Habitat Use Strategies Of Coilia Nasus In The Yangtze River Estuary Tracked By Otolith Microchemistry

The Yangtze River is the longest river in China and the third longest river in the world.It flows eastwards from the Tibet Plateau,to the East China Sea at Shanghai.The estuarine portion of the river provides a suitable growth area for a number of migratory fish species including Coilia nasus.In recent years,numerous studies have been carried out on the ontogenetic patterns of habitat use and the life history characteristics of C.nasus,in various rivers and lakes around China.The estuarine tapered tail anchovy is a highly commercial fish species distributed along the waters of China,Korea and Japan.In this study samples collected from the Yangtze River estuary around Chongming Island,China,were studied by analyzing the elemental signature in the otolith.The process of otolith microchemistry analysis was used to assay otolith elemental composition to determine the migratory patterns and life history characteristics of C.nasus.The conservative nature of the crystalline structure makes otoliths an ideal depository of life-history events that affect their structure and composition.The ratio of strontium(Sr)and calcium(Ca)(Sr:Ca ratios)concentrations were analyzed using Electron Probe Microanalyzer(EPMA).Otolith strontium(Sr)and calcium(Ca)signatures can be applied as a useful scalar to trace environmental history of fish,since the deposition of Sr and the Sr:Ca ratio positively correlates with the salinities of freshwater,brackish water,and sea water.The bluish,greenish or yellowish,and reddish regions in the otoliths were characteristics of freshwater(low salinity),brackish water(medium salinity)and sea water(high salinity),respectively.The anadromous life history characteristics observed in this study,suggest that C.nasus had much diverse and complex experience with different salinity habitat uses between freshwater and sea water.All the aforementioned evidences of life-history transect were supported by the otolith Sr mapping analyses of EPMA.In this studies C.nasus samples were collected in a uniform location at different times in April 2013 and December 2015,respectively.These samples were analyzed taking into consideration the different habitat use strategies of C.nasus in the Yangtze River estuary.During spring adult C.nasus migrates upstream for spawning and the offspring migrate back to the estuarine in about August.Comparative analysis was done on the length of the central bluish core and the fresh water coefficient,to determine the length of time an individual spent in fresh water habitats during their early life history.Samples with a larger central bluish core were estimated to have spawned further upstream from the river mouth.The results of this study suggest that the migration strategy of C.nasus in the Yangtze River estuary is much more flexible than previously stated.The estuary habitat was found to constitute multiple populations from different natal origins and with different migration histories.The life history patterns showed variation under restrictive analytical conditions.The fish of 2013 exhibited the most significant large-scale habitat shifts between different environments and salinity levels.Although previous studies have stated that the C.nasus anchovy is an anadromous fish that spawns and hatches in a freshwater habitat,the results of the present study indicates that they could also spend their entire early life history in an estuarine habitat.