Anthropogenic impacts on the distribution of wild and cultured Japanese eels in the Tone River watershed, Japan, from otolith oxygen and carbon stable isotopic composition

Abstract

Effective management decisions are vital for the declining Japanese eel Anguilla japonica. Elucidating the distribution range of wild eels (i.e., naturally recruited individuals) represents the first step in resource management, although stocking of cultured eels throughout the waters of Japan masks the original distribution range of wild eels. Herein, we applied a recently developed discrimination model based on otolith oxygen and carbon stable isotopic ratios (δ18O and δ13C) to determine the distribution range of wild eels throughout the Tone River watershed, which is the largest fisheries ground for this species. The model indicated that eels naturally recruit up to 200 km in the catchment area from the river mouth. Although natural recruitment was confirmed at all sampling sites, the percentages of wild eel occurrence at each site ranged from 56 to 100% of sampled individuals. The presence of wild individuals at sites above river-crossing structures indicates that some individuals successfully navigate through these structures. The operation of sluice gates and fish passes installed on artificial structures might facilitate the recruitment of individuals in areas above the structures and further upstream in the watershed. Difference in the percentages of wild eels among sampling sites might be a product of major variation in the abundance of cultured eels stocked at respective sampling sites and nearby areas.