Analytical research of spatial distribution of fish larvae and juveniles in summer and autumn in the eastern Chongming, Yangtze River Estuary

The CEB (Chongming Eastern Beach) is located on the crestal plane of the diluted water at the mouth of the Yangtze River. Under the action of strong currents and tides, it forms a wealth of food and provides a unique place for growth and development of fish larvae. To investigate the distribution and diversity of fish larvae, using big larval net (mesh size of 0.5 mm) to conduct 10 min horizontal trawl, while collecting real-time data of environmental parameters of the water column including temperature (SST), salinity (SSS), depth (DEP), turbidity (TUR), and chlorophyll a (Chl). A total of 10 925 fish larvae and juveniles, belonging to 52 species, 14 families, were collected. Among them, the most abundant were freshwater fish of 22 species (42%), followed by estuarine fish of 19 species, accounting 37% of total, coastal fish of 7 species (13%) and 4 species of offshore fish (8%). Whereas the most common species were Cyprinidae family, consisting of 14 species (27%); Gobiidae family had 12 species (23%); the families of Engraulidae and Osmeridae had both 5 species (10%). The dominant species were (Coilia mystus, 21.51%), (Colia nastus, 18.11%), (Hemiculter bleekeri, 16.34%), (Pseudolaubuca sinensis, 10.82%)and (Acanthogobius ommaturus, 9.38%), respectively. In all, the fish larvae of pre-flexion comprised 76.49% of the total catch, 11.94% of the flexion stage, 8.01% of the post-flexion stage, 3.43% of the juvenile stage. However, the density of fish larvae in the stages of pre-flexion and flexion had a declining tendency, while the density of fish larvae in the stage of post-flexion rose, as the season changed. The number of species, bio-diversity and the density of fish larvae were higher in the summer season than those in the autumn season. The Non-metric Multi-dimensional Scaling and the Cluster Analysis demonstrated that the fishes can be classified as two groups, rich in the freshwater type, followed by the estuary type. Based on the results of this research and other surveys of Coilia family larvae, it shows that there is a significant difference in the spatial distribution of Coilia nasus and Coilia mystus at the larval stage of the Yangtze River. This research showed that the Gobiidae was distributed in a scattered manner, and traces of the larvae and juveniles were found in the southern branch, northern branch of the Yangtze River Estuary and Dongtan water area. Like many estuaries in the world, the Gobiidae have special living habits, which enable them to adapt to the huge runoff and sedimentation of the estuary. At present, many nets and fyke nets for eel fishing have been set up along the Yangtze River Estuary. There is no effective method to assess the damage to larvae and juveniles. In addition, the feeding of larvae and juveniles and the protection of nurseries are the key to the protection and restoration of fishery resources. The CEB of the Yangtze River estuary is a buffer water area for Chinese sturgeon juveniles to descend and migrate to the sea. The density of larvae and juveniles in this water area will significantly affect the species preservation and continuation of Chinese sturgeon. Therefore, it is urgent to develop effective and systematic long-term tracking.