Developmental Characteristics And Temporal And Spatial Distribution Of Coilia Mystus Larvae And Juveniles In The Yangtze Estuary

Coilia mystus belongs to clupeiformes,Engraulidae and Coilia.It mainly inhabits in coastal shallow water or offshore,and is widely distributed in China.As the main economic fish in the Yangtze Estuary,C.mystus was one of the five famous fishing species in the Yangtze Estuary.However,due to the influence of human activities and environmental changes,the resource of C.mystus showed a downward trend in recent 20 years.Because of the more sensitivity,artificial breeding of C.mystus has not been successful.In order to protect and make rational use of C.mystus resources,the development characteristics and spatial-temporal distribution of C.mystus larvae and juveniles in the Yangtze Estuary were studied based on the survey data,as well as the correlation between the abundance of larvae and juveniles and the environmental factors.The main results are as follows:1.Developmental characteristics of C.mystus larvae and juveniles in the Yangtze EstuaryIn order to understand the developmental characteristics of the larvae and juveniles of C.mystus in the Yangtze Estuary,10 survey stations were set up in the coastal waters of the Yangtze Estuary from May 2019 to January 2021 to collect the samples of the larvae and juveniles.The morphological characteristics of the larvae and juveniles at different developmental stages were summarized.At the yolk sac stage,the mouth fissure has been formed and the yolk sac is oval;at the pre bending stage,the yolk sac bends and absorbs,leaving only a small pear shaped oil ball and a "V" shaped sarcomere;at the bending stage,the upper and lower jaw of the larvae are duck beak shaped,the oil ball bends completely disappear,the melanin from the posterior edge of the gill cap to the ventral fin,the ventral fin to the outer edge of the gluteal fin and the base of the caudal fin is obvious,and the posterior edge of the inferior caudal bone is not perpendicular to the body axis In the postflexion stage,the dorsal fin of the larva moved forward,the sarcomere changed into "W" shape,and the infracaudal bone was completely formed and perpendicular to the body axis;the body shape and appearance of the juvenile were very similar to that of the adult.The morphology of the larvae and juveniles of C.mystus has obvious changes at different developmental stages,which can be distinguished from the similar species by the disappearance of oil ball and the appearance time of scales.The changes of fatty acids and feeding habits of C.mystus larvae and juveniles at different developmental stages were further analyzed by the determination of fatty acids.A total of 22 fatty acids were detected in the larvae and juveniles of C.mystus,including 7 saturated fatty acids(SFA),5 monounsaturated fatty acids(MUFA)and 10 polyunsaturated fatty acids(PUFA).SFA accounted for 47.70%-52.02% of the relative content in pre bending stage and bending stage,while MUFA accounted for 42.18%-45.75% of the relative content in post bending stage and juvenile stage.In single fatty acid,c18:1n9c and c16:0 accounted for the largest proportion(22.9%-38.83%)and(22.27%-37.46%)respectively.The results showed that the ratio of DHA / EPA was between 1.57 and 1.75,and the trophic level was low.The trophic level increased with the delay of the development stage,and the content of characteristic fatty acids of phytoplankton decreased with the delay of the development stage,showing a trend of preference to zooplankton feeding habits.The larvae mainly ate zooplankton.2.Spatial and temporal distribution of C.mystus larvae and juveniles in the Yangtze EstuaryIn order to find out the spatial and temporal distribution of C.mystus larvae and juveniles in the Yangtze Estuary,the abundance distribution and its relationship with environmental factors were studied.The results showed that the main breeding season of C.mystus was from April to October,and the peak breeding season was from May to August.The larvae and juveniles of C.mystus were mainly distributed in the North Branch waters from May to September,and in the South Branch waters in October.The abundance of larvae and juveniles of C.mystus at high tide was higher than that at low tide.When the water temperature was high,they were mostly distributed in the middle water area.The abundance was higher at night at high tide,and higher at daytime at low tide.The coastal waters of Dongwangsha-Beibayao-Qilongxiang in the North Branch of the Yangtze Estuary are the main distribution waters of C.mystus larvae and juveniles,and are the main nursery grounds of C.mystus in the Yangtze Estuary.GAM analysis showed that the distribution of C.mystus larvae and juveniles in the Yangtze Estuary was mainly affected by salinity,temperature and dissolved oxygen.Three environmental factors had significant effects on the abundance of C.mystus larvae and juveniles(P < 0.05).The results showed that the optimum salinity and temperature were 5 ~ 12 and 20 ~ 28 °C,respectively.The suitable dissolved oxygen for the growth of C.mystus larvae and juveniles was about 5.2-8.0 mg/L.