Application of hydroacoustics to investigate the distribution, diel movement, and abundance of fish on Zhubi Reef, Nansha Islands, South China Sea

Abstract

A combination of traditional fish sampling methods (hand-line and gill net) and modern hydroacoustic techniques were used to study fish community structure, distribution, and diel movements of fish on Zhubi Reef to enhance understanding of the ecosystem. We collected 126 individuals from 29 species, 20 genera, 17 families, three orders, and two classes using traditional gear. Perciforms were the dominant group in terms of species richness and biomass. The acoustic data indicated that very small (target strength [TS], dB) -55 dB). The number of single-target detections was significantly higher during the night than during the day (P<0.05). The singletarget TS frequency distribution during the day was significantly different than during the night at depths of 4–8, 8–12, 12–16, and 16–20 m. Significant differences were observed among the 4–8, 8–12, 12–16, and 16–20-m-depth layers during day and night. Diel vertical movement was evidenced as fish began to spread and move upward just before sunset and began to assemble and descend shortly (15 min) after sunrise.