Habitat use and growth strategies of amphidromous fish “ayu” throughout a river system

Adult spawners are key contributors to fish population dynamics, and thus understanding their life history is essential for effective population management and conservation. The radiogenic Sr isotope ratios (⁸⁷Sr/⁸⁶Sr) and ring numbers of adult spawners' sagitta otoliths were measured to determine the habitat use strategy of ayu Plecoglossus altivelis. Six groups of habitat use were identified based on profiles of the otolith ⁸⁷Sr/⁸⁶Sr ratio, indicating high contributions of wild ayu (87.2%) and upper-to-middle mainstem habitats for spawning populations. Otolith ring number analyses showed that an early hatch date led to an early river entry and a larger fish size. The upper- and middle-segment (mainly mainstem) residents had earlier hatching and river entry dates than the tributary residents, with middle-segment residents having the largest fish size. These results indicate that early hatched and early river-entering ayu preferentially occupied mainstem habitats and grew large, whereas later-hatched and later-river-entering ayu tended to avoid mainstem habitats and enter tributaries as growth sites. This study can contribute to future habitat conservation efforts and fishery resource management for ayu.