Fish stock enhancement release is one of the core methods in the construction of marine ranches. The environmental DNA (eDNA) metabarcoding technology has advantages such as convenient sampling and accurate species identification. Moreover, it can well reflect the species biodiversity of fish and has a good correlation with the relative biomass of fish. Exploring the dispersion pattern of eDNA from the enhanced fish in the release area can facilitate the future use of eDNA technology to assess the effectiveness of fish stock enhancement releases. We conducted an experiment at Sanya Wuzhizhou Island Marine Ranch, having collected the water samples one day before (D1) and 1 (D2), 4 (D3), 7 (D4), 16 (D5) and 45 (D6) days after the fish stock enhancement. We applied eDNA metabarcoding technology to track the diffusion dynamics of Lutjanus sebae, L. erythropterus, and Epinephelus coioides. The results show that no enhanced fish were detected before the release, and L. sebae was only detected in water collected at IN2 site in the core area on D3, while the other two species were not detected at any time or site. There was no significant difference in the fish community structure obtained on D2 and D3 after the stock enhancement compared with D1 (p>0.05, r<0.08), while the fish community structure on D4, D5 and D6 differed significantly from that on D1 (p<0.05, r>0.7). The study indicates that the low detection rate of stock-enhanced fish in the area and the poor effect of stock enhancement may be related to the lack of domestication of the fish or the influence of environmental factors on the eDNA concentration of the released fish.

The fishery resource status in the distribution area of the cetaceans in the South China Sea is crucial for cetacean conservation. In this study, 122 water samples were collected during five cetacean surveys in the South China Sea from 2020 to 2023, during which 18 species of cetaceans were sighted. The environmental DNA metabarcoding focusing on mitochondrial Cytb were performed, and a total of 91 fish species were detected, which belong to 1 class, 11 orders, 38 families, and 75 genera. Among them, 23 species were found in both spring and summer, 19 species were unique to spring, and 49 species were unique to summer. Oxyporhamphus Micropterus (Beloniformes) was the dominant fish species in both seasons. There was a positive correlation between the occurrence of Mesoplodon hotaula and Grampus griseus with Exocoetus monocirrhus (Beloniformes), which suggested that cetaceans in the South China Sea may feed on Beloniformes fish. The abundance of Encrasicholina punctifer (Engraulidae) was high in the Xisha Archipelago, while the relative abundance of Hilsa kelee (Clupeidae) was high near Weizhou Island, which indicated that cetaceans prefer Clupeidae and Engraulidae fish. However, due to the abundance of Beloniformes in the Xisha Archipelago, they might also be a food source for cetaceans. Therefore, the fishery management in the Xisha Archipelago should focus on monitoring the resources of Beloniformes, Clupeidae, and Engraulidae fish to ensure the food supply for cetaceans and strengthen the protection.