Abstract: Estuaries are characterized by high primary productivity and steep environmental gradients along the freshwater–seawater mixing zone, providing critical habitats for migratory fishes to complete spawning, feeding, and juvenile rearing. The Yangtze River Estuary, situated at the boundary of the Yellow Sea and the East China Sea, is a typical large-scale estuarine system on the western Pacific coast. River discharge carries substantial nutrient loads into the coastal ocean, sustaining elevated primary productivity and abundant prey resources that support the migration, feeding, and early growth of commercially important fish species. Coilia nasus (Clupeiformes: Engraulidae) is a representative anadromous species in this system. Anadromous individuals typically feed and grow in nearshore waters and, upon reaching sexual maturity, migrate upstream to freshwater reaches of the middle and lower Yangtze River to spawn, primarily between April and July. Young-of-the-year (YOY) juveniles subsequently descend into the estuary and adjacent coastal waters for feeding and growth. Over recent decades, C. nasus has experienced marked population declines driven by overfishing, wetland loss, altered hydrodynamics, and reduced sediment discharge. A 10-year fishing ban took effect across key Yangtze waters in 2021, yet the habitat use patterns of different age groups within the estuary remain poorly understood. This study aimed to reveal the spatio-temporal distribution characteristics of age-specific groups of C. nasus in the Yangtze River Estuary and adjacent waters, and to evaluate the nursery function of the estuarine mouth zone and its environmental drivers. Bottom trawl surveys were conducted during spring (May), summer (August), and autumn (November) of 2024 and 2025 across 55 fixed stations (121°-124°E, 31°-32°N), totalling six seasonal cruises. All captured C. nasus were measured for standard length (SL, cm) and assigned to YOY (age 0) or non-YOY (age ≥ 1) groups based on overwintering bands on scales. Abundance was standardized as numerical density (ind./km²). Density-weighted centres of gravity, inertia ellipses, and the global index of collocation (GIC) were computed for each year × season × age-group combination. Bottom salinity and temperature were recorded at each station, and their effects on density were evaluated using Spearman rank correlations and negative binomial generalized linear models. Salinity fronts were identified within a latitudinal band covering the mouth zone by locating the maximum longitudinal salinity gradient. A total of 12 025 specimens were collected. Population composition shifted markedly across seasons: non-YOY individuals dominated in spring (78.6%), while YOY prevailed in summer (62.1%) and autumn (74.3%), reflecting the life-history pattern of spring spawning migration and subsequent summer–autumn juvenile recruitment. YOY density increased from (335 ± 757) ind./km² in spring to (1 454 ± 3 027) ind./km² in autumn, whereas non-YOY density declined from (1 680 ± 2 753) ind./km² in spring to (485 ± 1 085) ind./km² in autumn. Despite these contrasting trends, density-weighted centres of gravity of both groups remained within the estuarine mouth zone (approximately 122°E, 31.2°-31.4°N) across all cruises. Spatial overlap was highest in spring (GIC = 0.996) and declined in summer and autumn (GIC ≈ 0.940), indicating progressive spatial segregation while retaining a shared distribution core. Bottom salinity was the primary environmental constraint: density of both groups was negatively correlated with salinity across all seasons (Spearman ρ: −0.508 to −0.807, all P < 0.001). Salinity fronts were consistently located between 122.1° and 122.5°E, coinciding spatially with YOY distribution centres. The estuarine mouth zone functions as a stable nursery for C. nasus juveniles, as evidenced by consistently high densities and a spatially persistent distribution centre across seasons. Bottom salinity gradients and salinity front dynamics appear to be primary factors regulating the age-specific spatial distribution of C. nasus in the estuary. These results offer a quantitative basis for delineating critical nursery habitats and developing habitat-oriented management measures in the context of the ongoing fishing ban.