Reproductive status monitoring of Coilia nasus across four Yangtze River sections during the fishing ban period.

Catch-based indicators are widely used to monitor the Yangtze River fishing ban, but do not describe reproductive condition. We established a May-June 2024 reproductive status baseline for the globally Endangered Japanese grenadier anchovy Coilia nasus at four river sections. Drift gill nets with 40-mm mesh yielded 9455 fish, summarized in calendar 10-day intervals. Age and gonadosomatic index (GSI) were measured in a laboratory series of 1119 fish collected from 30 May to 4 June. Fish at macroscopic gonadal stages IV-V were classified as mature, with stage V analyzed separately. Anqing had the highest mean catch per event (129.5 fish) but the lowest mature proportion (8.9%). Chongming yielded fewer fish per event (95.3) but higher proportions of mature (54.2%) and stage-V fish (18.4%), and stage-V fish rose to 44.8% of the late June catch. Event-clustered models estimated approximately tenfold higher odds of maturity at Chongming than at Anqing; the contrast persisted during the 6-day common sampling window. At Anqing, the composition shifted seasonally from stage II toward stage III, but few reached stages IV-V, and Hukou was dominated by stage-III and stage-IV fish. Fish at both upstream sections were larger and in lower condition than those downstream, consistent with a contribution from long-distance migrants that mature later. Ages 2 and 3 accounted for 90.3% of the laboratory series, and GSI increased across macroscopic stages. Joint monitoring of catch rate and reproductive condition through the migration season provides a seasonal baseline for this Endangered species across estuarine, mainstream, and river-lake junction habitats.