton species belonging to 112 genera,58 families and 24 orders,were identified in this survey including 132 fish species with 92 genera,47 families and 22 orders,23 cephalopoda species with 20 genera,11 families and 2 orders.The number of species in the south investigating area was higher than that in the north area.The seasonal variations of species in number followed the order of spring?119?>summer?70?.During the survey,the proportion of catch appeared in the both of the two seasons was small,with a total of 34 species,accounting for 21.94%;the proportion of catch which occurred in only one season was higher,with a total of 121 species,accounting for 78.06%.2?The mean biomass of nekton in the investigating area was 707.96 kg/h.In general,the high-value area located to the investigating area of 40°⁴3°N,and the low-value area located to 35°³7°N.The abundance distribution is not the same as biomass distribution,the mean abundance was 6083 ind./h.The highest area was in at40°N and the lowest area was in at 36°N.The seasonal variations of biomass in weight follows the order of spring > summer ranging from 611.62 kg/h to 804.31kg/h.The seasonal variation of abundance follows the order of spring > summer ranging from 6113 ind./h to 6053 ind./h.3?The survey results showed that the dominant species was S.japonicus both in spring and summer.Analysis of the diversity showed that Margalef index?D?,Shannon-Wiener index?H'?and Pielou index?J'?changed in a large range at all stations,but the biodiversity showed that it was higher south than north,the biodiversity index of most stations was higher in spring than in summer,except for a few stations.Average of Margalef index?D?,Shannon-Wiener index?H'?and Pielou index?J'?was higher in spring than in summer.4?Samples of S.japonicus,Symbolophorus evermanni and so on captured during spring,the biomass and abundance distribution of which were analyzed emphatically,the results showed that S.japonicus was mainly distributed in the waters of 40°N,its length ranged from 44³90 mm,the average was 217.8 mm,its weight ranged from0.8⁶50.9 g,the average was 149.4 g;S.evermanni was distributed widely,which was captured in 39 stations and catch rate was higher in northern ocean,its length ranged from 23¹51 mm,the average is 96.3 mm,its weight ranged from 0.6¹09.0 g,the average is 11.1 g.Samples of S.japonicus,Sardinops sagax and so on captured during summer,the biomass and abundance distribution of which were analyzed emphatically,the results showed that S.japonicus was mainly distributed in the waters of 41°N,which was captured in 42 stations,its length ranged from 100⁴01 mm,the average was 243.9 mm,its weight ranged from 3¹000 g,the average was 214.5 g;S.sagax was captured in 19 stations,which was mainly distributed in 41°N,its length ranged from 81²56 mm,the average was 182.6 mm,its weight ranged from 2.4²05g,the average was 97.5 g.5?Based on catch data from a light-purse seine fishing vessel and temperature data in the Northwestern Pacific Ocean,the relationships between catch per net of S.japonicus,surface temperature and vertical temperature structure were studied.The results indicated that spring was the best fishing period for S.japonicus;the seasonal variation in the fishing grounds of S.japonicus was not so obvious,the catch was mainly concentrated in the waters near 40°N,151°¹53°E in spring,and in the waters near 41°,154°¹60°E in summer.The suitable sea surface temperature?SST?in the fishing grounds was 7¹9 ? in spring and 8²4 ? in summer,the optimum SST of S.japonicus was different,which was 11¹5 ? in spring and8¹2 ? in summer.The temperature gradient in fishing ground was different between spring and summer,the temperature gradient from 50 m to 100 m was analyzed,the catch per net of S.japonicus in the class interval of 0.10⁰.15 ?/m was highest in spring,and the others was lower significantly than it.However,the catch per net of S.Japonicus decreased with the increase in the temperature gradient from 50 m to 100 m in summer,and the catch per net in the class interval of-0.05⁰.00 ?/m was higher than the others.The degree of gray incidence?0.991?indicated that the temperature gradient form 50 m to 100 m was the greatest factor affecting the catch of S.japonicus,followed by the latitude,longitude and T₅₀.