Biological characteristics of silver-phase Japanese eels, Anguilla japonica, collected from Hamana Lake, Japan

The biological characteristics of 259 silver-phase Japanese eels, Anguilla japonica, were investigated in Hamana Lake, Japan from 2003 to 2007, to obtain basic data for this species at the downstream migration. Their silvering stage, condition factor, three internal (gonad-somatic index: GSI, hepato-somatic index and gut index) and two morphometric (fin index and eye index) indices were calculated. The overall sex ratio tended to be female (61%), but the sex ratio differed among years and months. Silver eels of S1 stage (early silver) were predominant than S2 (late silver) for both sexes (female: 67%, male: 77%). Female eels attained to larger size (mean: 659.3 mm) and older age (9.9 years) than males (527.7 mm and 8.3 years). The GSI increased from October to December whereas gut index decreased with months, but significant change in morphometric indices were not found within silver-phase. The results of this study suggest that silver-phase Japanese eels have a wide range of the biological characteristics, but significant differences can occur in sex ratio and the proportion of silvering stages among years, and in internal indices within silver-phase during their downstream migration season