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Ichthyoplankton assemblages and their relationship with environmental variables are investigated in waters off the Pearl River Estuary in spring and autumn of 2019. Of 80 ichthyoplankton taxa identified using DNA barcode and morphological methods, 61 are identified to species. The most abundance families (Carangidae, Trichiuridae, Mullidae, and Scombridae) account for 61.34% of the horizontal total catch in spring, while Menidae and Carangidae are the most abundant families identified in autumn, accounting for 89.72% of the horizontal total catch. Cluster analysis identifies three species assemblages in spring, and four in autumn based on horizontal trawls. Relationships between assemblage structure and environmental variables (in situ and remote sensed) are determined by canonical correspondence analysis. Ichthyoplankton assemblage structure appears to be strongly influenced by sea level anomalies, salinity, water depth, temperature at 10 m depth, and distance from shore. We demonstrate the efficacy of using DNA barcode to identify ichthyoplankton, and suggest how these data can be used to protect fish spawning grounds in waters off the Pearl River Estuary.

Summary The length‐weight relationships were determined for eight fish species [Zebrias zebra (Bloch, 1787); Zebrias quagga (Kaup, 1858); Amblyotrypauchen arctocephalus (Alcock, 1890); Gerres japonicus Bleeker, 1854; Apogonichthyoides pseudotaeniatus (Gon, 1986); Setipinna tenuifilis (Valenciennes, 1848); Bregmaceros rarisquamosus Munro, 1950; Bregmaceros nectabanus Whitley, 1941] belonging to six families. Fish samples were collected using gillnets (20 × 10 m, mesh size 0.5 cm) and cage net (200 × 10 × 15 cm, mesh size 0.5 cm) from mangroves of Guangdong, China. Samples were collected quarterly from June 2015 to March 2017.The allometric coefficient (b) of length‐weight relationship varied from 2.76 for Zebrias zebra to 3.38 for Setipinna tenuifilis. Length‐weight relationships for these 11 fish species were determined for the first time.

The Beibu Gulf is considered as one of China’s four major fishing grounds, although the substantial overexploitation of fisheries has led to the collapse of many fish stocks, and to changes to spawning grounds in recent decades. Classifying fish eggs is an important way to monitor the recruitment process and identify the spawning sites of fish. However, the lack of a basis for morphological identification and difficulties in correctly identifying fish eggs based on morphological characteristics has limited scientific studies. In the present study, we identified fish eggs using molecular detection of cytochrome c oxidase subunit I and cytochrome b fragments. Ichthyoplankton surveys were conducted in the spring and late autumn–winter of 2020 in the eastern Beibu Gulf. Among the DNA extracted from the 873 chosen fish eggs, we successfully obtained 541 high-quality cytochrome c oxidase subunit I sequences and 41 high-quality cytochrome b sequences. We successfully identified 212 fish eggs (36.4%) from 32 species; 291 eggs (50.0%) showed ambiguous species delimitation, and 79 eggs (13.6%) could not be identified. Among the identified species, we found 25 species in spring and 25 species in late autumn–winter, out of which 18 species occurred in both seasons. We also obtained high resolution photographs of fish eggs at the species level for further morphological analysis and identification. The present study confirms the efficacy of using molecular methods to identify fish species from eggs and provides valuable information for protecting the spawning ground of economically valuable fish and for managing fishery resources.

As a euryhaline marine teleost, the cobia, Rachycentron canadum, has been characterized by rapid growth, prominent meat quality and strong stress resistance to environmental conditions, making it one of the commercially important candidates of fish for mariculture in China (Zhao et al., 2020). Under the influence of extreme weather and climate change-driven precipitation variability, such as typhoon,