Identification of salmon redds using RPV-based imagery produces comparable estimates to ground counts with high inter-observer variability

Abstract

An integral part of population monitoring within fisheries is ground-based surveys of fish redds. Remotely piloted vehicles or drones (RPVs) could provide a complementary method but need verification due to a host of methodological differences. To compare methods, we counted summer Chinook redds (Oncorhynchus tshawytscha) (~6 m2 in size) using RPVs and compared them to ground-based counts in the Wenatchee River (WA, USA). We found individual aerial counts were many times twice the corresponding ground counts. We also found large inter-observer variability among aerial counters. The coefficient of variation among multiple aerial counts were 37%, 38%, and 50% across three sites, which are comparable to published variation in ground counts. We attribute inter-observer variability to inherent uncertainties in redd identification similar to ground counting, and importantly, we did not see evidence that the clarity of substrate in the image influenced observer bias. Overall, our data suggest that redd counting using RPVs is an effective method, particularly in high-density spawning locations. We conclude that RPV imagery accurately identifies redds in a clear, relatively wide (60 m) river, but suggest continuing research into increasing precision, limiting observer variability, and assessing the accuracy across methods and locations.