Silver eel downstream migration in fragmented rivers: use of a Bayesian model to track movements triggering and duration★

Abstract

-Obstacles in rivers are considered to be one of the main threats to diadromous fish. As a result of the recent collapse of the European eel, the European Commission introduced a Regulation, requiring to reduce all sources of anthropogenic mortality, including those caused by passing through hydropower turbines. Improving knowledge about migration triggers and processes is crucial to assess and mitigate the impact of obstacles. In our study, we tracked 97 tagged silver eels in a fragmented river situated in the Western France (the River Dronne). Using the movement ecology framework, and implementing a Bayesian state-space model, we confirmed the influence of river discharge on migration triggering and the distance travelled by fish. We also demonstrated that, in our studied area, there is a small window of opportunity for migration. Moreover, we found that obstacles have a significant impact on distance travelled. Combined with the small window, this suggests that assessment of obstacles impact on downstream migration should not be limited to quantifying mortality at hydroelectric facilities, but should also consider the delay induced by obstacles, and its effects on escapement. The study also suggests that temporary turbines shutdown may mitigate the impacts of hydropower facilities in rivers with migration process similar to those observed here.

Freshwater eels: A symbol of the effects of global change

“…Higher river discharge increases migration speed (Tesch, ; Vøllestad et al., ). Reduced discharge delays migration, and eels may even be stopped until the following year if environmental conditions are not favourable (Drouineau et al., ; Durif, Elie, Gosset, Rives, & Travade, ). At last, reduced discharge can lead to higher proportions of eels going through turbines, since at low flow a higher proportion of water is guided through the turbines, leading to higher mortalities (Bau et al., ; Jansen, Winter, Bruijs, & Polman, ).…”

“…Moreover, increased energy costs induced by obstacle passage may have a delayed impact on migration success and fecundity: Silver eels stop feeding during the spawning migration, and their lipid stores are crucial to achieve the oceanic migration and produce oocytes (Van Ginneken & van den Thillart, ). Delays induced by obstacles can impair escapement, especially when the environmental migration suitability window is limited (Drouineau et al., ; Verbiest, Breukelaar, Ovidio, Philippart, & Belpaire, ). At last, similarly to obstacles to upstream migration, obstacles to downstream migration affect specific types of individual: individuals that settle upstream of the obstacle (i.e.…”

“…The discharge regime is also modified by water extraction for human use, agriculture and other industrial processes (Postel & Richter, 2003;Verreault, Mingelbier, & Dumont, 2012). River discharge and rainfall are important triggers (direct or indirect) of silver eel migration (Acou, Laffaille, Legault, & Feunteun, 2008;Boubée et al, 2001;Bruijs & Durif, 2009;Drouineau et al, 2017;Durif & Elie, 2008;Reckordt, Ubl, Wagner, Frankowski, & Dorow, 2014;Trancart, Acou, Oliveira, & Feunteun, 2013). Higher river discharge increases migration speed (Tesch, 2003;Vøllestad et al, 1986).…”

Behaviour of Atlantic salmon smolts approaching a bypass under light and dark conditions: Importance of fish development

“…Passage through spillways or over weirs is almost always considered safe (Larinier and Travade, 2002). Nevertheless, the issue of migratory delay remains for all kinds of barrier, whether hydropower (Keefer et al, 2012) or not (Aarestrup and Koed, 2003;Drouineau et al, 2017).…”

River Continuity Restoration and Diadromous Fishes: Much More than an Ecological Issue

“…Recently, Mateo et al (2017) showed that obstacles to eel upstream migration can indirectly severely impact spawning stock biomass even without direct mortality. Regarding eel downstream migration, Drouineau et al (2017) pointed out that indirect impacts are potentially as important as direct impacts and, that they too, should be taken into account in the future when assessing the impact of obstacles. Restricting the impacts of mortality to direct mortality can lead to an inadequate quantification of those impacts and therefore to an inappropriate prioritization of management actions.…”