Upstream movement of river lamprey through a culvert retrofitted with spoiler baffles under experimental conditions

ABSTARCT Culverts used to convey river water under roads and embankments are one of the most common small-scale barriers to longitudinal fish movements worldwide. Using an open channel flume, this study assessed the ability of upstream migrating adult river lamprey (Lampetra fluviatilis) to ascend a pipe culvert when unmodified (control [C]) and retrofitted with spoiler baffles (treatment [T]) under three flow regimes (low discharge [L], high discharge [H] and high discharge with a raised downstream water level [HD]). Few lamprey attempted to ascend the culvert under low (11%) and high (21%) discharge in both the control and the treatments. Despite a greater percentage attempting to pass (75%) under HD, they frequently failed. Contrary to our predictions, upstream progress was impeded by the spoiler baffles and may reflect low motivation or avoidance of the physical and/or hydraulic conditions encountered. This study emphasizes the need to better understand the factors influencing the behaviour and motivation of fish as they ascend fish passage structures, and of the importance of reporting negative results as fish passage solutions that are promising for some species may be ineffective for others.