Install siphon fish passes

Supporting evidence from individual studies

1. A study in 2002 in a river in North Island, New Zealand (Boubée & Williams 2006) found that a siphon fish pass and bypass channel at a hydropower station dam were used by migrating silver eels of two species to travel downstream, although the siphon became blocked after two months. In 2002, a total of 544 silver eels (516 shortfin eels Anguilla australis and 28 longfin eels Anguilla dieffenbachii) used the siphon fish pass and bypass channel to pass the dam (separate results not provided). The siphon was blocked by a large longfin eel in late May 2002, and not used thereafter. On 29 April 2002, a siphon fish pass, consisting of a 120-mm flexible hose hung over the dam spillway, was installed. At the same time, a 100-mm diameter bypass channel was installed in the wall of a 3.5-m high concrete dam (1 m below the crest), consisting of a steel manifold and pipe connected to a wooden chute, opening into a pool downstream of the dam. Eels were captured in nets placed at the end of the siphon (in April–May 2002) and the wooden chute of the bypass channel (in April–June 2002). 

   Study and other actions tested

   Referenced paper

   Boubée J.A.T. & Williams E.K. (2006) Downstream passage of silver eels at a small hydroelectric facility. Fisheries Management and Ecology, 13, 165-176.

2. A replicated study in 2005 in two rivers in the Netherlands (Bult & Dekker 2007) found that siphon fish passes at locks were used by European glass eels Anguilla anguilla to travel upstream. Glass eels were captured exiting the siphon passes during each of 13 surveys (5–1,542 eels/survey) at one site, and 12 of 13 surveys (1–76 eels/survey) at the other site. Overall, greater numbers of glass eels were captured exiting siphon passes (0–1,452 eels/survey) than in eel traps placed in front of each lock (0–262 eels/survey). In spring 2005, a siphon fish pass (110-mm PVC pipe with vacuum pump and control valve) was installed over a navigational lock in each of two rivers, at the edge of estuaries. A conventional eel trap (1.5 x 1 x 1 m floating plastic container with freshwater pumped into it) was placed in front of the lock gates at each site. In April–May 2005, eels were captured in nets placed over the upstream end of the siphon passes and in the eel traps during 12–13 surveys over 10 nights (10 surveys at high tide, 2–3 surveys at low tide).  

   Study and other actions tested

   Referenced paper

   Bult T.P. & Dekker W. (2007) Experimental field study on the migratory behaviour of glass eels (Anguilla anguilla) at the interface of fresh and salt water. ICES Journal of Marine Science, 64, 1396-1401.

3. A study in 2007 in a river in southwestern Sweden (Calles et al. 2012) reported that a siphon fish pass at a hydropower station was used by low numbers of migrating European silver eels Anguilla anguilla to travel downstream. In total, three untagged eels and none of 16 tagged eels passed through the siphon pass. All 16 tagged eels passed through a bar rack across a turbine intake, four of which died (23%). In 2006, a siphon fish pass (200-mm diameter pipe) was installed at the bottom corner of a bar rack (90 mm bar spacing, angled 60° from vertical) across a turbine intake. 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 siphon fish pass in October–November 2007. 

   Study and other actions tested

   Referenced paper

   Calles O., Karlsson S., Hebrand M. & Comoglio C. (2012) Evaluating technical improvements for downstream migrating diadromous fish at a hydroelectric plant. Ecological Engineering, 48, 30-37.

4. A study in 2015 in an indoor channel in the USA (4) found that a siphon fish pass at a simulated hydropower forebay was used by more than two-thirds of American silver eels Anguilla rostrata to move downstream, and all eels that passed through it survived. Overall, 58 of 84 eels (69%) passed through the siphon fish pass. None of the eels that passed through died or had visible injuries. In November 2015, two screens were erected perpendicular to the water flow in a 6-m wide channel. A siphon fish pass constructed from steel and PVC pipe and fittings was installed in one of the screens, with the entrance (31 cm diameter) located 11 cm above the channel floor. The siphon fish pass aimed to transport eels 22 m downstream over a bulkhead to a submerged collection cage. A vacuum pump created a gravity siphon (see paper for details). During each of four trials, wild-caught silver eels (53–100 cm long) were radio-tagged and released in the channel (14–42 eels/trial). Eels were tracked with eight antennas and an underwater video camera for 3 h from dusk. Eels were monitored for signs of injury for 48 h after each trial.  

   Study and other actions tested

   Referenced paper

   Baker N., Haro A., Watten B., Noreika J. & Bolland J.D. (2019) Comparison of attraction, entrance and passage of downstream migrant American eels (Anguilla rostrata) through airlift and siphon deep entrance bypass systems. Ecological Engineering, 126, 74-82.