| The Northwest Pacific is one of the most important fishing grounds in the world,and its fishing output ranks among the top in the world.However,due to the changeable weather conditions and the particularity of the geographical location of the Northwest Pacific,the research and development of the Northwest Pacific in China is relatively late,and the relevant basic research is relatively weak compared with other countries.In particular,there is still little research on the composition,quantity distribution,diversity and dominant species of the nekton in the sea area.In order to have a more effective and comprehensive understanding of the fishery resources,this paper analyzed the data of the light falling net survey conducted in the Northwest Pacific Ocean from August 23 to September 26,2018,combined with the relevant water temperature data.The species composition and diversity characteristics of the nekton in the survey area were identified and analyzed,the species composition and quantity distribution of the dominant species of the nekton were analyzed.The relationship between population distribution and environmental factors of Sardinops melanostictus,the dominant species,was analyzed.In addition,the advantages and disadvantages of the traditional light falling net method and the environmental DNA metabarcoding technology in the study of fish diversity were discussed.The main research results are as follows:1.A total of 44 species,belonging to 36 genera,20 families and 9 orders,were identified by the light falling net.Among them,Chondrichthyes include 2 orders,2families,2 genera,2 species,Actinopterygii,6 orders,16 families,30 genera,38 species,and Cephalopoda,1 order,2 families,4 genera,4 species.At the order level,Perciformes had the largest number of species(25 species),followed by Myctophiformes(5 species)and Beloniforms(4 species).2.The average catch rates at all stations were 1126.64 kg/h and 41727.11 ind./h,of which the average catch rates for fish were 1111.81 kg/h and 41538.10 ind./h,and for cephalopod were 14.83 kg/h and 189.01 ind./h.The biomass of fish accounted for98.25% of the total catch,and the tail number of fish accounted for about 91.44%.Cephalopod biomass accounted for 1.75 % and mantissa accounted for 8.56 %.3.The mean value of Margalef index(D)was 0.36,which ranged from 0.00 to1.08.The average value of Pielou index(J ’)is 0.48,and the range is 0.00 ~ 0.97.The average Shannon-Wiener(H ’)was 0.70,and the range was 0.00 ~ 1.74.From the aspect of section,D is the highest at section 37°N,H ’ is the highest at section 37°N,and the highest value of J’ appears at section 39°N.The lowest values of D,J ’and H’ all appear at the 35°N.4.In autumn,Sardinops melanostictus were mainly distributed in the 43°N,and their resource distribution was closely related to environmental factors.In this survey,only 11 stations collected samples of Sardinops melanostictus,most of which were concentrated in the northern section of the surveyed sea area.The results showed that there was a significant relationship between chlorophyll a and temperature at 50 m in this season.5.A total of 20 species of fish,belonging to 13 genera,7 families and 4 orders,were detected in 36 samples from 12 environmental DNA sites by using common fish primers.Thirteen species of fish,belonging to 13 genera,7 families and 4 orders,were identified by the method of light falling net survey.Perciformes were the most common species detected in both survey methods.6.According to the results of environmental DNA sampling and light falling net survey in the Northwest Pacific,the results show that environmental DNA metabarcoding can reveal more groups at all classification levels.There is no significant correlation between the abundance of environmental DNA sequencing fragments and the occurrence rate(r=0.742,P=0.056)and catch(R2=0.052,P=0.625)of corresponding species in light falling net.This result shows that we can not reasonably infer the occurrence rate and biomass of corresponding species only from the abundance of environmental DNA sequencing.In addition,environmental DNA in seawater is influenced by physical factors such as temperature,salinity and current flow,so relatively accurate information on biomass and location is not available. |
