# Evaluation of Satellite Imagery for Monitoring Pacific Walruses at a Large Coastal Haulout
## 4 Discussion  
We found that publicly and commercially available satellite imagery can be used to detect the presence, areal size, and geographic position of walrus aggregations at a terrestrial haulout near Point Lay, Alaska. Not only were observers able to consistently determine when walruses were present, they also were able to delineate the extent of aggregations with considerable agreement and consistency with aerial surveys. These findings suggest that satellite reconnaissance may provide a tool for monitoring walrus haulout occupancy and dynamics, as may be required by resource managers. Both the  
Figure 4: Variation among observers in determining both the location and size of walruses from satellite images. The area of 3-way intersection among the observers’ delineations of walrus aggregations in each satellite image (y axis) are plotted on the area of the geographic union of the observers’ delineations. A perfect 3-way spatial agreement would attain a 1:1 ratio. Symbols: as in Figure 2. Linear regression slopes for each imagery type are displayed in Table 1. One Sentinel-1 EW outlier was excluded from this graph: x = 20.11, y = 10.75).  
reliability and precision of detecting walruses with satellite imagery are important criteria for monitoring, and both varied among the different types of imagery we evaluated. For optical imaging, reliability was limited by (1) the mission's revisitation frequency, a predictable factor (i.e., a factor that may be predicted at the start of a study), (2) adequate solar illumination, a predictable factor, and (3) clear-sky conditions, an unpredictable factor. For SAR imaging, reliability was limited by (1) the mission's revisitation frequency and (2) mission-controlled acquisition scheduling. Below, we discuss technical advantages and disadvantages of the different imagery types for monitoring walrus haulouts. Monitoring efforts can achieve increased temporal resolution by combining interpreted data from various imagery types, as evidenced by the temporal resolution of the haulout occupancy across the three study seasons (Figure 2). However, we do not attempt cost assessments of commercial imagery because pricing can vary widely, depending on factors like volume, latency, institutional licensing, and contractual arrangements.  
Coastal walrus haulout sizes can change substantially over short time intervals. During less than five hours, [32] reported a 46% reduction in the number of walruses hauled out at Point Lay. During the 2018 and 2019 UAS surveys, fluctuations of up to 60% in the area occupied were documented between UAS surveys collected 5 to 9 h apart (Figure 3, [25]). These fluctuations were not only due to changes in the peripheral extent of walrus aggregations on shore, but also due to the formation of holes within aggregations which may not be evident in some satellite imagery, depending on resolution. Among UAS surveys conducted within 9 h of a prior survey (n = 10), the estimated number of walrus individuals ranged from 1900 to 14,780 [25]. Such dynamics over the course of only a few hours confounded our attempts to directly compare haulout area estimates from satellite and UAS imagery because most were not collected near-coincidently (on the order of minutes, not hours).