Size‐structured monitoring and otolith chemistry reveal recurrent tidal‐freshwater use by juvenile Japanese grenadier anchovy Coilia nasus in the lower Yangtze River

Tidal freshwater reaches may provide early-life habitat for diadromous fishes, but long-term evidence of their use remains limited. Size-resolved records are needed as a reference for monitoring under the Yangtze fishing ban introduced in 2021. We analysed monitoring data from Jingjiang, 240 km upstream of the river mouth (2002-2017; 519 effective surveys; 10,560 measured individuals), combining size-based catch indices, environmental-window analyses and otolith Sr/Ca profiles from 115 Japanese grenadier anchovy Coilia nasus. Individuals <120 mm standard length were classified operationally as juveniles during August-October, when scale readings supported an age-0 interpretation, whereas individuals ≥180 mm were treated as a descriptive large-fish category throughout the year. Juveniles occurred in every survey year; annulus-free individuals also occurred in winter and spring. The Juvenile Recruitment Index (JRI) showed negative associations with tidal range across the April-July spawning and larval-transport period, although none of the 72 environmental-window correlations remained significant after Benjamini-Hochberg correction at a 5% false-discovery rate. Monthly models also showed a negative lagged association. None of the 40 autumn recruits sampled across four capture years had an accepted elevated Sr/Ca zone; such zones were more frequent in later developmental groups. Total catch obscured contrasting size structures: 2016 combined the series-high JRI with few large fish, whereas 2010 combined the series-high large-fish catch rate with a mid-range JRI. These findings document recurrent early-life use of a tidal freshwater reach and provide a size-resolved pre-ban reference, without establishing a disproportionate contribution to adult production.