Abstract
Despite increasing conservation concerns, information on diel, seasonal and ontogenetic changes in the movement patterns and habitat use in rivers remains limited for anguillid eels. Here, we used manual radiotelemetry to investigate these changes using 10 Indo‐Pacific eel Anguilla marmorata individuals of varying body sizes [364–1030 mm total length (TL)] in a small subtropical river in southern Japan over a period of up to 10 months. Our Bayesian models revealed that the eels exhibited strong site fidelity throughout the year, with noticeable shifts in daytime shelter positions occurring only occasionally. Additionally, the models showed that the nocturnal movement distances by eels from their daytime shelters were also limited and remained almost constant year‐round, although larger individuals moved farther than smaller ones. Regarding their habitat use, there were no consistent diel and seasonal changes in habitat types used across individuals. However, larger individuals clearly used deeper, more lentic microhabitats with smaller substrate sizes during the daytime in all seasons. These results suggest that despite the limited movements in small rivers, A. marmorata requires a variety of habitats in rivers throughout the life stages, underscoring the importance of maintaining habitat heterogeneity for effective conservation.