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.