Isotopic Evidence of Population Connectivity at Multiple Life Stages for Larimichthys polyactis in the Southern Yellow Sea and East China Sea

Larimichthys polyactis, a typically oceanodromous species found across the East China, Yellow, and Bohai Seas, is one of the most ecologically and commercially important species in East Asian countries. Moreover, over the past decades, L. polyactis has experienced significant changes in its biological characteristics under long-term intensive fishing pressure, suggesting a need for urgent protection. A robust understanding of connectivity at the spatiotemporal scale is critical for the effective and thorough management of L. polyactis. In the present study, the otolith stable carbon (δ¹³C) and oxygen (δ¹⁸O) isotope ratios of four adult L. polyactis groups (Y-1–4) sampled in the southern Yellow and East China Seas (SYS and ECS) were determined at multiple life stages, including the larval dispersal period (core), overwintering period (A1), and one whole life cycle except the post-spawning period (A2). Dunn’s multiple comparison test showed that there was no difference among the Y-1, Y-2, and Y-3 groups in the otolith core and A1 zones (p > 0.05); the otolith δ¹³C and δ¹⁸O of the Y-4 group were significantly higher than those of the Y-3 group (p < 0.05) in the three analysed zones. In the otolith A2 zone, δ¹³C and δ¹⁸O of the Y-1 group were significantly higher than those of the Y-3 group (p < 0.05). Combined with the biplot of otolith δ¹³C and δ¹⁸O, the quadratic discriminant analysis (QDA) of the otolith core, A1 and A2 zones showed that: (1) in the otolith core zone, two dominant larval aggregations occurred, likely distributed in the coastal waters of northern Lvsi and Haizhou Bay fishing grounds (Y-4 and part Y-1/2 groups) and Zhoushan and southern Yangtze Estuary fishing grounds (Y-3 and part Y-1/2); (2) in the otolith A1 zone, two clear aggregations were separated by Y-4 and Y-1/2/3, suggesting two dominant groups overwintering in the SYS and ECS, respectively; and (3) in the otolith A2 zone, two dominant aggregations were separated by Y-4/1 and Y-2/3. In addition to the long-term larval dispersal process, the migratory route between spawning and overwintering grounds may be diverse, explaining the non-significant genetic differentiation of L. polyactis from different groups. Combining these results with those of previous studies on the life history of L. polyactis, a schematic map of the migratory routes is depicted providing important references for effective resource management.