Modify operation of dams/barriers

Supporting evidence from individual studies

1. A study in 2000–2001 in a river in the Taranaki region, New Zealand (Watene & Boubée 2005) reported that a greater number of migrating wild longfin eels Anguilla dieffenbachii and shortfin eels Anguilla australis moved downstream past a dam when a spillway gate was partially opened compared to when it was closed, and most released eels that passed through the gate opening survived. esults are not based on tests of statistical significance. Overall, 39 wild longfin eels and 48 wild shortfin eels were caught downstream of a dam during 2.5 h when a spillway gate was partially opened, whereas 12 longfin eels and 12 shortfin eels (including eight dead) were captured during 9 h when it was closed. Seventy-two of 79 released shortfin eels (91%) survived after passing through the gate opening. Ten additional wild eels (seven longfin and three shortfin) caught alongside released eels showed no signs of injury. On 9 April 2000, a spillway gate in an 82-m high hydroelectric dam on the Patea River was opened by 150 mm from 19:30 to 22:00 h. Wild longfin (510–1,170 mm long) and shortfin eels (370–1,140 mm long) were captured 1.5 km downstream of the dam in a fyke net across the width of the river for 2.5 h during and 9 h after the gate was opened. In March–May 2001, on three occasions, tagged yellow and silver eels (18, 73 and 29 eels; 460–940 mm long) sourced from commercial fishers were released in front of the spillway gate (opened by 50–70 mm). Released eels were recaptured in a net across the base of the spillway. 

   Study and other actions tested

   Referenced paper

   Watene E.M. & Boubée J.A.T. (2005) Selective opening of hydroelectric dam spillway gates for downstream migrant eels in New Zealand. Fisheries Management and Ecology, 12, 69-75.

2. A replicated study in 1997–1998 in two canals in North Carolina, USA (Rulifson & Wall 2006) reported that slotted weirs were used by more adult American eels Anguilla rostrata than stainless steel flap gates to pass upstream through concrete barriers in canals. Results are not based on tests of statistical significance. In two sample years, 8–16 eels (0.006–0.010 eels/hour) passed through slotted weirs, while 0–8 eels (0–0.007 eels/hour) passed through stainless steel flap gates. However, authors noted that in 1998, flap gates in one of the canals were often clogged by floating debris, causing them to close. Two manually-operated slotted weirs (61 cm high x 30 cm wide x 61 cm deep, with a 4-cm wide centre slot) were installed in 1996 and two stainless steel flap gates (152 cm wide x 61 cm deep) were installed in 1989 at one barrier (water control structure) in each of two canals. Slotted weirs were open for the entire sampling period. Flap gates opened passively, depending on water flow. Barriers were located at the entrance to a lake. In February–May 1997 and 1998, adult eels were captured in a trap (made of 13-mm-bar wire mesh) deployed upstream (on the lake side) of each slotted weir and flap gate for 6–8 hours/day over 65–84 days. 

   Study and other actions tested

   Referenced paper

   Rulifson R.A. & Wall B.L. (2006) Fish and blue crab passage through water control structures of a coastal bay lake. North American Journal of Fisheries Management, 26, 317-326.

3. A site comparison study in 2009 at a river mouth in Nieuwpoort, Belgium (Mouton et al. 2011; same study site as Mouton et al. 2014) reported that a greater number of European glass eels Anguilla anguilla passed upstream through a partially opened tidal barrier compared to a closed tidal barrier. Results are not based on tests of statistical significance. An average of 632 glass eels/sampling event were caught upstream of a partially opened tidal barrier, whereas 3 glass eels/sampling event were caught upstream of a closed tidal barrier. Between March and April 2009, glass eels were caught during 12 sampling events at a partially opened tidal barrier (lifted by 0.1 m) and three sampling events at a closed tidal barrier. The barriers were located across the mouth of the Yser River where it joined an artificial basin, 3 km from the sea. Each sampling event began as the tide was rising and ended at high tide.Eels were captured in fyke nets upstream of each barrier (opening size: 2.0 x 2.1 m, length: 10.5 m, mesh size: 1 mm).  

   Study and other actions tested

   Referenced paper

   Mouton A., Stevens M., Van Den Neucker T., Buysse D. & Coeck J. (2011) Adjusted barrier management to improve glass eel migration at an estuarine barrier. Marine Ecology Progress Series, 439, 213-222.

4. A study in 2010 at a river mouth in Nieuwpoort, Belgium (Mouton et al. 2014; same study site as Mouton et al. 2011) found that more European glass eels Anguilla anguilla passed upstream through wider openings in tidal barriers than narrower openings, but opening two barriers rather than one had no effect. On average, more glass eels were caught migrating upstream through a wider opening in a tidal barrier (30 individuals/net, 0.031 individuals/m3) than a narrower opening (9 individuals/net, 0.017 individuals/m3) However, the number and density of migrating glass eels did not differ significantly when one barrier was open (23 individuals/net, density: 0.038 individuals/m2) compared to two (27 individuals/net, density: individuals/0.036 m2). Between March and May 2010, migrating glass eels were sampled 16 times upstream of two tidal barriers at the mouth of the Yser River, during tidal rise. During each sampling event, either one or two barriers were opened, either by 10 cm (narrow) or 20 cm (wide). Eels were captured using a fyke net that covered the entire opening of each barrier (opening size: 2.0 x 2.1 m, length: 10.5 m, mesh size: 1 mm). The volume of water sampled was estimated to calculate eel density. 

   Study and other actions tested

   Referenced paper

   Mouton A.M., Huysecom S., Buysse D., Stevens M., Van Den Neucker T. & Coeck J. (2014) Optimisation of adjusted barrier management to improve glass eel migration at an estuarine barrier. Journal of Coastal Conservation, 18, 111-120.

5. A study in 2015 in a river in Bavaria, Germany (Egg et al. 2017) found that increasing the opening width of an undershot sluice gate at a hydropower plant resulted in more European silver eels Anguilla anguilla migrating downstream through the gate. On average, more silver eels passed through the gate when it was opened to a width of 20 cm (23 eels/0.25 h) than when it was opened to a width of ≤10 cm (2 eels/0.25 h). In October 2015, an undershot sluice gate (6.25 m long, 3.75 m high) was opened at night to widths of 20 cm (42 x 0.25 h sampling periods) and ≤10 cm (3 x 0.25 h sampling periods) during eel migration. The gate was adjacent to a hydropower turbine with a fish protection screen and eel bypass tube in front of it. Silver eel activity was recorded using imaging sonars fixed to a boat in front of the gate.    

   Study and other actions tested

   Referenced paper

   Egg L., Mueller M., Pander J., Knott J. & Geist J. (2017) Improving European Silver Eel (Anguilla anguilla) downstream migration by undershot sluice gate management at a small-scale hydropower plant. Ecological Engineering, 106, 349-357.

6. A replicated study in 2018 and 2020 in a river in Lincolnshire, UK (Carter et al. 2023) reported that when electric pumps were turned off and a gravity sluice opened at night, most European silver eels Anguilla anguilla passed downstream through the sluice and none passed through the pumps. In 2018, 21 of 24 eels (88%) that approached the pumping station, when the pumps were switched off and gravity sluice opened, passed through the sluice. In 2020, sixteen of 18 (89%) eels that approached the pumping station passed through the sluice. The other 2–3 eels that approached in each year did not pass through the pumps or sluice. In both years, no eels passed through the pumps when they were switched off and the sluice was open, but five eels in total passed through the pumps when they were switched on and the sluice closed. In October–December 2018 and September–November 2020, electric pumps were turned off every night (17:00 to 07:00 h) and a gravity sluice was opened specifically for eel passage on 10–16 nights (during the new moon and full moon phase of the lunar cycle). The gravity sluice was opened for periods of 2 h 30 min to 4 h 10 min each night. Adult silver eels caught using fyke nets roughly 200 m upstream of the pumping station, were anaesthetised, tagged and released roughly 6 km upstream. Eels were tracked at the pumping station using acoustic receivers. 

   Study and other actions tested

   Referenced paper

   Carter L.J., Wright R.M., Thomas R.E., Reeds J., Murphy L.A., Collier S.J., Evans O., Baktoft H. & Bolland J.D. (2023) Timing is everything; operational changes at a pumping station with a gravity sluice to provide safe downstream passage for silver European eels and deliver considerable financial savings. Journal of Environmental Management, 347, 119143.