Fish size spectrum characteristics in offshore waters of southern Zhejiang Province

In order to improve the understanding of the spatio-temporal characteristics, seasonal variation and influencing factors of size structure of offshore fish communities in southern Zhejiang Province, we studied the characteristics of fish particle size spectrum and its spatiotemporal variation by means of particle size spectrum and multi-dimensional scale sorting based on the seasonal fishery survey data in 2019. The results show that the shape of annual Sheldon fish size spectrum was generally unimodal, and the size class ranged from −2 to 11, with the highest biomass occurring in the size classes of 6−7, mainly small fishes. The Sheldon fish size spectrum was basically unimodal in four seasons, and there were seasonal differences in the peak and maximum biomass corresponding to the size range and species composition. The fitted curves for the normalized fish size spectrum were all dome-shaped, and the curvature ranged from −0.18 to −0.09, with the maximum in winter (−0.09) and the minimum in spring (−0.18), indicating that the fish biomass was most vulnerable to external disturbance in spring but relatively stable in winter. The results of multidimensional scaling demonstrate that the fish community in this study can be classified into four subgroups in winter and spring, and three subgroups in summer and autumn. The analysis of size spectra by subgroups shows that the nearshore fish community is much affected by external disturbances than the far-shore fish community. In general, the fish community off southern Zhejiang Province is in a disturbed state, which may be related to environmental factors, seasonal migra-tory and sedentary fishes, as well as the closed fishing periods and fishing activities.