Effects Of Tide Onvertical Distribution And Drifting Mechanisms Of The Fish Larvae And Juvenilesin The Southern Branch Of The Yangtze Estuary

The Yangtze River estuary is the largest river estuary in China,has complex water nutrient factors and high diversity of biological community structures.It is a breeding ground for many fish,shrimp and crab organisms,and is the spawning or nursery ground for many economic fish.In recent decades,affected by domestic wastewater,ship pollution,farmland sewage,industrial pollution and other factors,the main dominant species in the Yangtze River estuary,has changed from high economic value fish to lower economic value fish.In order to further discussed the distribution mechanism of fish larvae and juveniles in the Yangtze River estuary,in this study,the fish larvae and juveniles were used as the research objects,and the diversity and the distribution with the tide was researched to provide a scientific basis for the protection and rational development and utilization of the fishery resources in the southern branch of the Yangtze River estuary.In this study,the fish larvae and juveniles were collected monthly at 2 stations(St.1:N31°35.500?,E121°20.004?;St.2:N31°33.500?,E121°20.000?)on the spring tide during May 2017 and April 2018 in the north-south longitudinal section of the southern branch of the Yangtze River estuary.Samples were conducted in North-Southern longitudinal section of the southern branch of Yangtze River estuary,using a larval net(1.3m mouth-diameter,0.5mm mesh).Moreover,according to the tidal changes(4 flood tides,4 ebb tides),each haul was made parallel to the surface layer and middle layer.A total of 51783 fish larvae and juveniles were collected,representing 32 species from 8 families.The five dominant species were Hemiculter bleekeri(87.05%),Carassius auratus auratus(3.45%),Culter alburnus(3.16%),Rhinogobius giurinus(2.16%),Repomucenus olidus(0.01%).Among the collected samples,the largest species isCyprinidae(15 species)accounting for 94.87%of the total collection,and Gobiidae(4species,2.21%),Callionymidae(1 species,0.98%),Engraulidae(1 species,0.77%),Bagridae(2 species,0.48%),Hemiramphidae(1 species,0.25%),Mugilidae(1 species,0.14%),Osmeridae(4 species,0.10%),Cynoglossidae(1 species,0.08%),Percichthyidae(1 species,0.07%),Moronidae(1 species,0.06%).20 species of freshwater fish,accounting for 97.61%of the total collection,8 species(2.21%)of brackish water fish,and 4 species(0.18%)of offshore fish.The pre-flexion stage was the dominant stage(95.21%),follows by flexion stage(4.54%),post-flexion stage(0.12%),juvenile stage(0.12%)and yolk-sac larval stage(0.01%).The results of average density analysis showed that the monthly average density was affected by the flood tide to a certain extent:average density of flood tide is larger than the ebb tide;the values of“flood tide and ebb tide”in July are the highest in the whole year(2609.3/1000 m~3 and 1316.1/1000 m~3,respectively);and the lowest average density were observed at flood tide of February and ebb tide of January-March(both 0.1/1000 m~3).The results of body length of fish larvae and juveniles analysis showed that the average body length of Hemiculter bleekeri,Culter alburnus and Rhinogobius giurinus had no significant correction in day and night or surface and middle layer.The Carassius auratus auratus and the Repomucenus olidus are mainly appeared on the surface of the day.From the perspective of the amount of appearance,Hemiculter bleekeri,Carassius auratus auratusand Repomucenus olidus at flood tide is greater than that of the ebb tide,while the amount of Culter alburnus at ebb tide is higher than flood tide.There was no change on the amount of Rhinogobius giurinus.In general,the fish larvae and juveniles of the southern branch of the Yangtze River estuary are drifting back and forth with the tides,and showed differences between species.The results of developmental stage analysis showed that:when the fish larvae developes from pre-flexion to flexion,the flexion stage of the five dominant species at flood tide were lower than those at the ebb tide.While when the fish larvae developes from flexion to post-flexion,the density of the post-flexion stage in ebb tide were lower than those at flood tide.The results of ecological types analysis showed that the total fish collection of freshwater,brackish water and offshore at flood tide were greater than that those at ebb tide.T-test results showed that there was a significant correlation between dissolved oxygen concentration,temperature and average density,but no significant correlation between salinity and average density.