The Yangtze River estuary is the main production area of Anguilla japonica
in China, as well as the only existing fishery area for adult eels. Japanese eels are distributed in the main rivers and many tributaries from the Yangtze River estuary to the upper Jinsha River, which extend to nearly 3 000 km. However, their migration behaviors remain relatively unknown. We analyzed the biological characteristics of 153 specimens of silver eels collected from the Jingjiang section of Yangtze River (31° 30° N, 120° 42'E) between September and November, 2008, and tested the sagittal Sr/Ca ratios of 27 specimens. Among the 153 specimens examined, 85 were female and 68 were male, which translated to a female-male ratio of 1 : 0.8. The ages of the female specimens ranged from 3 to 7 a (average 5.52) with an average total length (TL) of (669± 80) mm, average body weight (BW) of (555± 229)g, average condition factor of 1.77± 0.22, and average gonad somatic index (GSI) of 1.32± 0.31. The ages of the males ranged from 3 to 5 a (average:4.38) with an average TL of (518± 51) mm, average BW of (234± 76) g, average condition factor of 1.62± 0.18, and average GSI of 0.21± 0.11. All biological parameters of females were significantly larger than those of the male specimens (P< 0.05). According to the average Sr/Ca ratio (7.99± 1.05) x10-3
of the elver mark of sagitta, 17 individuals (62.96%) were river eels and 10 individuals (37.04%) were estuarine eels. Of 16 females, 13 individuals (81.25%) were river eels and 3 were estuarine eels, while of 11 males, 36.36% were river eels and 63.64% were estuarine eels. The analysis on Sr/Ca ratios for every growth layer group (GLG) indicated there were no significant differences between second-age males and females. However, significant differences were observed between the third-age, fourth-age, and migration-age male and female specimens. This was likely related to the fact that second-age eels of both sexes stay in the same inhabitation waters; however, as they grow older, they move to different areas.