长江口凤鲚幼鱼的耳石微结构、日龄与生长

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

2007年7月、8月和9月在长江口区域从小型拖网和张网渔获物中获取凤鲚幼鱼,摘取矢耳石进行耳石微结构分析,确定凤鲚幼鱼的日龄和孵化日期,分析个体早期生长过程。凤鲚幼鱼的体长范围为25.6～63.2 mm,矢耳石形状与成鱼耳石相似,为不规则扁椭球形,只有一个翼状突起。耳石横截面磨片上具有一个核和一个原基。耳石原基的直径为12.5～22.8μm,平均为(14.3±4.8)μm(n=111)。耳石核中心到第一个生长轮的距离为(24.9±6.4)μm(n=44)。生长轮在耳石长轴上排列较疏且不清晰,短轴上较密但清晰。凤鲚幼鱼的日龄为54～128 d,对应的孵化日期为2007年4月10日至7月23日,高峰期为2007年5月11日至6月26日。耳石半径(R)与标准体长(LS)间呈显著的线性相关关系,回归方程为LS=6.68+0.065R。凤鲚幼鱼前20个生长轮宽度较窄,并有逐渐变窄的趋势,第20日轮之后,生长轮宽度逐渐变宽,至第50日轮后生长轮宽度趋于稳定。

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.mystus juveniles 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（LS）of C.mystus juveniles was between 25.6 and 63.2 mm.Otolith shape of C.mystus juveniles 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（LS）was fitted to a linear function：LS=6.68＋0.065R（r2=0.81,n=111,P0.05）.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.