IMIS

The Seine River and its tributaries (France) historically hosted an abundant Allis shad (Alosa alosa) population. The construction of several navigation weirs, channelization works and deoxygenation due to pollution downstream of Paris led to the extinction of this population in the early 20th century. After almost a century of absence, shad has been recolonizing the Seine basin since the 2000s. Video monitoring at the first weir on the lower Seine River now records several hundred to a few thousand individuals each year, and dozens are also observed at the 12th weir, located 390 km from the sea on the Aisne River, showing that part of the individuals migrates far upstream on this axis. However, knowledge about this population is limited and its spawning grounds remain unidentified. Furthermore, the passability and behaviour of shad in front of navigation weirs is unknown. These structures present two potential passage routes for migrating shad: fishways and navigation locks. 
To improve our understanding of shad migration behaviour in this system, a telemetry study was conducted during the 2024 migration season in the Oise River, one of the main tributaries of the Seine basin. A total of 50 shad were caught by angling and with a cage-trap located in the fishway of the first weir. Individuals were tagged with Innovasea V9 69 kHz acoustic transmitters and their movements were monitored by a receiver array deployed around eight navigation weirs located within a 90km-long river stretch. Individuals had already migrated 290 km and passed four to five weirs from the sea before tagging, meaning that the study focused on individuals already in the later stages of their spawning migration.