Install vertical-slot fish passes

Supporting evidence from individual studies

1. A study in 1995–1997 in a river in Queensland, Australia (Stuart & Mallen-Cooper 1999) found that a vertical-slot fish pass at a tidal barrage was used by adult longfin eels Anguilla reinhardtii to travel upstream but not by longfin elvers, and eels used the fish pass more during the night than day. During 38 days, a total of 289 adult longfin eels travelled to the top of the fish pass. Longfin elvers were reported to be abundant at the bottom of the fish pass, but none were captured at the top. Adult longfin eels used the fish pass more at night than during the day (data not reported). In 1994, an existing pool-and-weir fish pass (in a concrete channel, 41-m long x 1.8-m wide) was modified by removing overflow baffles, extending the height of the channel to 1.7 m, and installing 17 vertical-slot baffles (each 0.15-m wide) made of marine plywood. The vertical-slot fish pass contained 16 pools (each 1.95-m long x 1.83-m wide) and one longer upstream pool (9.65 m long). From October 1995 to February 1997, eels were captured in traps placed at the top and bottom of the fishway for 24 h each on consecutive days (total 38 paired days).  

   Study and other actions tested

   Referenced paper

   Stuart I.G. & Mallen-Cooper M. (1999) An assessment of the effectiveness of a vertical-slot fishway for non-salmonid fish at a tidal barrier on a large tropical/subtropical river. Regulated Rivers-Research & Management, 15, 575-590.

2. A study in 2013 in a river near Visé, Belgium (Nzau Matondo et al. 2017) reported that a vertical-slot pass at a dam was used by fewer European eels Anguilla anguilla to travel upstream than a pool-and-weir pass. Unless stated, statistical significance was not assessed. Over six months, 164 eels (261–836 mm long) were captured in the vertical-slot fish pass, whereas 271 eels (196–765 mm long) were captured in the pool-and-weir fish pass. After tagging and releasing the captured eels downstream, there was no significant difference in the number of tagged eels passing through the two types of fish pass for a second time (vertical slot: 60 eels detected, 16 eels recaptured; pool-and-weir: 84 eels detected, 11 eels recaptured). Alongside a dam at a hydropower station, a pool-and-weir fish pass (48 m long) was installed in 1980 and a vertical-slot fish pass 305 m long) was installed in 1998. From April to September 2013, eels were captured in two cone traps in the-pool-and-weir pass and eight net traps in the vertical-slot pass. The vertical-slot pass had higher discharge, larger pools and deeper slots than the pool-and-weir pass (see paper for details). Captured eels were radio-tagged, released 0.3 km downstream of the dam and either recaptured or detected with an antenna upstream of each fish pass. 

   Study and other actions tested

   Referenced paper

   Nzau Matondo B., Benitez J.P., Dierckx A., Philippart J.C. & Ovidio M. (2017) Assessment of the entering stock, migration dynamics and fish pass fidelity of European eel in the Belgian Meuse River. River Research and Applications, 33, 292-301.

3. A study in 2014–2016 in a river near Bonn in Germany (Økland et al. 2017) found that a vertical-slot fish pass at a power station was used by low numbers of European silver eels Anguilla anguilla to migrate downstream. In two experiments, a vertical-slot fish pass was used by 7 of 91 (12%) and 9 of 74 tagged eels (8%) that were tracked passing the power station. The remaining eels passed via a spillway gate or ice gate (36–54 eels, 49–59%), a surface bypass via a rack and debris flushing channel (20–22 eels, 24–27%) or a natural fishway or canoe pass (2–3 eels, 2–4%). In 2014 and 2015, silver eels were captured in the river (134–136 eels/year, 60–112 cm length), fitted with radio tags, and released 10 km upstream of the power station. Stationary receivers were placed at migration routes past the power station, including a vertical-slot fish pass installed for upstream migrants (details not provided). Tagged eels were recorded passing the power station from October 2014 to July 2015 and October 2015 to May 2016. 

   Study and other actions tested

   Referenced paper

   Økland F., Teichert M.A.K., Havn T.B., Thorstad E.B., Heermann L., Sæther S.A., Tambets M. & Borcherding J. (2017) Downstream migration of European eel at three German hydropower stations. Norwegian Institute for Nature Research (NINA) report, 1355.