Mortality of silver eels migrating through different types of hydropower turbines in Lithuania

Abstract

Hydropower plants (HPP) are considered to be one of the major threats to the survival of European eels when they migrate downstream along inland water bodies during the early part of their annual journey to the spawning area in the Sargasso Sea. There are 98 HPPs in Lithuania and thousands throughout Europe. Numerous studies describe HPP induced mortality rates among European eels as variable depending on local, environmental, and technical factors. This heterogeneity in effect complicates theoretical extrapolation to eel mortality arising from specific types of HPP, necessary for effective management of local stocks. Silver eel mortality was estimated for 4 different HPPs in Lithuania. Mortality was estimated using RFID (Radio Frequency Identification, passive integrated transponders) tags and acoustic telemetry in a large HPP (>100 MW) with Kaplan turbines, a small HPP (<1 MW) with a Kaplan turbine and a fish passage, and for the first time in two small HPPs (<1 MW) with CINK turbines. The results supported a hypothesis that the mortality rate of migrating eels depends mainly on the type and size of the turbine. HPP induced mortality varied from 100% in a small CINK turbine down to 25% in the large HPP with Kaplan turbines. The importance of simple mitigation measures was highlighted by 34% of all tagged eels bypassing one of the HPP via an adjacent fish passage constructed for upstream migration of salmonids. The observed differences in mortality provide essential information for long term strategies designed to restore depleted eel populations in Lithuania and other European countries.

Mortality of downstream migrating European eel at power stations can be low when turbine mortality is eliminated by protection measures and safe bypass routes are available

“…Larinier () summarized turbine mortalities in different studies and concluded that turbine mortality is a minimum of 10–20% in large low‐head turbines and more than 50% in the smaller turbines used in most small‐scale hydroelectric power plants. Some turbines may even impose a 100% mortality (Dainys, Stakenas, Gorfine, & Lozys, ; Larinier & Travade, ). Mortality rates of eels when passing through turbines are generally higher than for instance for Atlantic salmon ( Salmo salar ) smolts because of greater body lengths (Larinier, ).…”

Behavioural responses of eel (Anguilla anguilla) approaching a large pumping station with trash rack using an acoustic camera (DIDSON)

“…Searching and recurrence behaviour and upstream swimming enhance the chance of finding these alternative routes if they are in the vicinity and have sufficient attraction by cues. Mortality rates can also be reduced when eels are hindered from entering the turbines, for example covering the entrance with racks with narrow bar spacings and at the same time supplying safe bypass routes (Dainys, Stakenas, Gorfine, & Lozys, 2018; Økland et al, 2019; Travade et al, 2010). A large, fine‐mesh trash rack, however, is technically challenging to construct and operate, and could result in fish mortality due to impingement (Calles et al, 2010; Fjeldstad et al, 2018).…”

Passage survival of European and American eels at Francis and propeller turbines

“…Stocks of European eel Anguilla anguilla (l. 1758) and American eel ( Anguilla rostrata LeSueur 1817) have declined over past decades (Bernaś et al ., ; Bruijs & Durif, ; Dainys et al ., ; Dekker, ; EPRI, ; Travade et al ., ). Among a host of causative factors ( e.g ., overfishing, dam construction, habitat loss, etc .)…”

“…Despite concerns on the fate of silver eels passing through hydro turbines, published literature is relatively sparse compared with that on emigrating clupeids and salmonids. EPRI's (, ) review and of others (Bernaś et al ., ; Bruijs & Durif, ; Calles, ; Dainys et al ., ; Larinier & Travade, ; Travade et al ., ) indicated that earlier passage studies primarily utilised netting systems to capture turbine‐passed emigrating silver eels. A large amount of variation (mortality of between 15 and 38% at large propeller turbines and 50 to 100% at small turbines for European eel) was noted across sites in these uncontrolled field studies (Bernaś et al ., ; Bruijs & Durif, ).…”

“…A large amount of variation (mortality of between 15 and 38% at large propeller turbines and 50 to 100% at small turbines for European eel) was noted across sites in these uncontrolled field studies (Bernaś et al ., ; Bruijs & Durif, ). More recently, however, telemetry techniques have been used to estimate mortality for downstream migration of silver European eel through turbines (Dainys et al ., ; Winter et al ., ). Reported mortality from these studies on European eel at Kaplan (propeller) turbines has ranged from 20 to 38%.…”