Extraction of transshipment information of offshore fish carrier vessels in Zhejiang Province of China

Fish carrier vessels are engaged in the transportation of catch for high efficiency and effort of vessels, as fishing intensity redoubles in recent years. This study aims to extract the characteristic data in the transshipment of fish carrier vessels at sea in Zhengjiang Province of China, and then indirectly determine the fishing intensity of vessels. Beidou Vessel Monitoring System (VMS) position signals were also used to set the threshold of speed, distance, and time during extraction. If the distance between fish carrier and fishing vessel at sea was less than 50 m, and the duration was longer than 3 VMS position records, the system assumed that a transshipment event possibly happened, where the duration of the encounter, the names of vessels, and the spatial location were also recorded in real time. As such, the possible transshipment events were identified using the Beidou VMS position data in 2018, and then statistical analysis was also made for verification. It was found that there were 28 916 transshipment events between 808 fish carriers and 3 548 fishing vessels. Specifically, 21.0% of transshipment events happened, when the fish carrier vessels were stopped, whereas, 53.7% of transshipment events happened when the fish carrier vessels were sailing at a low speed between 1-1.4 m/s. The transshipment events with a duration of less than 12.5 min accounted for 81.3% of the total. Furthermore, the distribution of hot spots was finally obtained for the transshipment of fish carrier vessels. Additionally, the longest cumulative duration of transshipment was 187 hours at the space grid of 122.5-123°E and 31.5-32°N, followed by 150 h at the space grid of 122-122.5°E and 28-28.5°N. Consequently, it is widely expected to analyze the spatial distribution and the cumulative duration of transshipment events at sea, thereby clarifying the temporal and spatial changes in fishing vessel efforts.