| Summary: | Abstract: Globally, populations of diadromous anguilliform fish, such as eel and lamprey, have experienced substantial declines, partly as a result of habitat fragmentation caused by river infrastructure. In the UK, a new configuration of bristle pass (sidemounted and vertically oriented) has been developed to help upstream moving European eel (Anguilla anguilla) negotiate low-head structural barriers such as gauging weirs. However, the efficiency of this type of anguilliform pass remains untested, despite regional implementation and recommendation of nationwide deployment in England and Wales. This study investigated the behaviour of European eel and river lamprey (Lampetra fluviatilis) as they attempted to pass an unmodified (control), or modified (treatment - with bristle passes installed) Crump weir, under experimental conditions. The experiment was repeated under three hydraulic regimes (low, medium and high velocity) that represent a range of conditions frequently encountered at Crump weirs in the field. Passage and delay were quantified and the influence of hydraulic regime and treatment assessed. Both species were highly motivated to explore their surroundings and move upstream during the trials. Bristle passes helped European eel and river lamprey pass the Crump weir, although interspecific variation in efficiency was evident. Passage metrics and behavioral reasons for interspecific differences are presented and discussed, as is the need for further research on design optimisation.
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