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Simple Summary: Coral reef fish diversity forms the foundation of their adaptability. A comprehensive understanding of coral reef fish diversity is crucial for the conservation of both coral reefs and their associated fish species. In this study, we employed three taxonomic diversity indices to analyze the characteristics and changes in taxonomic diversity within a representative coral reef (Meiji Reef) located in the South China Sea from 1998 to 2018. Fish species richness played a significant role in shaping the diversity of coral reef fish families and genera within the South China Sea region. Contrary to findings from 1998–1999, there had been a relative increase in fish family diversity at Meiji Reef during 2017–2018. Distribution patterns among different coral reef fish communities across taxonomic levels exhibited varying degrees of unevenness within the South China Sea region. The changes observed in the taxonomic distinctness index between 1998–1999 and 2016–2018 indicated greater dissimilarity among species relationships and reduced taxonomic uniformity. Coral reef fish are important groups of coral reefs, which have great economic and ecological value. Meiji Reef is a representative tropical semi-enclosed atoll in the South China Sea, with rich fish resources. Based on the data from hand-fishing, line-fishing, and gillnet surveys of fish in Meiji Reef from 1998 to 2018, this study summarized the fish species list of Meiji Reef and analyzed the species composition, inclusion index at the taxonomic level (TINCL), genus–family diversity index (G–F index), average taxonomic distinctness index (Δ+), and variation in taxonomic distinctness (Λ+) and their changes. The results revealed that from 1998 to 2018, there were 166 reef-dwelling fish species on Meiji Reef, belonging to 69 genera, 33 families, and 11 orders, of which 128 species were from 20 families of Perciformes, accounting for 77.10% of the total cataloged species. Regarding the dependence of fish on coral reefs, there were 155 reef-dependent species or resident species (accounting for 93.37%) and 11 reef-independent species or wandering species (accounting for 6.63%). The TINCL of the order, families, and genus of fish in Meiji Reef were very high. The genus diversity index (G index), family diversity index (F index), and G–F index of fish in Meiji Reef were very high, and the G index of fish in Meiji Reef in 1998–1999 was higher than that in 2016–2018. The Δ+ and Λ+ values of fish in Meiji Reef from 1998 to 2018 were 56.1 and 148.5, respectively. Compared with 1998–1999, Δ+ and Λ+ of fish increased during 2016–2018, reflecting that the relatives of fish in Meiji Reef became further distant, and the uniformity of taxonomic relationships among species decreased. The research findings indicated that fish exhibited a high taxonomic diversity in Meiji Reef; however, it also revealed significant fluctuations in the fish diversity of Meiji Reef over an extended period, emphasizing the urgent need for timely protection measures. This investigation significantly contributes to our comprehension of the intricate dynamics governing fish species within Meiji Reef and holds broader implications for biodiversity conservation in tropical marine ecosystems. [ABSTRACT FROM AUTHOR]

The diel vertical migration of mesopelagic fishes in the southern (S-May station and S-Nov station) and central (C-Jun station and C-Dec station) South China Sea was investigated through a series of continuous field surveys conducted in May, June, November, and December 2017. These surveys employed a combination of mid-water trawl and acoustics techniques. The diel migration process, vertical distribution, acoustic migration proportion, and migration pattern of mesopelagic fishes were analyzed. The results revealed that mesopelagic fishes initiated an upward migration before sunset, with the process concluding within 30–120 min after dark. Subsequently, they commenced a downward migration before dawn, which terminated within 10–50 min after sunrise. The mesopelagic deep-sea layers of mesopelagic fishes at the S-May, S-Nov, C-Jun, and C-Dec stations ranged from 360 to 700 m, 350 to 680 m, 350 to 520 m, and 300 to 700 m, respectively. The acoustic migration proportions of mesopelagic fishes at the corresponding stations were found to be approximately 44.5%, 25.7%, 29.8%, and 58.0%, respectively. There were seasonal and regional differences in the vertical migration and distribution patterns of mesopelagic fishes in the South China Sea. A total of 228 species were identified, including 203 fish species, 23 cephalopod species, and 2 shark species. Among these, a subset of 43 fish and cephalopod species exhibited extensive diel vertical migrating behavior. Specifically, this subset comprised 23 lanternfish species, 8 cephalopod species, and 12 other fish species. Lanternfishes were the predominant diel vertical migratory species, while cephalopods also played a significant role in diel vertical migration. The diel migration behavior was found to be slight for Diaphus lucidus, Melamphaes microps, Argyropelecus affinis, and six other fish species. Non-migratory behavior was observed in Sternoptyx obscura, Argyropelecus sladeni, Sternoptyx diaphana, and 13 other fish species. The diel migration habits of 178 additional species of fish, cephalopods, and sharks could not be definitively determined. [ABSTRACT FROM AUTHOR]

With the progression of overfishing along the coast, oceanic fisheries in the South China Sea have attracted increasing attention from bordering countries. Fishing with lights has developed rapidly over the past decade. In this study, we analysed the trend in fishing over the spring fishing season (March–April) from 2012 to 2020 in the open-SCS fishing zone based on nightly satellite. The results indicated that the number of fishing boats detected by satellites were apparently influenced by the phases of the moon. Using data from moonless nights, we estimated that the number of fishing boats increased from ∼400 to ∼2000 over the past decade. These fishing craft in the open-SCS could be classified into large falling net vessels with bright lights and tuna fishing boats with dim lights. The nightly images of large falling net vessels were studied further using records from a typical commercial fishing vessel and, on this basis, we established an algorithm to extract data for this type of craft, whose numbers were estimated to have increased from ∼20 to ∼200 over the past decade. Using this algorithm, we were able to trace the development of these fleets and map out their distribution patterns in the open-SCS. [ABSTRACT FROM AUTHOR]