APPLIED ISSUES: Size‐dependent mortality of migratory silver eels at a hydropower plant, and implications for escapement to the sea

Abstract

1. 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. 2. 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. 3. 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 reservoir was 30%, including both routes. 4. Annual catch data from 1957 to 2006 showed that the number of eels in the River Ä tran has decreased. Despite this decrease, escapement biomass has remained unchanged, because of the fact that the mean size of eels has doubled. Passage data from 2007 show that changes in size and abundance have resulted in a reduction of relative escapement to the sea to values that are 21-24% of what they were in 1957-66. However, this low level of 2167 escapement could potentially be rectified if appropriate measures facilitating HEP passage are successfully implemented, since the potential escapement biomass in the river, owing to the large size of the eels, has changed little since the 1950s.

Movements of dead fish in rivers

“…Box 5685, Sluppen, 7485 Trondheim, Norway Full list of author information is available at the end of the article Calles et al [2] performed one of the few studies in which fish was experimentally released to assess the extent of drifting by dead individuals. Based on release of six dead European eels Anguilla anguilla into hydropower turbines, drift of dead fish up to 4.2 km downstream was demonstrated.…”

“…The aim of telemetry studies is often to determine the fate and mortality rates of fish marked with electronic tags [e.g., 1,2]. A moving fish is usually regarded as alive, while a long-term stationary fish is regarded as deadand the site where it became stationary is regarded as the site where it died [e.g., 3,4].…”

Effects of Renewable Energy Production and Infrastructure on Wildlife

“…Because the risk posed by turbines increases with body length, long-bodied species such as freshwater eels (Fam. Anguillidae) often are more severely affected by the downstream barrier effect of hydroelectric infrastructures (Calles et al 2010 ).…”

Impact of Tide Gates on the Migration of Adult European Eels, Anguilla anguilla

“…To date, eel escapement research has focused on: (1) assessing the barrier effects of structures on upstream migration of juveniles (Knights and White 1998;Piper et al 2012), and (2) the impact of hydropower installations and success or failure of screening for downstream migrating adults (Calles et al 2013;Calles et al 2010;Pedersen et al 2012;Russon et al 2010). Intermittent barriers created by weirs, ramps, culverts, and tide gates, which are considerably more abundant than large structures such as dams (Lucas et al 2009), have received less attention.…”