Preferred gap structure within stone piles of fishing gear by Japanese eel Anguilla japonica at each life history stage: the search for an effective method to restore estuarine habitats

To conserve the Japanese eel Anguilla japonica, damaged estuarine habitats need to be restored, because the estuaries are essential for not only growth of the species, but also as areas for silver eels to acclimate to seawater prior to migrating into the ocean for spawning. As a first step towards improving and restoring estuarine habitats, this study aimed to reveal the preferred gap structure within stone piles by eels at each life history stage. Compari-son of the number of individuals among Ishikura nets with different-sized stones (large, 30 cm in major axis; medi-um, 20 cm; small, 10 cm) found that the immature adults (elvers and yellow eels) preferred those with small-sized stones the most. In contrast, the maturing adults in the middle of downstream spawning migration (silver eels) uti-lized only the Ishikura nets with large and medium-sized stones. These findings suggest the importance of estab-lishing estuarine habitats with various-sized stones to allow many eels to survive to maturity in stream areas.