Improving eel pass efficiency: The role of crest shape and water flow in facilitating upstream juvenile eel migration

River connectivity is crucial for the European eel (Anguilla anguilla) to complete its complex life cycle, which is vital for upstream recruitment to the declining population of this critically endangered catadromous fish. Eel passes, or ladders, are frequently installed on riverine structures, such as dams and weirs, to mitigate barrier effects and restore connectivity for upstream migrating eel. Efforts to optimise the effectiveness of passes have previously focused on the ascent section, quantifying the effects of climbing substrate, longitudinal slope, lateral slope and flow rate. However, conditions at the pass crest also impact the rapidity and success of upstream movements. Using controlled experiments and custom-built eel passes with contrasting crest shapes (curved vs. sloped) and flow directions (ascending vs. descending), we quantified the effect of crest conditions on the attempt success, passage efficiency and speed of ascending juvenile eel. Only three of the four treatments (sloped ascending, curved descending and curved ascending) demonstrated passage efficiencies significantly greater than 50%. Transit speed at the crest was significantly quicker (~3.5 min) in passes with a curved crest shape and ascending flow compared to the control. Our findings indicate that simple modifications to the shape of the pass crest and the configuration of flow delivery can help minimise delay and enhance passage efficiency. This, in turn, will increase upstream migration success and contribute to conservation and management goals, such as the EU Eel Regulation and The Eels (England and Wales) Regulations 2009.