The Tidal Variation Of The Fish Larvae Community In The South Branch Of Yangtze River Estuary

Based on the fish larvae community in the south branch of Yangtze River Estuary,we mainly discuss the species composition,spatial and temporal distribution,the relation between amount changes and environmental factors,try to illustrate the tidal variation of fish larvae in the south branch of Yangtze River Estuary and vertical distribution of fish larvae community.All above will provide an important scientific foundation for the evaluation and protection of early fish resources.Since March 2015 to February 2016,8 stations were set in the south branch of Yangtze River Estuary,and level trawls were performed almost 10 minutes at each station by using a large fish larvae net,the flow number was recorded after each trawling.The sampling process was carried out in two days,while St.6-St.8 were performed flood tide sampling after sunrise and ebb tide sampling before sunset simultaneously,St.7 was performed extraordinary middle layer trawl,St.8 was performed fixed trawling during the flood and ebb tide in the night.All samples will be fixed in 5 % formalin solution immediately,and the sorted fish larvae specimen will be kept in 80% ethanol solution.171 trawls were carried out in the whole year survey and over 74689 individuals were collected,belong to 39 species of 12 families.The Cyprinidae is the dominant family(8 species’ abundance is 79.12%),follows by 8 species of Gobiidae(14.9%),3 species of Engraulidae(0.75%),3 species of Osmeridae(4.86%),8 species belong to different families respectively,and 3 unknown species.Freshwater species were 17,estuarine species were 6,while anadromous species,marine species and marineestuary dependent species were 2 respectively.The preflexion stage is the dominant stage(84.96%),follows by flexion stage(9.98%)and postflextion stage(4.20%).The main six dominant species is Hemiculter bleekeri(53.4%),Rhinogobius giurinus(12.65%),Culter alburnus(8.38%),Parabramis pekinensis(7.18%),Mylopharyngodon piceus(6.55%)and Salanx ariakensis(4.63%).In the throughout survey,no catches were observed during January to March,the richness was higher during May to September,37 species were collected at flood tide and 24 species were collected at ebb tide.The amount changes of fish larvae display a distinct seasonal variation,the amount and density is higher in spring and summer,while no catches in winter.Two peak reproduction period exist,one is spring period including April to May,the main dominant species is Neosalanx tangkahkeii,another peak period is summer period including July to September,the main dominant species is Hemiculter bleekeri.Based on the diversity index,the richness is higher during May to September,the evenness is higher during May to October,the K-dominance curve is lowest in May and the diversity is best in May.Based on the whole year density distribution of fish larvae,basically,no catches were observed in the lower section of the south branch in April,while no catches were observed in the upper section of the south branch in December.The density of fish larvae in November is higher than the adjacent two months.Based on the density distribution at St.6 to St.8 during flood and ebb tide,the density is higher at flood tide than the ebb tide,but the diversity is lower than the ebb tide because of the extremely high abundance of some rarely dominant species at flood tide.Salanx ariakensis mainly occurred in November,as the distribution was relatively wide at flood tide and relatively intensive at St.8 at ebb tide.Furthermore,the density of Salanx ariakensis larvae collected at St.8 is higher in the nighttime than the daytime.The middle layer trawl is tentatively performed at St.7.During the flood tide,19 species were collected in the surface layer with a good diversity(H’=1.771,E =0.210),16 species were collected in the middle layer with a good diversity(H’=1.504,E=0.261).During the ebb tide,14 species were collected in the surface layer with a poor diversity(H’=0.863,E=0.261),9species were collected in the middle layer with a poor diversity(H’=0.596,E=0.071).In the whole year survey,Cynoglossus abbreviates,Lophiogobius ocellicauda and Odontamblyopus lacepedii were only occurred in the middle layer at St.7.Besides,Culter alburnus’ s abundance at flood tide is higher in the surface layer,while the abundance at ebb tide is higher in the middle layer.Thus interspecific differences are existing in the vertical distribution of different fish larvae.After a long period of evolution,Yangtze River Estuary form three branches and four mouths into the sea.The south branch locates at a very strategic geographical location linked the river to the sea,with a complex aquatic environment affected by tidal variation and landform limitation.The strong runoff brings in massive freshwater,which can breed some economical species like Coilia nasus and Salanx ariakensis.With the increasing depletion of fishery resource in Yangtze River Estuary,the amount changes of fish larvae and the study of drifting mechanism is becoming more and more important.This kind of study not only contribute to the protection of the ecological and biological diversity,but also contribute to the sustainable utilization and development of fishery resource.