## 4 Discussion
### Ground Truth Red Tide Map  
As shown in Figure 9, the red tide areas extracted from the RTI and U-Net models may differ from the daily red tide reports provided by the NIFS. The daily red tide report is generated by summarizing fishermen's reports and a daily field survey of red tide blooms [5]. The extent of red tide extent from daily reports can be regarded as the extent of red tide blooms that occurred throughout the day. Because PlanetScope images were acquired at approximately 10:40 GMT+9, the red tide areas extracted in this study may not represent the red tide distribution for the whole day. In addition, daily red tide reports provide information on areas where red tides occur frequently. Therefore, the information in the report is not indicative of red tide information that occurred all over the waters. The actual red tide areas can be recognized as non-red tide areas in the ground-truth data. Hence, these samples were used to train the U-Net model. The difference in the data format between the ground-truth red tide map and PlanetScope imagery may affect the performance of the RTI and U-Net models. While NIFS provides red tide spatial information in polygon format, PlanetScope image are given in raster format; thus, we converted the NIFS datasets in polygon format to raster format. Depending on the spatial resolution in raster format, the areas of red tide produced after conversion can fluctuate, causing uncertainty with regards to the extent of the red tide. Above all, _M. polykrikoides_ is characterized by vertical migration [2; 54; 55]. _M. polykrikoides_ is known to move on the sea surface in the morning (beginning at approximately 8:00 GMT+09), with the surface cell density peaking around 16:00 GMT+09, and decreasing thereafter. In this study, PlanetScope images were acquired at approximately 10:00 or 11:00 GMT+09. Hence, the acquisition time of the PlanetScope images did not correspond to the peak time of the red tide bloom. Therefore, this could explain the lower performance of the RTI and U-Net models.  
In addition to _M. polykrikoides_ blooms, a variety of harmful and non-harmful blooms occur frequently near the coast. The optical properties of these red tide species are very similar, and it is not easy to distinguish _M. polykrikoides_ from other red tide species. In order to distinguish these red tide species, it is essential to collect the ground truth for each red tide species. However, NIFS provides red tide information focused on _M. polykrikoides_ blooms because the red tide monitoring system was established after a massive fish-kill by _M. polykrikoides_ blooms in 1995. For this reason, information on the occurrence of other red tide species is difficult to obtain without a field survey. In fact, NIFS provided only information on _M. polykrikoides_ blooms between 24 July and 2 August 2018, when the PlanetScope images were acquired. If blooms were caused by red tide species not provided by NIFS during this period, this would cause a difference between the extracted areas and ground-truth areas. Although the red tide events were detected from PlanetScope images, the events were not monitored by NIFS. Therefore, it needs to be considered as a potential red tide area.