Direct and indirect impacts of pumping station operation on downstream migration of critically endangered European eel

Abstract

Downstream passage of European eel Anguilla anguilla (L.) in catchments with pump(s) for water level management is a major concern. Catchment‐wide acoustic telemetry revealed silver eels quickly migrated downstream through unobstructed reaches (n = 12; mean ± SD = 17.9 ± 1.9 km/day). Fourteen of 17 acoustic‐tagged eels detected at the pumping station (82.1%) retreated back upstream and ten (58.8%) passed through pumps after delays (9.5 ± 11.0 days). Multi‐beam sonar imaging across the intake screen (55‐mm gaps) revealed that peaks in migration occurred during the nights preceding the new moon but 76.7% retreated back upstream. All passive integrated transponder (PIT)‐tagged eels recaptured (n = 56) downstream of a large (2.23‐m diameter) mixed flow pump survived but 96.5% had minor injuries, reduced physical condition and/or abnormal behaviour. By contrast, 64.7% of PIT‐tagged eels recaptured (n = 17) downstream of a small (0.8‐m diameter) axial flow pump died. No acoustic‐tagged eels that passed through the small axial flow pump (n = 10) performed onward migration at sea, unlike “control” eels released downstream (n = 11). This evidence may help develop effective remediation measures, such as operational changes, to maximise escapement of catadromous eel species at pumping stations.

Environmental DNA metabarcoding provides enhanced detection of the European eel Anguilla anguilla and fish community structure in pumped river catchments

“…Despite these measures, the International Council for the Exploration of the Sea (ICES) Working Group on Eels reports that current A. anguilla recruitment remains consistently less than 2% in recent years, with recruitment at 1.9% and 1.4% in 2018 and 2019, respectively (ICES, 2019). Such declines, at least in part, are a consequence of anthropogenic impacts on rivers the focus here being societal reliance on land-drainage pumping stations for water level management (Bolland et al, 2019;Buysse et al, 2014Buysse et al, , 2015Solomon & Wright, 2012). These structures operate by pumping water against the natural gradient to a higher downstream river elevation, regulating river levels in the upstream catchment.…”

“…These structures operate by pumping water against the natural gradient to a higher downstream river elevation, regulating river levels in the upstream catchment. This is a requirement in many areas across the world to enable flood management, agricultural water supply and navigation (Bolland et al, 2019;ICES, 2019;Solomon & Wright, 2012). The overall ecological impacts of operating pumps are not fully understood however, and only recently have concerns regarding their adverse impacts on eels and whole fish communities been highlighted (Bolland et al, 2019;Buysse et al, 2014Buysse et al, , 2015Solomon & Wright, 2012).…”

“…This life cycle means that in pumped catchments, mature eels must pass through pumps to achieve escapement and spawn. It is the necessity to pass through pumps, in addition to their elongated morphology, that makes A. anguilla especially susceptible to entrainment at pumping stations (Bolland et al, 2019;Buysse et al, 2014). Buysse et al (2014) found that mortality rates were 97 ± 5% for a propeller pump, 17 ± 7% for a large Archimedes screw pump and 19 ± 11% for a small Archimedes screw pumpindicating that mortality rates differ between pump types.…”

Environmental DNA metabarcoding provides enhanced detection of the European eelAnguilla anguillaand fish community structure in pumped river catchments

“…While eDNA has been well validated in lentic systems (Hänfling et al, 2016;Handley et al, 2019;Li et al, 2019), it is acknowledged that downstream transportation of eDNA and fluctuations in river flow can influence eDNA detection and spatial resolution in lotic systems (Turner et al, 2015;Bylemans et al, 2018;Pont et al, 2018;Itakura et al, 2019;Milhau et al, 2019;Laporte et al, 2020). Pumped catchments are prone to binary fluctuations in flow, changing from a lentic to a lotic water body with pump operation to regulate river level (Solomon & Wright, 2012;Buysse et al, 2014;Bolland et al, 2019). As pump operation, in most instances, is influenced by rainfall, there are likely seasonal trends to consider as well as smaller scale temporal variation in pumping regimes with currently unknown influences on species detectability.…”

Research, management and conservation of anguillid eels—A global perspective

“…Gubili et al (2019) (2019) reported European silver eel escapement from a German regulated river at 63%. Further, Bolland et al (2019) found that about 4 in 10 migrating silver eels were blocked by pumping stations, with others delayed for several days. This highlights the importance of sharing knowledge across regions and species in relation to common threats.…”