Impact of tide gates on the upstream movement of adult brown trout, Salmo trutta

Tide gates, used to regulate tidal flow as part of land reclamation programmes, temporally block fish movement by closing during the flood tide. Their impact on the upstream movement of brown trout, Salmo trutta, and other fish species has received little consideration. The River Stiffkey, UK, discharges into the North Sea via three top-hung tide gates, one counterbalanced (Gate 1), and two not (collectively referred to as Gate 2). Three-hundred adult trout were caught between 0.5 and 6.0 km upstream from the gates on 20 separate days between July and December 2011 (n = 15 per day) and implanted with 23 mm half-duplex Passive Integrated Transponder (PIT) tags before being released 15 m downstream from Gate 1 where PIT antennas were located on either side. Overall, gate attraction (percentage of fish released that were detected by at least one antenna) and passage efficiencies (number of fish that passed Gate 1 reported as a proportion of those that approached) were 96.7% and 92.4%, respectively. The operation of an orifice, installed to improve connectivity for adult trout and juvenile eels, did not influence passage efficiency or delay. Of the fish that passed Gate 1 when the orifice was operational, 42.6-55.7% approached the orifice entrance and 70.6-92.3% of these passed through. Individuals that passed through the orifice were larger than those that did not. Movement past the tide gates (median duration = 6.04 h) took 6 times longer than passage through two unimpeded reaches upstream. Duration of passage through the gates was predominately related to the mean angle of gate opening during the time prior to passage, followed by water temperature. Overall, a counterbalanced top-hung tide gate delayed the upstream movement of brown trout, highlighting a need to assess and potentially mitigate the impact of gates with more restrictive opening apertures and durations.