Rapid and Accurate Monitoring of Intertidal Oyster Reef Habitat Using Unoccupied Aircraft Systems and Structure from Motion

Oysters support an economically important fishery in many locations in the United States and provide benefits to the surrounding environment by filtering water, providing habitat for fish, and stabilizing shorelines. Changes in oyster reef health reflect variations in factors such as recreational and commercial harvests, predation, disease, storms, and broader anthropogenic influences, such as climate change. Consistent measurements of reef area and morphology can help effectively monitor oyster habitat across locations. However, traditional approaches to acquiring these data are time-consuming and can be costly. Unoccupied aircraft systems (UAS) present a rapid and reliable method for assessing oyster habitat that may overcome these limitations, although little information on the accuracy of platforms and processing techniques is available. In the present study, oyster reefs ranging in size from 30 m(2) to 300 m(2) were surveyed using both fixed-wing and multirotor UAS and compared with ground-based surveys of each reef conducted with a real-time kinematic global positioning system (RTK-GPS). Survey images from UAS were processed using structure from motion (SfM) stereo photogrammetry techniques, with and without the use of ground control point (GCP) correction, to create reef-scale measures of area and morphology for comparison to ground-based measures. UAS-based estimates of both reef area and morphology were consistently lower than ground-based estimates, and the results of matched pairs analyses revealed that differences in reef area did not vary significantly by aircraft or the use of GCPs. However, the use of GCPs increased the accuracy of UAS-based reef morphology measurements, particularly in areas with the presence of water and/or homogeneous spectral characteristics. Our results indicate that both fixed-wing and multirotor UAS can be used to accurately monitor intertidal oyster reefs over time and that proper ground control techniques will improve measurements of reef morphology. These non-destructive methods help modernize oyster habitat monitoring by providing useful and accurate knowledge about the structure and health of oyster reefs ecosystems.