Ontogenetic Habitat Shift of the Endangered Benthic Freshwater Fish: Unravelling the Role of Drifting Larval Dispersal in a Small Spring‐Fed Stream

ABSTRACT Understanding habitat shifts across life stages is crucial to conserve Lefua echigonia , an endangered freshwater fish in Japan. This species has a drifting period in its larval and juvenile stages, resulting in high mortality. Therefore, it is essential to understand the habitat requirements in particular during these stages for effective conservation. In this study, the microhabitats of this species across six total length (TL) classes were selected to identify ontogenetic habitat shifts. Monthly fish sampling and environmental measurements were conducted over a three‐year period along a 1.3‐km spring‐fed stream, from the downstream end to the upstream end. Habitat characteristics of this species were analyzed using random forests at a point‐based microhabitat scale. In addition, the relationship between the TL and the water velocity at each cross‐section was examined. The habitat characteristics of this species vary markedly among drifting larvae and juveniles, and benthic adults. Shallow water depth is an important factor for drifting larvae and juveniles, as it provides refuges from predators. In contrast, adult fish prefer low‐velocity environments, indicating that microhabitat use is associated with body size and swimming ability and low‐velocity refuges is formed by streambed substrates in fast‐flowing reaches. The drifting larvae and juveniles were unable to use these refuges and were predominantly found around vegetation, which highlights the importance of vertical and lateral velocity moderation by vegetation for their effective conservation.