Gonadal development and silvering of the Japanese Eel (Anguilla japonica) in the Yangtze River during downstream migration

The Yangtze River and its branches are important habitats for the Japanese Eel (Anguilla japonica), however, there are few data on gonadal development and silvering indices of eels during the downstream spawning migration. This study investigates gonadal development in 121 Japanese eels caught downstream from the Jingjiang Reach of the Yangtze River in October 2012. We found that in females the ovaries of most of the out-migrating population developed earlier than the testes in males, that is, the ovaries reached stage 3 (F3) or early stage 4 (eF4), while the testes were at stage 2 (M2) or stage 3 (M3). Gonadal development in eels collected in 2012 was earlier than that of samples caught from the same region in 2007–2008 and from the Pearl River Estuary in 1972–1973. The average age, body size and gonadal-somatic index of females were significantly greater than those in males. For both females and males, there were no significant differences between the two stages of gonadal development in any of the morphological indices, which indicates that gonadal development of the Japanese Eel is independent of body size. These results help us to understand the spawning population of Japanese Eels in the Yangtze River.