When “safe” dams kill: Analyzing combination of impacts of overflow dams on the migration of silver eels

Abstract

1. The drastic decline in European eel Anguilla anguilla stock is now widely recognized. However, while various causes for this decline have been identified, the relative importance of each cause remains unclear. 2. During the catadromous migration of silver eels, the negative impact of dams is frequently highlighted, but mainly for powered dams (with turbines) or where connectivity is completely ruptured. Mechanical impact due to turbine blades is often considered the most important cause of mortality of silver eels during downstream migration. Consequently, non-powered dams equipped with spillways are often considered safe for the passage of migrating silver eels. 3. We hypothesized that, to understand the negative impacts of dams, a much wider context must be considered than turbine mortality alone. Using an acoustic telemetry survey of silver eels, we demonstrated the negative effects of non-powered dams on downstream migration. 4. Five main impacts on eel populations were highlighted: (i) the attenuation or loss of triggering factors, leading to an absence of or delay in migration; (ii) extra delays and extra distances travelled when crossing the dam; (iii) extra energetic costs of the additional distance traveled as result of exploring the dam and the reservoir to find other escape passages; (iv) the selection of a more risky behavioral phenotype, i.e., bold eels; and (v) direct blocking once migration has started. Mortality was evaluated as a supplementary impact. Some of these effects (attenuation of triggers, extra delays to cross the dam) might be more important than the same effect from powered dams, probably due to the constant high water discharge required for turbines that facilitate the passage of eels. 5. As these "safe" dams are very widespread, they must be considered a potential threat to effective eel conservation. Keywords. acoustic telemetry, silver eels, migration, mortality, non-powered dams, turbines drinking water intake 1. Introduction Diadromous species are fishes that regularly migrate between freshwater and marine This study was funded by the 'Agence de l'Eau Loire Bretagne,' the 'Region Bretagne,' and the 'Syndicat Eau du pays de Saint-Malo.' The study was conducted by the Museum National d'Histoire Naturelle. We thank Fish Pass (Yohan, Francois and Mathieu) and Museum National d'Histoire Naturelle teams, as well as everyone who helped with sampling and collecting data. Institutional and national guides for ethical care and use of laboratory animals were followed.

Towards transferability in fish migration models: A generic operational tool for predicting silver eel migration in rivers

“…A gap between predicted and actual migration can also occur when migration delays are induced by obstacles and management actions (e.g. sluice gate operations, dams, and water reservoirs; Trancart et al, 2020), e.g. in the Vilaine River (dam generating an abrupt discontinuity in the estuarine-fluvial gradient) or in the Somme River (recurrent artificial discharge peaks).…”

“…While river obstruction affects accessibility of catchments during upstream migration, dams and hydropower turbines are also sources of mortality during downstream movement (Eyler et al, 2016;Feunteun et al, 2000;Song et al, 2019). Even without power plants, reservoir dams can temporarily or permanently stop fish migration (Acou et al, 2008;Besson et al, 2016;Trancart et al, 2020). By accounting for the influence of environmental cues or behavioural settings, several predictive models have been developed to predict migration events for several diadromous species, including white sturgeon (e.g.…”

“…Among a range of conservation measures, this plan was recommended to reduce turbine mortality, and a number of fish passage facilities were built for that purpose (Nieminen et al, 2017). However, numerous river obstacles remain without fish passage facilities because of structural impossibilities or economic constraints, and thus continue to limit escapement of silver eels to their reproduction areas (Acou et al, 2008;Trancart et al, 2020). In these locations, active solutions can be implemented without modifying the dam structure, e.g.…”

Migrating silver eels return from the sea to the river of origin after a false start

“…The species is considered critically endangered [7]. Their decline is associated with the drastic negative effect of habitat loss, hydropower, overfishing, climate change, pollution, parasites and diseases on spawning stock size [7,8]. Eel spawning stock is determined by the number of migrating silver eels (escapement) from the waters of North Africa to the northern tip of Scandinavia [9,10].…”

“…are favourable and the predation risk is minimal [4]. Failures in migration timing, including delays caused by man-made obstacles, can be costly [8]. It has been shown that the timing of migration is adjusted according to the conditions in freshwaters, estuaries and marine waters [12–16].…”