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Allis shad (Alosa alosa L., 1758) is an anadromous fish globally listed as «critically endangered» (IUCN, 2024). In France, it is estimated that shad has lost 69% (7,000 km) of its historic distribution due to human activities, especially the disruption of connectivity between growth areas at sea and reproduction areas in rivers. After an absence of almost one century, signs of its return in the Seine River have been recorded for the past two decades. Despite scarce data, specimens of both adult and juvenile Allis shad were caught by anglers or recorded at video-counting stations up to 400 km from the sea. To acquire knowledge about the migratory behaviour and routes of these recolonising shads in the navigable lower reaches of the Seine River, an acoustic telemetry study was conducted. Nineteen shads were captured, tagged and released at the first weir from the sea and tracked over a 188 km channelized reach, fragmented by five navigation weirs and locks, up to Paris. Most individuals quickly resumed their upstream migration, providing an opportunity to analyse their behaviour in the face of cumulative obstacles and multiple crossing routes. Our results highlighted the interest of quantifying, in a complementary way, the overall crossing rate, the time spent below weirs waiting for crossing and the cumulative effects of navigation weirs on migration delays. This is the first time that the presumed low effectiveness of these fishways, assessed using a structural analysis by experts, has been confirmed by the low crossing rate observed (7%). Nevertheless, 16% of shads reached the Oise tributary or Paris by using navigation locks voluntarily, as no fish lockage are operated in the Seine River. While all day long trajectories were direct in open-river reaches with median cruising speeds around 0.8 m.s -1 , time delays varied downstream navigation weirs (median time of 27h to 87h) and crossings were observed during daytime. This study revealed that both short-and longdistance repeated retreats and attempts downstream weirs occurred during the upstream migration of shad, probably in an endeavour to locate alternative passage routes. Individual variation in time delay or retreat and attempt movements at navigation weirs affected neither the subsequent upstream migration speed nor the final upstream extent of their migration. The behavioural variation among the recolonising shads, composed of multi-aged and mixedexperience groups, could help to explain their return to the lower reaches of the Seine River.