Install bypass systems alongside exclusion devices
Overall effectiveness category Evidence not assessed
Number of studies: 9
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Supporting evidence from individual studies
A study in 1999–2002 in a river in Halsou, France (Gosset et al. 2005) reported that a bypass system located on the river bottom alongside a bar rack at a hydropower station was used by more European silver eels Anguilla anguilla for downstream migration than a bypass located at the surface. Results are not based on assessments of statistical significance. Over three years, out of 70 tagged eels, 15–19 eels (21–27%) used the bottom bypass and 3–5 eels (4–7%) used the surface bypass, while 10–13 eels (14–19%) passed through the turbines, 3–4 eels (4–6%) passed over the weir and one eel (1%) used a fishway. Over three years, a total of 637 eels were captured after passing through the surface and bottom bypasses. A surface bypass (flap gate; 1.4 m length, 0.9 m width) and bottom bypass (‘motorized gate’; 1.2 m length, 1.3 m width) located on the right bank of the forebay (4-m depth) were opened alternately every other day to allow passage of eels. An angled bar rack (‘trashrack’; 30 mm bar spacing, 20° from the vertical, 20 m length and 3 m height) was located adjacent to the bypasses. Between October and January in 1999–2002, a total of 70 silver eels were captured, fitted with radio tags, and released upstream of the power plant. Untagged eels were caught in a trap below the bypasses in 1999–2001.
Study and other actions testedA study in 2002–2003 in a river in North Island, New Zealand (Boubée & Williams 2006) found that a bypass system alongside a bar rack, and a siphon fish pass, at a hydropower station dam were used by shortfin Anguilla australis and longfin Anguilla dieffenbachii silver eels migrating downstream. In 2002, a total of 544 silver eels (516 shortfin eels and 28 longfin eels) used the bypass system and siphon to pass the dam (separate results not provided). In 2003, a total of 744 silver eels (309 shortfin eels, 43 longfin eels, and 392 unidentified eels) used the bypass system. In April 2002, a 100-mm diameter bypass system was installed in the wall of a 3.5-m high concrete dam (1 m below the crest). The bypass consisted of a steel manifold and pipe connected to a wooden chute, opening into a pool downstream of the dam. An additional siphon fish pass, consisting of a 120-mm flexible hose hung over the dam spillway, was installed in April 2002. The siphon was blocked by a large longfin eel in late May 2002, and not used thereafter. A vertical bar rack (‘trash rack’) with 30-mm spacing covered the turbines. Eels were captured in nets placed at the end of the siphon (in April–May 2002) and the wooden chute of the bypass system (in April–June 2002 and March–June 2003).
Study and other actions testedA study in 2004–2007 in a river in Nouvelle-Aquitaine, France (Travade et al. 2010) reported that two of four fish bypass systems alongside bar racks at a hydropower station were used by some silver European eels Anguilla anguilla migrating downstream. During three years, 16 of 116 tracked eels (14%) used a larger bypass to migrate downstream, and one eel (1%) used a small surface bypass. No eels used two other small bypasses (one at the surface, one ‘deep’). The other tracked eels passed through the bar racks and turbines (48 eels, 41%), spillway gates (48 eels, 41%) or an old ‘Denil’ fish pass (3 eels, 3%). A 57-m long dam at a hydropower facility had two small surface bypasses (0.8 m wide, 0.65 m deep), one small ‘deep’ bypass (0.8 m wide, 1 m high, depth not reported) and one larger surface bypass (2.0 m wide, 1 m deep). In the second year, the larger surface bypass was altered to become a ‘deep’ bypass, with the opening 7 m below the water level (see paper for details). Bar racks (‘trashracks’) covered the main turbine intake (30 mm bar spacing, 30° from the river axis, 40 m long and 5 m deep) and a second, smaller turbine intake (20 mm bar spacing, 7.4 m long, 4.6 m high). The dam also contained an old 80-m long upstream ‘Denil’ fish pass. Silver eels from nearby fisheries were surgically tagged and released 2.8–7.5 km upstream of the dam. Tagged eels (average 610–840 mm long) were tracked as they migrated downstream past the dam from October to April for three consecutive years in 2004–2007 (37–40 eels/year).
Study and other actions testedA study in 2007 in a river in southwestern Sweden (Calles et al. 2012) reported that a bypass system alongside a bar rack at a hydropower station was used by some European eels Anguilla anguilla migrating downstream but almost a quarter of tagged eels died passing through the bar rack and turbine. In total, 43 untagged eels and none of 16 tagged eels passed through the bypass system. All 16 tagged eels passed through a turbine intake covered by a bar rack, four of which died (23%). In 2006, a surface gate (3.3 m wide) connecting to a bypass system was installed adjacent to a turbine intake covered by a bar rack (90 mm bar spacing, angled 60° from vertical). In October 2007, forty-two silver eels were caught in the river, radio-tagged, and released 24 km upstream of the hydropower station. Sixteen radio-tagged eels were tracked as they passed one of two powerhouses at the hydropower station. Eels were captured in a trap on the bypass in October–November 2007.
Study and other actions testedA study in 2015–2016 in a river in Bavaria, Germany (Egg et al. 2017) found that a bypass system alongside a protection screen at a hydropower was not used by European eels Anguilla anguilla migrating downstream. Over two years, none of the 1,323 eels that passed the hydropower station used the bypass system, despite approaching a screen located directly adjacent to the bypass entrance. Instead, eels passed through an opening in a sluice gate. The eel bypass (a zig-zag shaped tube) was located on the riverbed in front of a horizontal fish protection screen (15-mm gap size) installed directly upstream of a hydropower turbine. An adjacent sluice gate (6.25 m long, 3.75 m high) was opened at night to widths of ≤10 or 20 cm. During eel migration in October–November 2015 and 2016, silver eel activity was recorded using imaging sonars fixed to boats in front of the screen and gate. A fyke net was placed at the outlet of the bypass to collect eels passing through.
Study and other actions testedA study in 2014–2015 in a river in Falkenberg, Sweden (Calles et al. 2021) found that a bypass system alongside a bar rack at a hydropower station was used by almost half of tagged silver European eels Anguilla anguilla migrating downstream and no tagged eels entered a turbine intake. Over two years, 27 of 59 tagged eels (46%) used a bypass system to pass a hydropower station. The other tagged eels used a nature-like fishway (29 eels, 49%) or failed to pass the hydropower station and remained in the reservoir (3 eels, 5%). None of 39 tagged silver eels that entered the turbine intake channel passed through the bar rack and into the turbines. In 2013, a ‘full-depth’ bypass system was installed adjacent to a turbine intake with a 40-m angled bar rack (15 mm bar spacing, 30° angle relative to the intake banks) at one of two powerhouses at a hydropower station. A hydraulic gate at the bypass entrance had open slots at the top (30 cm wide x 65 cm high) and bottom (20 x 20 cm), and was fully opened periodically to clear debris. A nature-like fishway was installed at a second powerhouse in an adjacent channel. In each of two years, 30 migrating silver eels captured 13–70 km upstream of the hydropower station were tagged and released at dusk either immediately upstream (during four nights in September 2014) or 24 km upstream (during three nights in September–October 2015). Eels were tracked passing the hydropower station using an array of eight antennas. One eel was not detected due to tag failure.
Study and other actions testedA study in 2017 in a river in Falkenberg, Sweden (Kjærås et al. 2023) reported that a bypass system alongside a bar rack at a hydropower station was used by some silver European eels Anguilla anguilla to migrate downstream. Unless stated, statistical significance was not assessed. In total, 38 of 90 eels (42%) used a bypass system alongside a bar rack to migrate downstream. Of those, 22 eels (24%) used a bottom slot in the bypass system, while nine (10%) used a surface slot (slot use could not be determined for seven eels). The remaining 52 eels (58%) used a nature-like fishway to pass the hydropower station. In 2013, a ‘full-depth’ bypass was installed adjacent to a turbine intake with a 40-m angled bar rack (15 mm bar spacing, 30° angle) at one of two powerhouses at a hydropower station. A hydraulic gate at the bypass entrance had open slots at the top (30 cm wide x 65 cm high) and bottom (20 x 20 cm), and was fully opened periodically to clear debris. A nature-like fishway was installed at a second powerhouse in an adjacent channel. On 23–25 September 2017, downstream-migrating silver eels (98 eels, 64–100 cm long) caught in four traps, located 13–17 km upstream of the hydropower station, were tagged and released 20 km upstream. Eels were tracked passing the hydropower station using 33 hydrophones.
Study and other actions testedA study in 2017–2019 in a river in southwestern France (Tétard et al. 2023) reported that a bypass system alongside a bar rack was used by two-thirds of European eels Anguilla anguilla that approached an intake channel at a hydropower station when migrating downstream and most eels did not enter the turbine. Overall, 28 of 45 tagged eels (62%) that approached the intake channel used a bypass system. The other tagged eels passed through the bar rack and turbine (2 eels, 4%) or a 17-m wide gate (14 eels, 31%), or turned back and passed through a dam (1 eel, 2%). In 2015, a bypass system accessed via a flap gate (2 m wide, 1.1 m long) was installed alongside a 17-m wide ‘bear-trap’ gate used to regulate water levels. The gates were located 6 m downstream of an angled bar rack (20 mm bar spacing, 5° angle from vertical, 60 m long and 1.75 m high) across a turbine intake channel. A dam (with a spilling weir and two 15-m wide flap gates) was located 100 m upstream on the opposite bank. Wild silver eels (96–98 eels/year) were trapped in a river, radio-tagged and released 20.5 km upstream of the hydropower station. Forty-five tagged eels were tracked by an array of underwater and aerial radio antennas and by manual radio-tracking in winter 2017–2018 and 2018–2019.
Study and other actions testedA study in 2017–2019 in a river in southwestern France (Tomanova et al. 2023) reported that bypass systems alongside bar racks at four hydropower stations were used by all European eels Anguilla anguilla that approached intake channels when migrating downstream and no eels entered the turbines. At each of four sites, all tagged eels that approached the intake channels (total 52–74 eels/site) passed through entrances to bypasses located in inclined bar racks. None passed through the bar racks into turbines. On average, tagged eels took 1–5 minutes to pass through the bypasses. Four hydropower stations along an 8-km stretch of river had inclined bar racks (20 mm bar spacing, 26° from the horizontal, surface area 71–118 m2) installed across their turbine intake channels. Each bar rack contained three surface entrances (0.5–1 m wide) connecting to a bypass system. Wild silver eels (96–98 female eels/year) were trapped in a river, radio-tagged and released 600 m upstream of the first hydropower station. Tagged eels were tracked using arrays of radio antennas at each of the four hydropower stations in winter 2017–2018 and 2018–2019.
Study and other actions tested
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This Action forms part of the Action Synopsis:
Eel Conservation in Inland HabitatsRelated Actions
| Actions | Effectiveness | Studies | Category | |
|---|---|---|---|---|
Use temporary/seasonal dams Action Link | No evidence found (no assessment) | 0 | Synopsis Link | |
Reduce water abstractions Action Link | No evidence found (no assessment) | 0 | Synopsis Link | |
Install nature-like fishways Action Link | Evidence not assessed | 3 | Synopsis Link |
