Study On Annual Resource Variation Of Larvae And Juveniles In The Southern Branch Of The Yangtze River Estuary

The hydrological environment of the Yangtze River estuary is relatively complex.It is an important spawning,breeding and nursery ground for many fish,and plays a vital role in fishery and ichthyology research.In this study,juvenile larvae were used as the research object.The species composition of juvenile larvae,the number of dominant species,and the changes of resources of Coilia nasus larvae and juveniles were analyzed and compared with 2015 and 2014.It aims to provide scientific basis for the protection of early fish resources in the Yangtze River estuary and subsequent related research.To clarify the distribution and abundance of larvae and juveniles,There are 8stations were set from June to October 2018.During the two days,the flood tide sampling was performed on St.1-St.8 during the day,the ebb tide sampling was performed on St.6-St.8 during the day,and the flood and ebb tide sampling was performed on St.8 at night.The collected samples were fixed on-site with 5% formalin solution,and the water temperature and salinity were measured.A total of 24824 individuals of larvae and juveniles were collected in the 58 trawls at the spring tides in the surface layer by a larval net(1.3 m mouth-diameter,mesh aperture 0.5 mm).These individuals belong to 41 species of 13 families.The Cyprinidae contains 14 species of fish and abundance is 70.21%,and there are 10 species in Gobiidae,include 2369 individuals,Osmeridae(4 species,105 individuals),4 species of Engraulidae(4623 individuals),There are one species of other species,and 1 unknown species of Engraulidae.The larvae and juveniles collected in the past three years are the most freshwater fish,and the number is also more than other types of fish,followed by migratory fish and estuary fish,and the percentage of marine fish is the lowest.The Fishes of Cyprinidae and Gobiidae are the main species.The dominant stage is preflexion larvae,which abundance is 91.31%,and the postflextion larvae and flexion larvae followed,which accounted for5.42% and 2.67%.Both yolk-sac larvae and juveniles did not exceed 1% in 2018.The dominant species in 2018 were Hemiculter bleekeri,Coilia nasus,Culter alburnus,and Rhinogobius giurinus.The number of Hemiculter bleekeri accounts for 54.24% of the total collection,followed by Coilia nasus(18.58%),Culteralburnus(7.88%),and Rhinogobius giurinus(4.36%).The above four types of fish accounted for 85.03% of the total catch.The catches of Hemiculter bleekeri and Coilia nasus were the highest in June.With the increase of month,the number of the larvae and juveniles declined overall.The three-year common dominance species were Hemiculter bleekeri and Rhinogobius giurinus.A total of 4611 Coilia nasus larvae and juveniles were collected in 2018.The body length ranged from 3.1-61.74 mm,with developmental stages from preflexion larvae stage to juvenile stage.The most dominant developmental stages was preflexion larvae whose body length ranged from 3.1-18.12 mm,accounting for86.19% of this total number,followed by flexion larvae whose body length ranged from 4.9?24.05 mm for 8.5%,postflexion larvae whose body length ranged from9.75?30.36 mm for 4.6% and juveniles whose body length ranged from24.22?61.7mm for 0.72%.A total of 4271 individuals of Coilia nasus were collected at flood tide during the day,the average body length was 11.18 mm,the average density was 0.169 ind./m3.336 individuals of Coilia nasus were collected at ebb tide during the day,the average body length was 10.52 mm,and the average density was0.029 ind./m3.At St.8,the average density was 0.191 ind./m3 at flood tide and0.005 ind./m3 at ebb tide during the day.The average density was 0.005 ind./m3 at flood tide and 0.002 ind./m3 at ebb tide at night.Development stage showed a similar tendency and preflexion larvae were dominant in 2015 and 2018,however yolk-sac larvae were dominant in 2014.Most Coilia nasus larvae and juveniles were collected from June to August in each year,and rarely collected from September to October.The density was not significantly different(P> 0.05)among stations.