Sagittal otolith microstructure,daily age and early growth of Coilia mystus juveniles in the Yangtze Estuary

Coilia mystus is a commercially important fish in the Yangtze Estuary.Annual capture averaged 2 000 tonnes in the 1980s,accounting for almost half of the total catch in this estuarine area.However,this species has declined dramatically since the late 1990s.C.mystusjuveniles were sampled from the Yangtze Estuary using trawls and set nets in July,August and September 2007.Sagittal otoliths were analyzed to estimate the daily age and early growth of the juveniles.Standard length(L_S)of C.mystusjuveniles was between 25.6 and 63.2 mm.Otolith shape of C.mystusjuveniles was similar to that of adults:anomalistic and elliptic,with a single rostral protrusion.There was one nucleus and one primordium on the transverse plane of the otolith.Primordium diameters were between 12.5 and 22.8 μm,averaging (14.3?4.8)μm(n=111).The distance from the central nucleus to the first increment was (24.85?6.38)μm(n=44).The increments in the long axis were thin and unclear,but those in the short axis were thick and clear.Estimated ages of collected specimens ranged from 54 to 128 days.Hatch dates fell between 10 April and 23 July 2007,with the peak between 11 may and 26 June.The relationship of otolith radius(R)to standard length(L_S)was fitted to a linear function:L_S=6.68+0.065R(r²=0.81,n=111,P<0.05).The incremental width was narrow and decreasing in the first 20 increments;after that,the incremental width increased to the 50th increment,and then remained constant.