Aerial photogrammetric monitoring on spawning migration of pink salmon Oncorhynchus gorbusha using consumer–class UAVs applied to topographic conditions of the rivers in Sakhalin

Methodology for the aerial photogrammetric counting of pacific salmon spawners with serial unmanned aerial vehicles (UAVs) is developed and tested for the rivers of southeastern Sakhalin on example of pink salmon Oncorhynchus gorbuscha. In total, 22 water streams with the total length of about 230 km were surveyed during the spawning migrations in July-August of 2022–2024. Traditional methods of the counting are labor-intensive and ineffective in this area because of hard relief and high afforestation of the river shores, but operational monitoring with consumer-class UAVs (DJI Phantom 4 Pro V2.0, DJI Mini 2, DJI Matrice 300 RTK) is available. The optimal parameters for aerial survey have been established: the flight altitude 20–100 m, UAV speed up to 6 m/s, longitudinal overlap of the images ≥ 80 % and the transverse overlap ≥ 40 %. The materials of 88 flight missions are processed in Agisoft Metashape Professional software package and orthophotoplans with resolution of 1.0–1.5 cm/pixel are obtained, suitable for visual identification and counting of fish using the geoinformation system NextGIS QGIS. Effectiveness of different UAV models is compared. The main limitations of the method concerned to weather conditions and the riverbed cover are defined. The developed methodology is an effective and economically feasible tool for operational control of spawning that can be used for the fishery regulation and evaluation of reproduction efficiency for pacific salmon in the Sakhalin Region.