## 4 Discussion  
### The Need for Fishing Vessel Monitoring with Nighttime Remote Sensing  
The effective management of fishing vessels hinges on precise monitoring. Compared with coastal regions, obtaining comprehensive, effective, and accurate data on the operating locations of fishing vessels in the open ocean is significantly more challenging [32]. Currently, most commercial fishing companies operating on the high seas are equipped with vessel monitoring systems, AIS (Automatic Identification System), and fishing logbooks. While vessel monitoring systems and AIS can record vessel positions, the frequency of position updates is limited, sometimes providing only one or two locations per day [31; 32; 33]. This makes it difficult to accurately pinpoint the operating locations of light fishing vessels based solely on monitoring data, especially during transition of different fishing grounds (Figure A4). Furthermore, illegal, unreported, and unregulated (IUU) fishing vessels often deliberately disable their monitoring systems, resulting in untrackable positions [33]. The overlay of typical nighttime remote sensing imagery with AIS data reveals discrepancies between the monitored positions and the actual operating locations of vessels (Figure A4). Figure 8: Dynamic changes in the centroid position of fishing vessels (**A**) and the number of operating vessels (**B**) during the summer fishing seasons from 2012 to 2023 in the central-east stock of the North Pacific. On the other hand, the coverage of vessel monitoring data was sparse in earlier years, with AIS satellite coverage also being low. As equipment coverage increased and satellite constellations were established, data coverage has become more comprehensive in recent years [34; 35]. Only after 2018 did the data coverage become sufficient to allow for a good match with nighttime remote sensing imagery. This discrepancy means that data from vessel monitoring systems cannot be used for long-term quantitative studies, as the coverage in earlier years was comparatively low, whereas nightly remote sensing data can be used for long-term continuous comparative studies, as they adhere to a unified standard. Fishing logbooks not only record operational locations but also contain rich information on yields [28]. However, they lack near-real-time capabilities. Moreover, acquiring data from vessel monitoring systems, AIS, and fishing logbooks is challenging, especially in fishing grounds jointly developed by multiple countries and regions, where data sharing between different countries is often difficult. Nighttime remote sensing offers a publicly accessible and comprehensive coverage technology for monitoring light fishing vessels. To date, research on the dynamic changes in fishing grounds in the study area has primarily relied on the integration of positional data from Japanese commercial fishing vessels' logbooks with ecological and environmental factors [4; 6; 7; 36]. Fishing vessels from different countries and regions often operate in the same areas, and having comprehensive operational information from multiple nations could significantly enhance the ability to forecast fishing ground conditions. Using nighttime light remote sensing, we observed that the number of fishing vessels operating in the central-east North Pacific during 2020 peaked at around 80 vessels, which is closely aligned with the vessel counts recorded by the NPFC. Similarly, AIS data also reported approximately 80 active vessels, further supporting the reliability of the nighttime light-based observations. This strong consistency between remote sensing and AIS-derived ground truth demonstrates the effectiveness of nighttime light remote sensing as a complementary monitoring tool for fishing activities. However, a daily comparison between AIS and nighttime light remote sensing revealed that AIS reported significantly higher vessel counts under cloudy conditions, whereas in moonless conditions, nighttime light remote sensing was able to detect nearly all active vessels.