Differences in the maturation level among life histories of the Japanese eel Anguilla japonica in the Nagata River, Japan

In order to understand the silver eel migration behavior of the Japanese eel Anguilla japonica, the life history of both the yellow stage (immature stage) and the silver stage (early maturing stage) was examined during the silver eel migration period, using otolith microchemical analysis of strontium (Sr) and calcium (Ca) concentrations. The mean Sr:Ca ratios in A. japonica from the elver mark to the otolith edge indicate eels of several general categories of migratory histories, including sea eels (average Sr:Ca ratios, ≥ 6.0 × 10^-3), estuarine eels (average Sr:Ca ratios, 2.0 × 10^-3 ≤ Sr:Ca < 6.0 × 10^-3) and river eels (average Sr:Ca ratios, < 2.0 × 10^-3), based on the criteria reported previously in A. japonica. A significant negative correlation was observed between the gonadosomatic index (GSI) and the mean Sr:Ca ratios during the growth phase after recruitment. Gonads of river eels were more developed compared to eels with other migratory histories. It thus appears that the maturation level for the silver eel migration to the open ocean varies depending on the life history.