Declines in catches of Japanese eels in rivers and lakes across Japan: Have river and lake modifications reduced fishery catches?

Catches and recruitment of the Japanese eel, Anguilla japonica, have declined for unknown reasons in recent decades, so examination of possible factors contributing to the declines is needed for resource management and conservation. One factor that has not been examined is the effect of habitat loss and alterations to aquatic community structure resulting from modifications of riverbanks and lakeshores. This study examined if there may be a relationship between riverbank and lakeshore revetment constructions and declines of Japanese eel catches in rivers and lakes in Japan, because levels of revetment may be a useful indicator of the degree of modifications to each aquatic system. Long-term trends in catches showed that although eel catches decreased in most locations and in Japan overall, large differences were observed in the rates of declines among locations. Catch reduction rates in 18 rivers and 9 lakes were positively correlated with the revetment rates of the shorelines of the rivers and lakes, suggesting that higher reductions in eel catches occurred in areas with high levels of revetment or other types of modifications. Evidence about the effects of revetment on aquatic community structure also suggests that revetment shorelines would negatively affect juvenile growth habitats for eels. Other types of habitat modifications or the creation of barriers to upstream movement by eels have also likely affected fisheries catches of Japanese eels, so detailed research is needed to determine how modifications have reduced eel production and if some kinds of habitat restorations may help to conserve this species.