Upstream passage windows for anadromous fish at a large barrier in the Rhine-Meuse estuary, 1996-2018

Passage behaviour at barriers of migratory fish is well studied in rivers, whereas little is known about fish movement at estuarine barriers. In response to climate change, more estuarine barriers are likely to be erected in the future, further compromising connectivity for diadromous fish. As these fish species are already severely endangered, there is an urgent need to better understand passage behaviour at barriers to facilitate connectivity. These fish face challenges at estuarine sluice gates, where suitable passage windows occur only infrequently. To enhance fish migration, sluice management at the Haringvliet barrier, located at the mouth of the Rhine and Meuse rivers, was recently adjusted to allow controlled inflow from the sea ("Kier management"). We assessed upstream passage windows for large anadromous fish prior to this adjustment, using long-term (1996-2018) telemetry (NEDAP Trail System^®) and environmental data. Of 1,751 tagged Atlantic salmon (Salmo salar), sea trout (Salmo trutta), and sea lamprey (Petromyzon marinus), 516 (29%) entered the rivers and for 68 fish we could determine the exact conditions during sluice passage. Our results show the barrier effect of the sluices and indicate the importance of river discharge to attract anadromous fish. Sluices discharged only during low tide, with flow velocities depending largely on water level differences between Haringvliet and the sea. Fish passage occurred primarily (72%) at the end of discharge periods, when the incoming tide reduced water level differences and flow velocities were lowest. Our findings also underline species-specific differences in passage conditions. Sluice passage for salmon and sea trout was limited to daylight hours, whereas sea lamprey showed no diel preference. Sea trout took, on average, more time between release and successful passage and showed a wider range in conditions, while sea lamprey had a limited seasonal range, and salmon appeared most selective in their passage conditions. The frequency of windows in time with potential passage conditions was strongly limited for all species under these unnatural conditions. These findings highlight the need for species-specific management to enhance connectivity at estuarine barriers. We recommend prolonging sluice openings during small water level differences to improve upstream passage for anadromous species.