Abstract? The ability of individual and groups of 30 migrating adult river lamprey Lampetra fluviatilis L. to pass a Crump or flat-v gauging weir under two discharge regimes (moderate and low) was assessed in an experimental channel. Despite repeated attempts by the lamprey, the Crump weir remained impassable during all trials. Lamprey passage over the flat-v weir occurred only during group trials at low discharge (5.73 ± 0.19 L s?1) and only as a single burst swimming event via the deeper water passing over the centre of the weir. Lampreys generally approached the weirs along the channel walls and particularly favoured the true right wall associated with elevated velocities during low discharge and reverse currents at moderate discharge (68.06 ± 2.41 L s?1). Time spent immediately below the weir was lower than expected compared with further downstream. Rate of weir approach, attempts to pass (absolute number and as a proportion of the total approaches) and time spent immediately downstream of the weir were highest for the Crump weir at low discharge and the flat-v weir at moderate discharge. The present study suggests that gauging weirs may severely impede the movements of migrating adult river lamprey under low to moderate discharges.<br/><br/
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