International Journal of Marine Science, 2025, Vol.15, No.3, 118-129 http://www.aquapublisher.com/index.php/ijms 122 complement or even expand the list of species surveyed in traditional surveys. On the other hand, traditional methods still have certain advantages in quantitative terms. At present, eDNA readings can only roughly reflect the relative abundance of species, and it is difficult to directly correspond to the number of individuals or biomass in the community. Visual census can estimate information such as population density and body length structure. Therefore, some scholars advocate combining the two methods and complementing each other's strengths (Muenzel et al., 2024). For example, Muenzel et al. (2024) proposed to use eDNA to quickly screen species lists, supplemented by fine visual surveys of key species to obtain quantitative parameters, so as to take into account breadth and ensure accuracy in protected areas planning. Of course, there are also a few cases where eDNA fails to detect visually discovered species. In Gold and other studies, a few quasi-perids fish cannot be distinguished because they share the same 12S sequence, and a very small number of cave-born fish that only appears visually (maybe due to extremely low DNA release). This suggests that the species resolution and specificity of eDNA primers, as well as the species DNA generation and diffusion mechanism, will affect the detection results. 4 Current Status of Research on Fish Diversity in Coral Reefs in Hainan Island 4.1 Overview of fish community structure and diversity The composition of coral reef fish in Hainan Island is affected by geographical location and environmental conditions, showing the characteristics of tropical Western Pacific fish species, and is similar to other coral reefs in the South China Sea. According to previous surveys, coral reef fishes along the coast of Hainan mainly include Dambescaridae, Snake family, Grouper family, Parrotaceae, Floyceae, and Floyceae. They include small-scale coral reef settlements, as well as migratory economic fish occasionally entering the reef area (Zhang et al., 2021). The overall species richness is quite high. Shi et al. (2022) used eDNA technology to detect 41 coral reef fish at one time in the waters near the West Island of Sanya. These species cover all major vegetative layers of tropical coral reef ecosystems, including algae-eating fish, plankton-eating fish, benthic invertebrate eaters, and top predators. Traditional field surveys also confirm the high diversity of coral reef fish in Hainan Island: underwater visual census in the reef areas of Sanya and Lingshui recorded about 200 species of fish, including damselfish, mullets and snapperidaceae as dominant groups. However, species composition varies across regions and habitats. Coral reefs around islands (such as West Island and Fenjiezhou Island) are significantly higher in fish species and numbers than those on the mainland shore reefs (Li, 2024). Research in the West Island waters provides the first list of Hainan coral reef fish based on eDNA, indicating that there are common species widely distributed in the Indo-Western Pacific, such as the large blacktip shark (Carcharhinus melanopterus) and the humphead wrasse (Cheilinus undulatus) have been observed, along with some species that are endemic to or rare in the South China Sea, such as the Hainan surgeonfish and the Chinese golden trevally.. 4.2 Key protection and distribution of endemic fish Hainan Island coral reefs are home to a variety of rare and endangered fish and endemic species in China, which have important conservation value. Among them, humphead wrasse (also known as Napoleon fish, scientific name Cheilinus undulatus) is one of the largest coral reef fish in the world. It has been listed as an endangered species by IUCN due to overfishing and is a national second-class protected animal in China. Humphead wrasse has historically been distributed in the sea areas of Sanya, Xisha, Hainan, and other waters. At present, the density of wild populations is extremely low. Its juvenile fish often live in coral clusters in lagoons and lagoons. Traditional diving surveys occasionally record adult fish individuals, and eDNA technology is expected to detect trace DNA, thereby confirming the existence of these occult species. In addition to the humphead wrasse (Cheilinus undulatus), large groupers, such as the brown-marbled grouper (Epinephelus fuscoguttatus) and the camouflage grouper (Epinephelus polyphekadion), also serve as important apex predators within the coral reef ecosystems of Hainan., but due to the fishing of egg-spawning groups, the population has dropped significantly (Zhang et al., 2021). Most of these groupers are national second-class protected aquatic animals or are included in the China Aquatic Wildlife Protection List, and they need to focus on monitoring their population dynamics. There are also some small fish unique to the South China Sea around Hainan Island. Because these endemic species are small in size and difficult to detect, there was a lack of information in the past.
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