Assessing the Efficiency of an Elver Ladder Using a Multi‐State Mark–Recapture Model

Abstract

The decline of the European eel (Anguilla anguilla) stock has led the European Commission to enforce a regulation (Council Regulation N°1 100/2007), in which each member state was required to establish an eel management plan. Various measures in the French plan aim at restoring river connectivity by mitigating the impact of obstacles on the colonization of continental water by eels. Consequently, many obstacles are going to be equipped with elver ladder in the near future. In this context, a method to assess the passability of an obstacle seems essential. In this study, we developed a tag-recapture method, appropriate to glass eels and elvers, and an associated multi-state mark-recapture model (i) to assess the passability of a ladder and (ii) to quantify the effect of various environmental factors on this passability. An application to a specific obstacle is applied as an illustrative example that demonstrates the relevance of the assessment method, and how the results can be used to propose technical solution to improve the efficiency of the ladder. Nine tag-recapture campaigns were carried on this obstacle, and about 4400 young eels were tagged. The model demonstrates that the efficiency of the ladder was rather low, especially during low river flow periods, mainly because of accessibility problems. The model also demonstrates the major influence of the river flow on the probability for an eel to pass the ladder; consequently, managing river flow during the migration period can be a relevant measure to improve river connectivity and facilitate colonization of the watershed.

Coastal river connectivity and the distribution of ascending juvenile European eel (Anguilla anguilla L.): Implications for conservation strategies regarding fish‐passage solutions

“…Despite the results presented here, indicating a low efficiency of eel ramps in facilitating upstream migration, the literature contains several reports of efficient implementations (Briand et al, ; Knights & White, ; Legault, ; Solomon & Beach, ). Studies also show that manipulating certain design features can have fundamental impacts on the success of eel ramps (Anwar, ; Drouineau et al, ; Jellyman et al, ; Kerr et al, ; Piper et al, ; Watz, Nilsson, Degerman, Tamario, & Calles, ). This knowledge needs to be transformed, however, into conservation action.…”

Freshwater eels: A symbol of the effects of global change

“…Given this plasticity in habitat use, the consequences of obstacles on upstream migrations are difficult to assess. Methods have been proposed to assess the passability of obstacles (Briand, Fatin, Feunteun, & Fontenelle, 2005;Drouineau et al, 2015;Tremblay, Cossette, Dutil, Verreault, & Dumont, 2016). Densities of eels are higher downstream of obstacles.…”

“…Densities of eels are higher downstream of obstacles. This leads to (a) increased competition between individuals, which can subsequently result in lower survival (Bevacqua, Melià, de Leo, & Gatto, 2011;Vøllestad & Jonsson, 1988), (b) increased susceptibility to predation (Agostinho, Agostinho, Pelicice, & Marques, 2012;Drouineau et al, 2015;Garcia De Leaniz, 2008;Larinier, 2001) and overfishing (Briand et al, 2005;Dekker, 2003c), and (c) possible modification to the sex ratio, as sex determination is density-dependent (Davey & Jellyman, 2005;De Leo & Gatto, 1996;Poole, Reynolds, & Moriarty, 1990;Roncarati, Melotti, Mordenti, & Gennari, 1997;Tesch, 2003).…”

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

“…After a period varying between 3 and 15 years in duration, yellow eels metamorphose into silver eels, migrate back to the sea, and travel across the ocean to the Sargasso Sea (van Ginneken and Maes, 2005). River fragmentation can therefore impact both the upstream migration of glass-eels (Briand et al, 2005;Mouton et al, 2011;Piper et al, 2012;Drouineau et al, 2015) and downstream migration of silver eels (Acou et al, 2008;Piper et al, 2013;Buysse et al, 2014). As a result of a population collapse (Dekker et al, , 2007, observed on both recruitment (Castonguay et al, 1994;ICES, 2014;Drouineau et al, 2016) and spawning biomass (Dekker, 2003), the European Commission introduced Council Regulation N°1100/2007, which requires a reduction in all sources of anthropogenic mortality, including death caused when passing through hydroelectric turbines during downstream migration.…”