Tests of two types of bypass for downstream migration of eels at a small hydroelectric power plant

Abstract

Efficiencies of two types of bypass, a surface and a bottom sluice, were tested for the natural downstream migration of silver eels Anguilla anguilla at a small hydroelectric power plant at Halsou, on the River Nive in France. Naturally migrating eels were caught after their passage through either bypass. A total of 637 eels were trapped during the three-year study. Total efficiency for both bypasses, evaluated on the basis of downstream movement of radiotagged eels, ranged from 56% to 64%. Given a bias due to hydrological conditions at the time of the runs, the precise efficiency of each separate bypass was not calculated. However, preferred passage through the bottom bypass for both tagged and untagged eels was confirmed by telemetry, as three to four times as many eels transited through the bottom bypass compared to the surface one. The behaviour of 74 individuals released in the forebay was observed by radiotelemetry. Close to half of the radiotagged eels returned up the headrace after their release, and most eventually migrated downstream over the dam with appropriate environmental conditions. Upon arrival at the power plant, eels displayed foraging behaviour in the forebay with frequent displacement interrupted by long resting periods in zones with low current. The repulsive effect of the trashrack located in front of the turbine intake increased with increasing turbined discharge. The study indicated that a trashrack with a smaller bar-spacing (around 20 mm), associated with an appropriate bypass, could deflect a large proportion of the female eels from the turbines. However, this solution needs to be tested on site to quantify the risk of mortality due to impingement on the trashrack.

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

“…Eels did not pass through the orifice, which was situated half way up the gate, supporting the results of other studies which report eels to be principally benthic oriented during freshwater migration (Jonsson 1991;Tesch 2003) exhibiting a preference for undershot pathways (Gosset et al 2005;Russon and Kemp 2011a;Russon and Kemp 2011b). Reduced delay when the orifice was operational may have been due to the gates being open on more occasions when eels initially approached them, or the lower upstream salinity that may have resulted from the lower tides that occurred during these periods, as diadromous fish may need to adapt to abrupt salinity gradients (e.g.…”

Forecasting animal migration using SARIMAX: an efficient means of reducing silver eel mortality caused by turbines

“…Currently, the European Union Directive 1100/2007 of 18 September 2007 recommends that mitigation measures be introduced by hydropower facilities. One option is to prevent the migrants from passing by turbines, by orienting them to a safe route using controlled spillage (Watene & Boubee 2005), eel-suitable downstream bypasses (Gosset et al 2005) and mechanical and behavioural deflection technologies (Russon et al 2010). A second option is to develop fish-friendly turbines to limit mortality (Hecker & Cook 2005).…”

Escapement, route choice, barrier passage and entrainment of seaward migrating European eel, Anguilla anguilla, within a highly regulated lowland river

“…In year 2 a 1 large piece of woody debris became lodged upstream of the undershot sluices and 2 although the gates remained partially open for much of the time, eels were delayed 3 for long periods and few ultimately passed. Silver eels have been observed to exhibit 4 predominantly benthic-oriented movement at barriers during their migration 5 downstream, and prefer undershot routes at structures (Behrmann-Godel and 6 Eckmann, 2003;Gosset et al, 2005;Russon and Kemp, 2011a;Russon and Kemp, 7 2011b). Although benthic oriented passage was possible for eels at Flatford despite 8 the debris, the abrupt velocity gradients near the constricted openings may have 9 induced the avoidance behaviour observed (Coutant and Whitney, 2000).…”