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

Abstract

Many diadromous fish populations are declining and at risk of collapse. Lack of river connectivity is a major contributor to these declines, as free migration routes between marine and freshwater habitats are crucial for life‐history completion. For the conservation and ultimately recovery of such species, it is imperative that remedial measures aimed at increasing connectivity are effective. This study investigated the distribution patterns of ascending juvenile European eel (Anguilla anguilla L.) in rivers in south‐western Sweden, with a focus on the effects of barriers and measures that aim to reduce the impact of barriers, i.e. fish‐passage solutions (FPSs). Eel occurrence data were spatially and temporally integrated with the national databases of dams and FPSs in a Geographic Information System (GIS) environment to evaluate their effect on ascending eel distribution. The types of barriers assessed were: (i) dams with nature‐like fishways; (ii) dams with eel ramps; (iii) dams with technical fishways; and (iv) dams without FPSs. Dams fitted with eel ramps or technical fishways, as well as dams without FPSs, produced a significant negative effect on the probability of eel occurrence upstream. This negative effect was not found for dams fitted with nature‐like fishways, indicating that these solutions may function better than the other FPS types in this study. The probability of eel occurrence decreased with distance from the sea and increased with area sampled, number of electrofishing runs, water temperature, and with the size of the bottom substrate. We suggest that future conservation strategies for improving the natural immigration of juvenile eels should include optimizing FPS function (e.g. placement and design), the continued maintenance of FPSs, the construction of nature‐like fishways, and preferably the removal of dams, which will also benefit the downstream migration of maturing eels as well as restoring other ecosystem services.

Ecological and Evolutionary Consequences of Environmental Change and Management Actions for Migrating Fish

“…Although fishways improve possibilities for spawning migration (Gouskov et al, 2016;Tamario et al, 2018), they seldom result in the desired level of connectivity restoration (Brown et al, 2013;Foulds and Lucas, 2013;Birnie-Gauvin et al, 2018;Silva et al, 2018;Tamario et al, 2019) and are not fully and equally permeable for all species, ages and phenotypes (Haugen et al, 2008;Volpato et al, 2009;Birnie-Gauvin et al, 2018). The altered severity of migration caused by fishways, and other types of partial dispersal barriers (Newton et al, 2018), may thus impose selection by favoring certain phenotypes and disfavoring others, and thereby impact on the phenotypic and genetic composition (Figure 5).…”

Occurrence and habitat use of European eel (Anguilla anguilla) in running waters: lessons for improved monitoring, habitat restoration and stocking

“…overexploitation (Moriarty and Dekker 1997;Jacoby et al 2015), ocean climate change (Castonguay et al 1994;Durif et al 2011), pollutants (Feunteun 2002) and the effect of parasites (Sjöberg et al 2009). In freshwater, the main causes of the decline are suggested to be poor function and placement of passes for eel immigrating to freshwater (Tamario et al 2019;Watz et al 2019), loss and deterioration of freshwater habitat (Feunteun 2002) and delay and mortality of maturing eel passing dams and power plants (Besson et al 2016).…”

“…The maintenance and monitoring of these have, however, been inadequate (cf. Pärlklint and Bengtsson 2015) and their efficiency low (Tamario et al 2019). Due to the large number of dams and the relatively few eel passes present, freshwater eel management has for decades relied mainly on assisted migration of elvers to lakes and streams upstream in the catchments.…”

“…The dispersal variable was evaluated as distance from the river mouth, representing a migration cost and general population thinning due to successive mortality and settling of eel in available habitats downstream of each site (e.g. Ibbotson et al 2002;Feunteun et al 2003;Imbert et al 2008;Acou et al 2011;Tamario et al 2019). Sampling effort should affect detectability of a species, particularly at low densities, and should thereby be a key factor for reliable assessment (e.g.…”

Climbing the ladder: an evaluation of three different anguillid eel climbing substrata and placement of upstream passage solutions at migration barriers

“…A recent study by Tamario et al . () reports that existing upstream passage solutions targeted at juvenile eels in general have suboptimal performance, indicating a need for further research on passage design. In this study, we evaluated and compared the performances of studded, open weave geotextile and bristle climbing substrata (Solomon & Beach, ).…”