Seven decades of transformation: evaluating the dynamics of coral reef fish communities in the Xisha Islands, South China Sea

Abstract

The Xisha Islands constitute the largest coral reef archipelago on the northern margin of the Coral Triangle and are the largest coral reef archipelago among the islands in the South China Sea. This study employed various survey methods including underwater spearfishing, gill netting, handline fishing, longlining fishing, underwater visual surveys, and environmental DNA (eDNA). Additionally, historical data from 1956 to 2023 were reviewed to comprehensively analyze the adaptation characteristics of the fish community structure in the Xisha Islands. The Xisha Islands have documented a total of 874 coral reef fish species, categorized into 27 orders, 102 families, and 337 genera. Of these species, the order Perciformes demonstrated the greatest species richness, which represents 71.05% of the total fish species documented. The structure of coral reef fish communities in the Xisha Islands has undergone significant changes. Firstly, a significant decline in the population of large-sized and carnivorous fish species was observed. The proportion of large-sized fish species diminished by 6.02% (62 species), and the medium-sized fish species saw a reduction of 3.09% (51 species). In contrast, there was a surge of 9.12% in the small-sized fish species population. The carnivorous fish species experienced a decrease of 4.73% (102 species), while an increase was noted in the population of herbivorous and omnivorous fish species. Secondly, the numbers of orders, families, and genera have also significantly decreased, with noticeable declines in the average taxonomic distinctness (Delta +), the variation taxonomic distinctness (Lambda +), functional richness (FRic), and quadratic entropy (RaoQ). Thirdly, the similarity between fish species at different time periods is relatively low. Among different functional groups, large-sized and carnivorous fish species exhibit the lowest similarity, whereas herbivorous fish species exhibit the highest. The turnover of live coral fish species is also evident. Overall, the coral reef fish in the Xisha Islands are showing a trend towards herbivory, miniaturization, and a simplification in species composition and functionality. This study contributes to a better understanding and prediction of the adaptation trends in fish species composition in the Xisha Islands, which is crucial for ensuring the ecosystem services of coral reefs and for the conservation and restoration of ecosystems.