| The South China Sea creates conditions for the breeding of various and abundant species due to the unique geographical environment.The central and southern South China Sea is rich in nekton resource with broad application prospect.However,the information about the species composition of nekton and the biological and ecological charactistics of the dominant species is relatively limited.For this propose,the study systematically collates the species composition of nekton in the central and southern South China Sea based on the data obtained from the survey conducted in the autumn of 2012 and spring of 2013.On this basis,biomass and abundance distribution of nekton,dominant species composition,diversity characteristics and their seasonal variations are analyzed.In addition,relationship between the spatial and temporal distribution of dominant species of nekton and the environment is preliminarily discussed in this study.The findings include:(1)A total of 125 species(including 6 undetermined species)of nekton were captured in two surveys,belonging to 97 genera,51 families and 14 orders.Among them,118 fish species belonging to 91 genera,46 families and 12 orders(including 2species of cartilaginous fishes belonging to 2 genera,2 families and 2 orders,and116 species of bony fishes belonging to 89 genera,44 families and 10 orders)accounted for 94.40%of the total speices of nekton.There were 7 species of cephalopods,belonging to 6 genera,5 families and 2 orders,accounting for 5.60%.Among all the fishes,Perciformes,Tetraodontiformes and Beloniformes are the most numerous.There was little difference in the species number of nekton between two seasons.In autumn,86 species of nekton were caught,including 82 species of fishes and 4 species of cephalopods.There were 85 species of nekton caught in spring,including 80 species of fishes and 5 species of cephalopods.Among them,43 species of fish and 2 species of cephalopods were caught both in autumn and spring,including Auxis thazard,Priacanthus macracanthus,Navodon tessellates,Symplectoteuthis oualaniensis and Thysanoteuthis rhombus.The rate of seasonal change of nekton species was 64.29%,which indicated that there were obvious seasonal variations of nekton species in the study area.(2)The mean biomass of nekton were 95.46 kg/h in autumn and 174.89 kg/h in spring,and the mean abandunce were 748 ind./h and 2234 ind./h,respectively.The mean biomass and abandance in spring were higher than those in autumn.The biomass and abandance distribution of nekton in two seasons was slightly different.In autumn,the capture rate of the stations to the north of 12°00?N was higher than those to the south of 12°00?N.Besides,the highest capture rates occurred at the sections of 13°00?N and 15°00?N.In spring,there was no obvious latitude trend in the distribution among stations.The upwelling and the reef effects in Xisha and Zhongsha Islands as well as the effective implementation of midsummer fishery management may be the reasons for the difference in the distribution of nekton in autumn.(3)The dominant species of nekton in the study area were concentrated,and there was a slight difference between two seasons.Rare species accounted for the largest proportion of all species.The dominant species in spring included S.oualaniensis,A.rochei and Decapterus macarellus,the remaining species were common and rare species.The dominant species in autumn were only S.oualaniensis and Katsuwonus pelamis,the remainings with 6 common species,7general species and the rest of rare species.S.oualaniensis accounted for absolute advantage according to the capture groups,while Scombridae and Decapterus were also in the front row.Most of the rare species in two seasons were unique species in autumn or spring,while the species with higher IRI values appeared in both seasons,indicating that the species with higher IRI values maintained the community stability.(4)The mean value of richness index(D),diversity index(H')and evenness index(J')of nekton in the study area was 1.39,0.38 and 0.17 respectively.In general,the index values in autumn of 2012 were higher than those in spring of 2013.The index were various in different latitudes,mainly around the 12°00?N section as the boundary line,which generally presented a trend of higher in the north and lower in the south.It is speculated that the difference of geomorphology may be one of the reasons that diversity in the sea area was higher in the north and lower in the south.Average taxonomic distinctness(?~+)of fish in the study area was the closest to that of Xisha and Zhongsha islands,probably because the stations with more catches were located near Xisha and Zhongsha islands,where made them have similar species composition.Compared to the nearby continental shelf areas,average taxonomic distinctness(?~+)of fish in the study area was lower.The relatively little variation of water temperature and salinity gradient may lead to the close morphology and genetic relationship of the fish species inhabiting and distributing in this area.(5)S.oualaniensis presented high frequency of occurrence in different seasons,indicating that the species was widely distributed in the sea area.In spring,the biomass and abundance of each station were generally higher than those of autumn.One of the reasons may be death of individuals after breeding in autumn and formation of recruitments in spring.S.oualaniensis with high capture of rate in autumn mainly concentrated between 110°00?E?114°00?E and the north of 11°00?N,showing a trend of north higher and south lower.In spring,each station was relatively uniform in the capture rate,and the distribution tended to move southward.GAM results show that the spatial effect has a great influence on the population distribution of S.oualaniensis,while other hydrologic environmental factors have little effect.It may be due to the presence of other potential hydrologic environmental factors not included in the model.The biomass and abundance of K.pelamis were not evenly distributed in the survey area and were obviously various with the seasons.The species mainly distributed between 112°00?E?114°00?E and north of 11°00?N in autumn,and almost not captured to the south of 11°00?N.K.pelamis with scattered distributions appeared at stations to the south of 11°00?N during the spring survey,indicating that the species migrated from north to south in different seasons.The species with high capture rate appeared in the sections of 13°00?N and 15°00?N in both seasons.GAM fitting results showed that the population distribution is influenced by the factor Month to a certain extent.Meanwhile the species was distributed in the salinity range of 33.0?34.0 and the temperature range of 28.5??29.5?.According to the survey results,A.rochei was scatteredly distributed in the survey area,and the capture rate was low in both seasons except for several stations.The environmental factors including sea water temperature,Chlorophyll-a concentration and longtitude had an effect on the population distribution.The species was distributed in the temperature about 29.0?and Chlorophyll-a concentration of0.10mg/m~3.Furthermore,it showed a trend of decreasing with the increase of longitude.The population distribution of D.macarellus in the study area was significantly different from season to season.The species achieved a magnitude growth of the biomass and abundance.Meanwhile,D.macarellus migrated from 15°00?N in the north of the survey area to 5°00?N in the south.The results of GAM showed that latitude had a great influence on the distribution of D.macarellus in the sea area.According to the study,the water temperature of the whole survey area was roughly the same in autumn.The distribution of the above species was mostly concentrated in the area near 13°00?N?15°00?N,where was close to the upwelling area with rich nutrients.Upon completion of the preparation for migration,under the stimulation of external conditions,the above species were concentrated in the waters with higher water temperature in the southern low latitudes by migration in spring. |
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