Author(s): R. D. Hedger; A. Foldvik; K. O. Gjelland.
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Abstract: High-resolution aerial photography, whether from traditional high-altitude aircraft or from low altitude UAVs, may be useful in surveying salmon habitats. However, challenges exist with regards to both, matching the remote sensing platform imaging abilities to the spatial and temporal scales of the habitat, and the many confounding factors associated with applying optical remote sensing to rivers. We address this issue and provide suggestions as to how this approach can be optimized. We first examine how well the spatial and temporal properties of aerial survey data relate to the spatial and temporal properties of salmon habitat. We then illustrate some of the potentials of aerial photography to qualitatively and quantitatively describe salmon habitat using traditional aerial photography (AP) and UAV images for selected rivers within Norway. We then examine how the light environment of salmon rivers may limit the ability to extract information. We found that the spatial extent over which UAVs can be legally operated was appropriate for surveying spatial heterogeneity within the individual mesohabitat unit. The flexibility of operating UAVs also meant that they were more suited to monitoring of the dynamic changes in salmon habitat, and the spatial resolution of UAV imagery was sufficient for resolving fine-scale features. However, UAVs are limited in spatial range so are of limited value for compartmentalizing the full watercourse of most salmon rivers into separate mesohabitat units. We also found that the ability to characterize salmon habitat from aerial surveys is constrained by a poor light environment caused by low solar elevation, resulting in low direct irradiance and an increased propensity to shadow from topography and riparian vegetation (although the operational flexibility associated with UAVs could be used to minimize these problems). Additionally, both traditional AP and UAVs were limited in the ability to resolve salmon habitat characteristics by multiple confounding factors (e. g. river bed features being obscured by attenuation or scattering within the water column, or surface white-water; extraction of depth being limited by shadows or reflectance of riparian vegetation). We conclude therefore that the most informative approach to aerially surveying salmon habitat is to combine traditional AP and UAV imaging together, and to conduct the survey in a manner that minimizes problems resulting from the poor light environment.
Year: 2018