Artificial Shelters That Promote Settlement and Improve Nutritional Condition of Japanese Eels in a Human-Modified Estuary

Various aquatic organisms are suffering from alterations of estuarine environments, where human activities are generally concentrated. The commercially important but endangered Japanese eel (Anguilla japonica) is one of the species most affected by estuarine habitat loss, because it mainly uses estuarine benthic environments for growth before long migrations to an offshore spawning area. To enhance their estuarine habitats, this study aimed to explore what type of shelter structures promote medium- to long-term settlement of this species and verify whether such shelters improve their growth in a riverine estuary in Japan whose channel was heavily lined with concrete revetment. A survey of the recapture rate of elastomer-tagged eels from the fishing gear “Ishikura net (IKN),” which consists of dozens to hundreds of stones, showed that the rate was higher when their first captures were from IKNs with fine pore structures formed by approximately 10-cm-sized stones than from those with coarser pores. Furthermore, the girth length (nutritional condition index) of recaptured individuals increased during their settlement in the IKNs. The use of the IKNs may have reduced energetic costs of behaviors such as predation avoidance and feeding, considering that a large amount of prey animals (e.g., crabs) were found, especially in the IKNs with fine pores. This study showed that the establishment of artificial shelters with fine pore structures is effective not only for the temporary residence of eel individuals but also for improvement of their body conditions through promoting their settlement and perhaps gathering their prey species.