The Bidasoa is a river in the Basque Country, North of the Iberian Peninsula. The watershed's area is 684 km2 and the total length of the main river is 66 Km, although there are 105 other permanent streams and rivers that number 497 km. It runs largely from south to north through the province of Navarre, with its downstream last 10 km forming the border between the regions of Gipuzkoa and Nouvelle-Aquitaine. It flows into the Cantabrian Sea in the bay of Txingudi, between the towns of Hendaye and Hondarribia. It is home to the only diadromous fish populations in Navarre, among which Atlantic salmon (Salmo salar) is the most important species, although shad (Alosa alosa), sea lamprey (Petromyzon marinus) and eel (Anguilla anguilla) are also present. The counter was installed by the Regional Government of Navarre as part of an Automatic Migratory Fish Tracking Station in 2025 at the fishway of the first obstacle (Nazas Hydropower Dam) that the fish encounter in their upstream migration from the sea.