Size-dependent mortality of migratory silver eels at a hydropower plant, and implications for escapement to the sea

• Published in: Freshwater Biology
• Main Authors: CALLES O., OLSSON I. C, KEMP P. S, BLUNDEN L, SCHMITZ M, GREENBERG A., COMOGLIO, Claudio
• Other Authors: Calles, O., Olsson, I. C., Comoglio, Claudio, Kemp, P. S., Blunden, L, Schmitz, M, Greenberg, A.
• Format: Article in Journal/Newspaper
• Language: English
• Published: 2010
• Subjects: Anguilla anguilla; catch data; passage succe; rack impingement; telemetry
• Online Access: http://hdl.handle.net/11583/2370249; https://doi.org/10.1111/j.1365-2427.2010.02459.x

The European eel population has decreased drastically during recent decades, and new EU-legislation calls for measures to change this negative trend. This decline has been attributed to a number of factors, including habitat fragmentation by structural barriers that prevent eels moving between freshwater and the sea. The success of downstream migrating adult silver eels migrating past a hydroelectric plant (HEP) in Sweden was examined by radio-telemetry, and the results were considered in a historical context by analysing catch data from the river for 1957–2006. The choice of routes and passage success were quantified for three treatment groups and one control group of silver eels. The first treatment, the reservoir group (n = 50), was released into the reservoir upstream of the HEP, and these fish could proceed downstream by passing through the HEP (20 mm rack and turbines) or by entering the spill gates into the former channel, bypassing the HEP. The second treatment group (inside rack, n = 15) was released downstream of the 20-mm rack and had to pass through the turbines to continue migration to the sea. The third treatment group consisted of dead radio-tagged eels (n = 6) that were released into the turbines to study the extent of drifting by dead individuals. Finally, the control group (n = 50) was released downstream of the HEP to test for effects of confounding factors. Most live individuals displayed migratory behaviour and continued to proceed downstream after release. Only 8% of the fish released in the reservoir or downstream of the HEP (control) did not migrate. The probability of reaching the next HEP, 24 km further downstream, was high for the control group (96%) and the reservoir-released individuals that passed the HEP via the spill gates and the former channel (83%). Survival was low and size-dependent for the individuals that passed the turbines (40%) and even lower for the individuals that had to pass through the rack and the turbines (26%). The overall passage success for eels released in the ...