Capture and transport eels around dams/barriers (‘trap and transport’)

Supporting evidence from individual studies

1. A study in 2009–2011 in a river in Schleswig-Holstein, Germany (Prigge et al. 2013) reported that European silver eels Anguilla anguilla that were captured and transported downstream of a hydropower station were detected migrating to the sea and migration speed did not differ between eels released in spring or autumn. Results are not based on tests of statistical significance. After 14 months, 28 of 29 captured and transported eels (97%) were recaptured in the sea and fjords beyond the river mouth and one (3%) in the river. Average daily migration speeds did not differ significantly between eels released in spring (0.1–2.3 km/day) and autumn (0.1–3.3 km/day). From September 2009 to April 2011, eels were caught during their downstream migration in a trap in a fish pass at a hydropower station located 9 km upstream of the river mouth. Authors assumed that all eels in the river originated from stocking due to two hydropower stations blocking upstream migration since 1904. Eels were tagged with two T-Bar anchor tags from March–June and September–November each year and released downstream of the hydropower stations (8 km upstream of the river mouth). Of 274 tagged eels (46–105 cm long), 29 (11%; 18 released in spring, 11 released in autumn) were recaptured by fisherman up to 14 months after release.  

   Study and other actions tested

   Referenced paper

   Prigge E., Marohn L. & Hanel R. (2013) Tracking the migratory success of stocked European eels Anguilla anguilla in the Baltic Sea. Journal of Fish Biology, 82, 686-699.

2. A controlled study in 2011–2014 in a river system in Ontario, Canada (Béguer-Pon et al. 2018) found that American eels Anguilla rostrata that were captured and transported downstream of dams were detected migrating to the ocean in similar proportions to eels that were not transported around dams. Overall, the proportion of eels detected migrating from the river system to the ocean did not differ significantly between eels captured and transported around dams (9%, 10 of 106 tagged eels) and eels not captured and transported (4%, 6 of 138 tagged eels). Each year in 2008–2014, large (>800 mm long) female yellow American eels were captured in commercial hoop nets (1,000–1,800 eels/year), tagged (in 2009–2011), and transported and released downstream of two large hydroelectric dams. In September–November 2011–2014, silver eels (106 transported, 138 not transported) were captured migrating downstream, tagged with acoustic transmitters, and released.  In 2011–2014, tagged silver eels were detected by acoustic receivers (154–186 receivers/year) deployed across a channel connecting the river system to the ocean.  

   Study and other actions tested

   Referenced paper

   Béguer-Pon M., Verreault G., Stanley D., Castonguay M. & Dodson J.J. (2018) The migration of stocked, trapped and transported, and wild female American silver eels through the Gulf of St. Lawrence. Canadian Journal of Fisheries and Aquatic Sciences, 75, 2024-2037.

3. A controlled study in 2014–2015 in a river in eastern England, UK (Piper et al. 2020) found that European silver eels Anguilla anguilla that were captured and transported from reservoirs migrated to the sea in similar proportions to resident eels that were not transported, but migration was slower for transported eels. Overall, the proportion of eels that migrated to the sea was similar for eels captured and transported from reservoirs (69 of 80 eels, 86%) and resident eels that were not transported (27 of 30 eels, 90%). Transported eels took longer to reach the estuary than resident eels (data not reported). In October–December 2014, silver eels were captured with fyke nets in two reservoirs (total 80 eels, average 937–942 mm long), tagged, and released in a river, 9.5 km upstream of the tidal limit. Thirty silver resident eels (average 633 mm long) were captured in the river, tagged, and released at the same location. Eel movements were monitored with an array of 25 acoustic receivers positioned along the river until February 2015. 

   Study and other actions tested

   Referenced paper

   Piper A.T., Rosewarne P.J., Wright R.M. & Kemp P.S. (2020) Using ‘trap and transport’ to facilitate seaward migration of landlocked European eel (Anguilla anguilla) from lakes and reservoirs. Fisheries Research, 228, 105567.

4. A study in 2015 in a river in Ontario and Quebec, Canada (Twardek et al. 2021) found that after capturing and transporting American eels Anguilla rostrata upstream of a hydropower station, most eels remained upstream, and numbers that returned downstream did not differ significantly between two release locations. After three months, 31 of 40 eels (78%) captured and transported upstream of a hydropower station remained upstream. The other nine eels returned to the river downstream, one within 24 h of release. The number of eels that returned downstream did not differ significantly between those released 60 km (two eels) or 166 km (seven eels) from the capture site. On 15 July 2015, forty juvenile eels (440–640 mm long) were captured, tagged and transported upstream of a hydropower station. Half of the eels were released 2 km upstream of the hydropower station (60 km from capture site), and half were released 6 km downstream of a second hydropower station located further along the river (166 km from capture site). Tagged eels were detected by acoustic receiver arrays deployed along a 120-km length of the river for three months after release. 

   Study and other actions tested

   Referenced paper

   Twardek W.M., Stoot L.J., Cooke S.J., Lapointe N.W.R. & Browne D.R. (2021) Fate of translocated American eel (Anguilla rostrata) in the lower Ottawa River and passage behavior at a multichannel barrier. River Research and Applications, 37, 1413-1423.