Long-term variations in body length and age at maturity of the small yellow croaker ( Larimichthys polyactis Bleeker, 1877) in the Bohai Sea and the Yellow Sea, China

Abstract

This study focused on changes in the maturation schedules, as depicted in maturity ogives, of female small yellow croaker ( Larimichthys polyactis) in the Bohai Sea and the Yellow Sea during the 1960–2010 spawning seasons. The two stocks studied were the northern Yellow Sea-Bohai Sea stock (the NYBS, n = 1283) and the southern Yellow Sea stock (the SYS, n = 2024). Body length ( L 50) and age at median sexual maturity ( A 50) were estimated by an arcsin-square-root (ASR) transformative logistic model and an inverse von Bertalanffy growth function, respectively. The results show that L 50 decreased from 152.8 mm to 105.3 mm between 1960 and 2003–2005 in the NYBS and from 184.4 mm to 110.1 mm between 1960 and 2010 in the SYS. Over the same period, A 50 decreased from approximately 1.5 years in the NYBS and 2.4 years in the SYS to about 1 year in both stocks. Significant intrastock changes ( P < 0.01) in length–maturation curves were found in both stocks over long time periods (≥4 years); however, there were no significant changes ( P > 0.05) over short time intervals (<4 years). Significant interstock changes ( P < 0.01) were observed in length–maturation curves in corresponding sampling years. Significant positive correlations were found between the instantaneous rate of maturation ( δ) and the growth potential index ( ω) in the NYBS and between L 50 and asymptotic body length ( L ∞) in the SYS. Significant negative correlations were found between L 50 and the sea surface temperature (SST) daily rise rate in the NYBS and between δ and the mean monthly SST in the SYS. These correlations suggest that declines in length and age at maturation primarily reflect changes in growth associated with overfishing and rising SST. Additionally, a fisheries-induced evolutionary response has contributed to changes in maturation schedules in both stocks. The principal pressures are the stress of continuously higher fishing intensity and an increasing proportion of yearling fish in the catches over time.