The Return of the Allis Shad ( Alosa alosa ) to the Lower Navigable Reaches of the Seine: Locks Play a Role

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ABSTRACT In France, it is estimated that shads ( Alosa spp.) have lost 69% of their historical range, mainly due to disrupted river connectivity. After an absence of one century, signs of their return in the Seine River have been recorded over the past two decades: adult and juvenile specimens have been caught by anglers or recorded at video‐counting stations up to 400 km from the sea. To investigate the migratory behaviour and routes of allis shad in the lower navigable reaches of the Seine, an acoustic telemetry study was conducted. Nineteen individuals were captured, tagged and released at the first weir from the sea and tracked along a 188‐km channelized reach, fragmented by five navigation weirs and locks, up to Paris. Seventeen individuals promptly resumed their upstream migration. Our results underscore the importance of quantifying the overall passage rate, the time fish spend waiting downstream of weirs before passing, and the cumulative effects of navigation weirs on migration delays. Low rates of passage through fishways were found, with most shad (93%) passing through locks without any specific management measures for this species. Throughout the day, trajectories were direct in open‐river reaches, with median cruising speeds of approximately 0.8 m s −1 . Time delays varied downstream of navigation weirs (median time of 27–87 h) and passage was observed during daytime. Ultimately, 16% of individuals reached Paris, passing five weirs with a cumulative delay of 6–12 days.

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