Age Structure, Growth, and Sex Ratio of Coilia nasus in the Lower Yangtze River and the Estuary During the Initial Stage of Fishing Moratorium

Abstract

The comprehensive 10-year fishing moratorium implemented in the Yangtze River Basin since 2021 provides a unique opportunity to evaluate the recovery of threatened fish populations. This study assessed the recovery status of Coilia nasus in the lower reaches of the Yangtze River during the fourth year of the moratorium through analysis of age structure, growth parameters, and sex ratio. Field investigations were conducted at two strategic locations (Chongming and Taizhou) during the migration season (May–June 2024), yielding 3077 specimens. Results revealed significant improvements in population parameters, including increased mean age (2.64 years) and standard length (28.5 cm), alongside a higher proportion of age 3 and age 4 individuals (63.3%). Sex ratio analysis showed a pronounced female bias (3.19:1), with spatial variations between estuarine (4.99:1) and upstream (2.04:1) sites. Growth parameter analysis indicated a shift toward natural growth dynamics, with increased asymptotic length (L_∞) and altered growth rates (k) compared to premoratorium periods. Despite these positive indicators, the absence of five-year-old individuals and lower average length compared to historical records suggest incomplete recovery. Our findings provide evidence for the effectiveness of the fishing ban while highlighting the need for continued protection.