A silvering index for the Japanese eel Anguilla japonica

Abstract

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 of the eels collected in a coastal area of Japan (Mikawa Bay). The four silvering stages are characterized by the colorations of pectoral fins and ventral skin as follows: (1) Y1, yellow eel without a metallic hue at the base of pectoral fins, (2) Y2, late yellow eel with a metallic hue at the base of the pectoral fins but without melanization at the tip of pectoral fins, (3) S1, silver eel with complete melanization at the tip of pectoral fins but without full pigmented belly in black or dark brown, and (4) S2, late silver eel with black or dark brown belly. The body size, eye diameter and sexual maturity of each stage increased in the order of Y1, Y2, S1 and S2 stages, whereas the digestive tract degenerated in the same order, suggesting a sequential development of these ontogenetic stages identified in the study. The Y1, Y2 and S1 stages could be also distinguished by canonical discriminant function analysis using three internal (gonad-somatic index, GSI; hepato-somatic index, HIS; and gut index) and two morphometric (condition factor and eye index) parameters, supporting the significance of these stages. This method of staging for the silvering process of the Japanese eel appeared to be applicable to all specimens of this species, since this index used only simple external characteristics that would be easy to observe during field surveys.

Japanese eel Anguilla japonica do not assimilate nutrition during the oceanic spawning migration: evidence from stable isotope analysis

“…A total of 72 adult Japanese eels Anguilla japonica at the initial phase were collected from 10 locales in Japan (Fig. 1, positions 1-10), which were classified into 37 yellow and 35 silver stage eels (Okamura et al 2007). All initial phase eels (IPE) were adult (60.5 ± 13.0cm in body length) and were either caught downstream and in river estuaries, or in coastal lakes and other coastal areas.…”

First observations of the oceanic migration of Japanese eel, from pop-up archival transmitting tags

“…Eels were anaesthetized and their total lengths, body weights and silvering indices (see Okamura et al 2007) were measured (Table 1). Surgical tagging was carried out following the method of Jellyman & Tsukamoto (2002) with some slight modifications.…”

Dispersal of yellow phase Japanese eels Anguilla japonica after recruitment in the Kojima Bay-Asahi River system, Japan

“…Sex was determined by visual inspection of gonad morphology. The growth stages of eels (yellow-eel or silver eel stages) were confirmed by the examination of the color of the fish body and pectoral fins according to the silvering index of Okamura et al (2007).…”