Larval and juvenile fish assemblages in the sandy beach surf zones of Nanji Island, China

Xiaodong Wang^1,2Dubin Zhao¹Qiang Liu¹Tianyu Lu¹Junsheng Zhong^1*Wandong Chen²Shangwei Xie²Shun Chen³

• ¹Shanghai University Key Laboratory of Marine Animal Taxonomy and Evolution, Shanghai Ocean University, Shanghai, China
• ²Nanji Islands National Marine Nature Reserve Administration, Wenzhou, China
• ³National Research Center for Marine Facility Aquaculture Engineering and Technology, Zhejiang Ocean University, Zhoushan, China

The Nanji Islands, a nationally protected marine reserve in China, sustain high biodiversity and productive marine ecosystems. However, fisheries monitoring has revealed significant resource depletion in recent years. To describe the community structure for larval and juvenile fishes, monthly samples were collected from five surf zone stations along the coast of Nanji Island from March 2023 to February 2024. All sampling was conducted during daytime (8:00-17:00). Larval and juvenile fishes were collected using a seine net (1m × 4m, 1 mm mesh aperture), with three replicates performed at each station. A total of 8568 fish were collected, belonging to 35 species, 28 genera, and 21 families. Most of the collected fish were postflexion larvae (89.41%). Engraulidae was the most abundant family, with Engraulis japonicus being the most dominant species, accounting for 91.30% of the total number of larval specimens collected. Valamugil speigleri was the common species, while others were rare species. The highest species number was observed in August (16 species), and the highest catch per unit of effort (CPUE) was recorded in May. Few larval and juvenile fishes were collected from December to March. Margalef’s richness index, Pielou’s evenness index, and Shannon–Wiener’s diversity index showed similar tendencies, with the highest values in August and the lowest in January. Cluster analysis of the data from 12 months indicated a a strong, though not statistically significant, ecological pattern of community separation. The five sampling stations were divided into two groups (group I on the west coast of the island and group II on the east coast). The species number, Margalef’s richness index, and Shannon–Wiener's diversity index showed a positive correlation with temperature, but salinity showed a negative correlation with species number, CPUE and Shannon–Wiener's diversity index. The research contributes essential data necessary for the conservation and sustainable management of Nanji Island's fisheries.