Extraction method of stow net fishing intensity distribution in Zhejiang Province

Fishing intensity is one of the important parameters in the field of fishery resources management and assessment. The traditional calculation method cannot satisfy the needs of real-time, large-scale, fast statistical spatial-temporal distribution. This paper proposes to divide the operation stages of fishing vessel production trip based on VMS data; combined with the operating characteristics of stow net fishing vessels, to use the threshold division and density clustering algorithm to extract the net position, and to calculate the fishing duration of each trip; the geographical grid is divided and accumulated within its range. The average fishing duration per square kilometer (h/km²) of each grid was used as the quantitative basis for the fishing intensity of the stow net, and the spatial distribution of fishing intensity was visualized. In this research, the Beidou VMS position data of the stow net fishing vessels belonging to Zhejiang Province in 2017 were taken as the research object. A total of 733 effective operating trips and 6021 net positions were extracted, and the total fishing time was 736 761.78 h. Geographic grids of 0.1° × 0.1° are divided in the range of 121.6°-126.5°E and 27.6°-33.9°N. The stow nets in the first half of 2017 were relatively scattered, and the average fishing intensity of each grid was 11.26 h/km²; relatively concentrated in the second half of the year, with an average fishing intensity of 12.83 h/km²; with the annual average of 14.76 h/km². Among them, in the range of 125.4°-126.1°E and 31.2°-32.1°N there exists the highest fishing intensity, with an average of 28.51 h/km². The net position extraction, fishing duration and spatial distribution of fishing intensity designed in this paper may provide new research ideas for analyzing the operation status of fishing stow nets and the quantification of fishing intensity.