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

Abstract

The abundance of the European eel has seriously declined during recent decades. Hydropower production is one of the main threats, and solutions at power stations are needed to reduce the mortality of the downstream migrating silver eel. We examined the mortality, migration routes, and behavior of silver eel at a power station in Germany, after the power station was rebuilt to reduce the mortality of downstream migrating fish. Of 270 eels implanted with radio transmitters and released upstream of the power station, 222 eels passed the power station, primarily in October and November, although some descended during winter and spring. Most eels followed the main flow and passed over the spillway gate (59% and 49% in the 2 study years) or followed the route toward the bar racks in front of the turbines (24% and 27%), where they were guided to a route outside the turbines via the flushing channel. Some eels used the vertical slot fish passage (12% and 8%), whereas few used a nature‐like fishway, canoe pass, or custom‐made bypasses for eel. The eels showed large individual variation in migration timing, migration speeds, and choice of bypass. No eels were killed in the turbines, as none passed through them, likely due to the narrow bar spacing of the racks (10 mm). The results demonstrated that the mortality of eel passing power stations can be low (0–4% and 0–8% in the 2 study years) when the turbine intake is covered by racks hindering eels from entering turbines and safe bypass routes are available. Mortality estimates are given as ranges because the fate of 4% and 8% of the individuals could not be determined. Potential mortality could have been related to injuries in the bypass routes or increased predation risk, but there were no indications of injuries caused by installations in any of the bypass routes.

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).…”

“…Bottom bypasses have been suggested over surface bypasses for eel (Dumont & Hermens, 2012; Gosset, Travade, Durif, Rives, & Elie, 2005; Klopries, Deng, Lachmann, Schüttrumpf, & Trumbo, 2018); however, surface bypasses have also been reported to benefit eel migration (Travade et al, 2010). Redirection would, however, only work if there are other possibilities to migrate and it should be taken into account that eels follow migration routes with the largest portion of water flow (Jansen et al, 2007; Økland et al, 2019; Trancart et al, 2018; Travade et al, 2010). Only 5%–9% of the eels migrating through a river power station in Germany (Økland et al, 2019) entered a specially built side bottom bypass and 1% a side bypass.…”

“…Redirection would, however, only work if there are other possibilities to migrate and it should be taken into account that eels follow migration routes with the largest portion of water flow (Jansen et al, 2007; Økland et al, 2019; Trancart et al, 2018; Travade et al, 2010). Only 5%–9% of the eels migrating through a river power station in Germany (Økland et al, 2019) entered a specially built side bottom bypass and 1% a side bypass. Most eels, however, passed over a spillway gate (59%–49% in two study years) or continued the migration towards the turbines (24%–27%), where they were redirected to a flushing channel.…”

Migration of silver eel, Anguilla anguilla, through three water pumping stations in The Netherlands

“…Fish‐friendly pumps will increase fish survival, but could still form a blockage during migration for some of the eels. Fish passages have been installed at many locations near man‐made structures, such a turbines of hydropower stations, to benefit fish migration (e.g., Dainys, Stakenas, Gorfine, & Lozys, 2018; Fjeldstad, Pulg, & Forseth, 2018; Klopries, Deng, Lachmann, Schüttrumpf, & Trumbo, 2018; Økland et al., 2019; STOWA, 2012). However, the efficiency of fish migration facilities is not always as high as anticipated (Egg, Mueller, Pander, Knott, & Geist, 2017; Silva et al., 2018).…”

Deep Learning for Automated Detection and Identification of Migrating American Eel Anguilla rostrata from Imaging Sonar Data

“…The dramatic decline of these eel populations is attributed to several factors including the construction and operation of hydroelectric facilities, degradation and pollution of habitats, commercial harvest, and changes in ocean-atmospheric conditions affecting their marine life stages [7]. Hydroelectric facilities affect the species in at least two ways: (1) the dams impede upstream migration of juveniles [8], and (2) adult migrants are exposed to risk of injury and mortality when they pass downstream via hydroelectric turbines [9,10].…”