Acoustic assessment of fishery resources and spatial distribution in Nan'ao Island area

Based on the acoustics and bottom trawl survey data of fishery resources in the Nan'ao Island area in autumn 2014 and spring 2015, we analyzed the species composition, dynamic and spatial distribution of fishery resources in that area. The dominant species vaired in different seasons. Priacanthus macracanthus was dominant in autumn and Secutor ruconius was dominant in spring. The average quantity and biomass densities of species assessed by acoustic detection were (26 806±39 968) ind·n mile^–2 and (403.68±601.9) kg·n mile^–2 in autumn 2014, and (34 416±38 497) ind·n mile^–2 and (124.5±139.26) kg·n mile^–2 in spring 2015, respectively. Significant difference existed in the quantity and biomass density of fishery resources (P=0.007<0.05; P=0.004<0.05) in single or mixed-fish species captured in autumn 2014. However, the difference in spring 2015 was not significant (P=0.113>0.05; P=0.310>0.05). The difference might be affected by the degree of dominance of the species assessed by acoustics in different seasons. The horizontal distribution of fishery resources was dramatically asymmetrical due to the seasonal schooling behavior of fish species like S.ruconius et.al. Furthermore, variation of TS (target strength) frequency and vertical distribution of single echo detection were obvious in different seasons with the depth of 15 m as the boundary.