Study On The Vertical Distribution And Changes During Day And Night Of Coilia Nasus Larvae And Juveniles Depending On Flood And Ebb Tide In The Southern Branch Of Yangtze River Estuary

In order to clarify the vertical distribution and changes during day and night of Coilia nasus larvae and juveniles depending on flood tide and ebb tide in the southern branch of Yangtze River estuary.Distribution characteristics of Coilia nasus larvae and juveniles in the surface and middle layers were studied.The relationship between the juveniles of Coilia nasus larvae and juveniles and the environmental factors was discussed.Further research on the distribution of Coilia nasus larvae and juveniles at flood tide and ebb tide,during day and night,in the surface and middle layers.Providing scientific basis for the sustainable use of fishery resources in the Yangtze River estuary.According to the tidal changes,monthly 4 flood trawls and 4 ebb trawls were set at the spring tides at two stations(St.1:N31°35.500′,E121°20.004′;St.2:N31°33.500′,E121°20.000′)(surface layer and middle layer)from May of 2017 to April of 2018 in this study by using a larval net(1.3m mouth-diameter,0.5mm mesh-aperture)againsting the current at every station.Which fastened by a flow meter on the mouth.fixed in a fishing-boat for 10 minutes,the trawling speed of the boat were 2 sea mile/hour.Specimen were fixed in 5% formalin solution,and measured the water temperature,dissolved oxygen,PH,and salinity at the same time.In this study,the salinity and temperature were analyzed for the correlation between Coilia nasus larvae and the juveniles.Using the software IBM SPSS Statistics21,the average water temperature of each month was correlated with the CPUE of surface layer and the middle layer of the scorpionfish in each month.The average water temperature and CPUE showed significant differences(P<0.01).In the middle of the month,the CPUE of the scorpionfish was analyzed,and the salinity was not significantly different from the CPUE(P>0.05).The number of preflexion and flexion of Coilia nasus during June and July is large because of temperature change at the same time.During the survey from May 2017 to April 2018,379 of Coilia nasus larvae and juveniles were collected,the number of preflexion larvae is 253(66.75%)and the number of flexion larvae is 58(15.30%).the number of postflexion larvae is 55(14.51%)and the number of juveniles is 13(3.43%),and there were relatively more preflexion larvae and flexion larvae.According to the data analysis,the average density of Coilia nasus larvae and juveniles in the surface layer is greater than the average density in the middle layer.In the comparison of surface data,the average density was the highest in June,followed by July,and the other months were relatively small.In the middle data comparison,the average density of Coilia nasus larvae and juveniles was the highest in July,followed by June,and other months were relatively small.In the surface at the St.1,the average density of Coilia nasus larvae and juveniles at the flood tide is greater than the ebb tide in each month.the average density of Coilia nasus in the surface layer is greater than the middle layer,and the average density during the day is greater than the average density at night.The density of Coilia nasus was large at flood tide at night in August.The result in the surface layer and middle layer at St.1 are different.The difference is that the density of Coilia nasus larvae and juveniles during the flood tide at daytime high tide in July is greater than other months.In the surface at the St.2,the average density of Coilia nasus larvae and juveniles at the flood tide is greater than the ebb tide in each month.the average density of Coilia nasus in the surface layer is greater than the middle layer,and the average density during the day is greater than the average density at night.In the flood tide,the average density in June was the highest,followed by July,and the other months were relatively small.These results were consistent with the surface results of St.1,and the total data was also consistent.The density of Coilia nasus larvae and juveniles was higher Coilia nasus at ebb tide during the nighttime in August.In the St.2 station,the density of Coilia nasus larvae of the juveniles was higher at flood than that ebb tide.The density of Coilia nasus larvae and juveniles was the highest at flood tide during the daytime in July,followed by June,the density of Coilia nasus larvae and juveniles in June and July in the surface was greater than in the middle layer.Most of the Coilia nasus larvae and juveniles are concentrated in these two months.In the comparison in the surface at the St.1 station and in the surface at the St.2 station,the average density of Coilia nasus larvae and juveniles at St.1 station is greater than at St.2.According to the data analysis of the Coilia nasus larva eand juveniles with different development stage at flood tide during the daytime,at ebb tide during the daytime,at flood tide during the nighttime,at ebb tide during the nighttime.Under at flood tide during the daytime,at ebb tide during the daytime,at flood tide during the nighttime,at ebb tide during the nighttime conditions,The CPUE of preflexion larvae of Coilia nasus larvae and juveniles in the surface layer is larger than the middle layer,The CPUE of flexion larvae of Coilia nasus larvae and juveniles in the surface layer is basically the same as the middle layer at ebb tide during the daytime.The postflexion and juvenile of Coilia nasus larvae and juveniles tend to the middle layer at flood tide during the daytime,and tend to drift to the surface layer at ebb tide during the daytime.The CPUE of postflexion larvae of Coilia nasus larvae juveniles in the surface layer is larger than the middle layer at flood ftide during the nighttime,the CPUE of postflexion larvae of the Coilia nasus larvae and juveniles in the surface layer is smaller than the middle layer at ebb tide during the nighttime.The CPUE of juvenile of Coilia nasus larvae and juveniles in the surface layer is larger than the middle layer at flood tide during the nighttime,the Coilia nasus larvae and juveniles were not collected at ebb tide during the nighttime which is still to be studied.In the survey,larvae of yolk sac were not found.A total of 253 preflexion larvae of the Coilia nasus larvae and juveniles whose body length range(3.85-15.95mm),the number of flexion larvae of Coilia nasus larvae and juveniles is 58 whose body length range(14.44-21.98mm);the number of postflexion larvae of Coilia nasus larvae and juveniles is 55 whose body length range(16.27-35.56mm),the number of juvenile of Coilia nasus larvae and juveniles is 13 whose body length range(20.69-61.34mm).The body length of Coilia nasus larvae and juveniles did not increase significantly between June and July.Between July to August and August to September,the body length of Coilia nasus larvae and juveniles were distributed in the surface layer and the middle layer,the day and night,at flood tide and at ebb tide have increased significantly.However,the different sizes of Coilia nasus larvae and juveniles were different in different time periods.Between August and September,it was found that there was no significant change in the body length of Coilia nasus larvae of the juveniles at ebb tide during the daytime.At night,no matter at the flood tide or at ebb tide,the body length of the Coilia nasus larvae and juveniles has obvious growth.There may be more Coilia nasus larvae and juveniles in this period.With the further development of the larvae and juveniles,their ability to cruise is stronger.At night,it is more conducive to the survival of the larvae and juveniles at this stage of development.As the temperature rises,the average water temperature also gradually increases,which is more suitable for the growth of the larvae and juveniles.The preflexion and flexion of the larvae and juveniles tend to live in the surface,and postflexion of Coilia nasus larvae and juveniles in the middle layer increases with the developmental stage.Preflexion larvae are more likely to be in the surface layer at flood tide during the daytime,at flood tide during the nighttime and at ebb tide during the nighttime.Most of the preflexion larvae tend to rise during the daytime.At flood tide During the daytime,the preflexion larvae and flexion larvae are relatively more distributed in the middle layer.Most of the postflexion of Coilia nasus larvae and juveniles appear at flood tide during the nighttime in thesurface layer,the postflexion of Coilia nasus larvae and juveniles are basically the same in the surface layer and the middle layer at ebb tide during the nighttime.The postflexion larvae of Coilia nasus larvae and juveniles are distributed more at night than during the daytime The postflexion of Coilia nasus larvae and juveniles only appeared in the surface layer and the juvenile only appeared in the middle layer at ebb tide during nighttime.