# Evaluation of Satellite Imagery for Monitoring Pacific Walruses at a Large Coastal Haulout
## 3 Results
During the three study years, we could estimate the start and end dates of the walrus haulout season based on the first and last detections of walrus aggregations: 26 August-16 October 2018, 10 August-21 October 2019, and 2 August-4 October 2020. Periods of persistent cloudiness introduced temporal gaps into the haulout area estimates from optical imagery, but those gaps would often be filled with estimates from SAR imagery. Excluding data from the coarse Sentinel-1 EW images, we detected walrus aggregations that ranged in size from 0.14 to 11 ha. The vertical bars in Figure 2 represent the full range of areas occupied by walruses, as delineated by the three observers. Variability among the observer interpretations was typically small relative to the mean area occupied across all imagery types, except for those from the coarse Sentinel-1 EW. The area occupied by walruses, as interpreted from satellite imagery, qualitatively agreed with the UAS surveys (Figure 2). The magnitude of day-to-day fluctuations in the area occupied, as documented by UAS surveys, emulated estimates obtained from satellite imagery. The size of the area occupied varied from zero to as much as eight or more hectares in each study year (Figure 2), excluding the Sentinel-1 EW estimates. Reasonable agreement between satellite imagery types was often evident when images were collected within a day of one another (again, disregarding the Sentinel-1 EW estimates). Figure 2: Size of the area occupied by walrus aggregations hauled out at Point Lay Alaska during 25 July–15 November 2018, 2019, and 2020 interpreted from various imagery types. Colored symbols indicate the mean estimated area among the three observers and shaded vertical bars indicate the full range of estimates. Symbol color denotes optical (blue) or SAR (red) imagery. The size of areas interpreted from UAS surveys are displayed as gray dots with an embedded “X”. Images interpreted to have no walruses present are plotted with a white circular background to distinguish them from images with a small area of walruses. Eighteen satellite images were collected within 24 h of a UAS survey, and because UAS surveys were often conducted in both the morning and afternoon of the same or consecutive days, 26 UAS surveys were collected within 24 h of another UAS survey (Figure 3). All walrus aggregations apparent in the satellite images were also present at similar locales in the matched UAS imagery, and vice-versa. However, we found substantial differences in walrus aggregation size, even between pairs of UAS images collected \(<\) 9 h apart, revealing just how dynamic the walrus aggregations can be over short time intervals. UAS surveys of walrus aggregations collected within 5-9 h of another UAS survey documented proportional changes in the area occupied by as much as 50% (Figure 3). That magnitude of deviation between paired UAS surveys was evident and somewhat greater when survey times differed by 12-24 h. Notably among the optical satellite imagery types, the range of deviations between the satellite-derived and UAS-derived estimates followed the same pattern as that between the UAS surveys themselves, indicating that the former deviations (those between optical satellite and UAS imagery) could be entirely due to the dynamics of walruses shifting on and off the haulout and/or shifts in haulout density during the relatively short intervening time between data collections. Four SAR images were collected within 24 h of a UAS survey (three Sentinel-1 IW images matched to 8 UAS surveys, and one Sentinel-1 EW image matched to a single UAS survey), and all produced mean area estimates of walrus occupancy that exceeded  
Figure 3: Deviation (percent) between satellite-derived areas of walrus occupancy and those obtained from matched UAS surveys, plotted on the amount of elapsed time between the two surveys (symbols, as in Figure 2).