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|a As part of flood protection and land reclamation schemes, tide gates allow rivers to discharge to sea when open, and prevent salt water intrusion when closed. Their impact on diadromous fish migration between essential spawning and rearing habitats, and the effectiveness of mitigation measures, have received little consideration. The River Meon, UK, discharges to sea through four top-hung counterbalanced tide gates. In March 2012, the gates were replaced with new ones of the same design, but with an orifice installed in two of them partly to improve fish passage. Sixty downstream migrating juvenile sea trout, Salmo trutta, were trapped approximately 4.9 km upstream of the tidal limit and tagged with acoustic transmitters in April 2011 (n = 30) and 2012 (n = 30). Tagged individuals were detected by acoustic receivers placed near the tide gates before (year 1) and after (year 2) orifice installation. Of the fish that approached the tide gates, 95.8% and 100.0% successfully passed in years 1 and 2, respectively. The speed of migration at the gates was slower than for upstream and downstream reaches, and was positively related to percentage of time the gates were open. Presence of the orifices did not influence delay. Overall, top-hung tide gates delayed sea trout migration, potentially increasing the risk of predation and energy expenditure during the vulnerable juvenile life stage.
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