As a wild resource, the large yellow croaker Larimichthys crocea has been seriously threatened since the mid-1980s. Owing to the implementation of protection measures, such as the establishment of a protection zone, fishing prohibitions, restocking programs and successful mariculture, its resources have gradually recovered year by year. Limited by the low spatial resolution and incomplete spatial coverage of sampling stations, the spatial genetic structure and diversity of large yellow croakers from the southern Yellow Sea and north-central East China Sea remains unclear. In order to evaluate the genetic diversity status of this wild stock, 22 wild sites were collected from the southern Yellow Sea and north-central East China Sea and analyzed by investigating genetic variability and its population structure using mitochondrial COI sequence in this study. Among the 662 sequences, a total of 71 different haplotypes were defined. The haplotype diversity (h) and nucleotide diversity (π) values were 0.644~1.000 and 0.00220~0.00473 respectively. The highest h and π occurred in the southern Yellow Sea (YS). AMOVA analysis showed no genetic differentiation among those 22 sites after Bonferroni correction. By comparing with previous studies, the croaker has maintained relatively steady genetic diversity in recent years. Our result also suggested that the croakers in the South Yellow Sea and north-central East China Sea belonged to the same group. Thus, they can be released as a management unit without regard for heterogenicity among those in the sea area. The YS populations can serve as parents for released fish fries in the South Yellow Sea and north-central East China Sea.

In order to understand the diel vertical migration and transport pattern during their early life stages, the vertical distribution of larvae and juveniles of the small yellow croaker (Larimichthys polyactis) in the Yangtze River estuary was investigated. Four sampling cycles were carried out at the station (122°39′ E, 31° N) during May and June 2015. The samples were collected by vertically discrete plankton hauls in three strata of the water column: surface (0~2 m depth), middle (8~10 m depth), and bottom (18~20 m depth). The total number of small yellow croaker larvae was 5523, of which 65.2% and 25.8% were postflexion larvae and juveniles, respectively. Most of the individuals were collected in mid-May, and the sum of the quantity in May accounted for 94.8%. In all cruises, the highest abundance occurred at the bottom, while the surface maintained the lowest abundance. The flexion larvae, postflexion larvae and juveniles moved up to the surface and middle at night, but there was no significant difference between night and daytime for preflexion larvae in each stratum. The postflexion larvae and juveniles showed low temperature responses and a preference for the bottom habitat, respectively, while the preflexion and flexion larvae had weaker selectivity to the water layer and occurrence time. The abundance of larvae and juveniles in the middle layer was significantly higher at flood tide than at ebb tide, and it was more dominant at night than during the day. The study indicates that larvae and juveniles of the small yellow croaker change their located depth with diel vertical migration and utilize the faster speed of the mid-upper layer, which contributes to their ingress into the Yangtze River estuary nursery ground.