长江口日本鳗鲡降海产卵洄游群体的银化指标研究

以长江口降海生殖洄游途中的日本鳗鲡为研究材料, 对繁殖群体银化过程中体色、年龄、全长、体重及银化指标等特征作了分析。结果显示, 依据“体侧黑色素沉积是否蔓延至胸鳍下缘”这一体色标准, 可将日本鳗鲡洄游群体分为银化前期和银化期二个发育期。日本鳗鲡银化过程中, 体型、胸鳍长、眼径、性腺指数呈递增趋势, 消化道指数则呈递减趋势。主成分分析亦表明, 银化期较银化前期的眼径和胸鳍增大, 还具有较高的性腺指数和较低的消化道饱满度。由于日本鳗鲡的产卵场位于3000 km之遥的马里亚纳海沟, 这些形态的修正可能是为了适应即将起始的长距离降海生殖洄游。根据银化指标所建立的判别方程, 对雌、雄群体银化前期和银化期的判别分析均取得了95%以上的成功率。研究认为, 长江口日本鳗鲡银化期的野外快速判别, 亦可采用“眼径指数大于5”作为识别依据。

SILVERING INDEXES FOR SPAWNING MIGRATORY POPULATION OF THE JAPANESE EEL ANGUILLA JAPONICA IN THE YANGTZE RIVER ESTUARY,CHINA

To establish a simple and reliable index for determining silvering stages of the Japanese eel, Anguilla japonica, we observed the colorations of various body parts and biological characteristics on the reproductive stock of the eels from the Yangtze River estuary before starting their oceanic migrations. The body color, and age, total length, body weight, and silvering index were analyzed in the silvering process of the Japanese eel. The two silvering stages are characterized by the colorations of trunk skin as follows: (1) Pre-silver stage: pigments deposited completely on the upper edge of pectoral fin on the lateral sides of trunk, (2) Silver stage: pigments deposited completely on the lower edge of pectoral fin on the lateral sides of trunk.The body size, eye diameter, pectoral fins length and gonad-somatic index were increased from pre-silver stage to silver stage, and digestive tract index were decreased in contrast. The principal component analysis demonstrated that the silver stage eels had larger eye and pectoralfins, higher sexual maturity and the degenerated digestive tract compared to pre-silver stage eels. Without feeding, the various modifications of the body and physiological changes indicated that silver eels developed various adaptations to prepare the long oceanic migration. The silver stage and pre-silver stage could be distinguished by canonical discriminant function analysis using five characters with a 95% reliability for correct classification. Our results revealed that an eye index of 5.0 is the cut off point for distinguishing the silver stage from the pre-silver stage of the Japanese eels for the field survey.