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

Abstract

To improve the management of the European eel (Anguilla anguilla) in freshwater, it is essential to define important lotic habitats. Electrofishing data from 289 wadeable, hard-bottom sites in 69 Swedish coastal rivers and streams, originally surveyed for salmonid monitoring, were used to evaluate the effects of sampling-and habitat-related factors on eel occurrence. Probability of eel occurrence, as influenced by sampling procedure (sampled area, number of consecutive runs and ambient water temperature) and habitat characteristics (size of catchment, dominating bottom substrate, shade, water velocity, mean depth), was evaluated for small (total length B 150 mm) and large ([ 150 mm) yellow eels. Data were analysed in a mixed presence/absence generalized linear model with dispersal (distance to mouth from sampled site), habitat and samplingrelated variables as covariates. The two models explained variation in occurrence to 81.5% for small eel and 76.2% for large eel. Probability of eel occurrence decreased with distance from the river mouth, and increased with sampled area, number of runs, water temperature, coarser substrate and size of river. We suggest that future eel habitat restoration should focus on lower reaches of larger rivers with suitable coarse bottom habitats. Stocking of young eel should be carried out in comparable accessible habitats in the upper reaches where eel densities are low. The results also strongly indicate that eel may be sampled together with young salmonids with DC electrofishing in wadeable habitats.

A comparison of European eel Anguilla anguilla eDNA concentrations to fyke net catches in five Irish lakes

“…Lough Owel (Shannon catchment) was also the site of a commercial fishery but small in scale, and recent fyke surveys already indicated a declining population. Lough Muckanagh (Degerman, Tamario, Watz, Nilsson, & Calles, 2019) but the focus of this study was designed to test the eDNA method with specific levels of eel abundance (high, medium, and low). Within all lakes, the net depths were set <20 m, with the majority set <5 m ( Table 1).…”

Substrate‐size choice in European eel (Anguilla anguilla)elvers is not altered by piscivore chemical cues

“…Our results are comparable with previously reported elver choice of substrate sizes (Christoffersen et al ., 2018; Degerman et al ., 2019) and, specifically, show that the choice of coarse gravel persists also in the presence of chemical cues from a piscivorous fish. River substrate composition is an important predictor of elver occurrence (Degerman et al ., 2019; Silberschneider et al ., 2004), likely due to elver active habitat choice considering, for example, foraging opportunity, preferred environmental conditions and refuge from predation.…”

“…Elvers have been indicated to prefer coarse and heterogeneous habitats ( e.g ., Christoffersen et al ., 2018; Degerman et al ., 2019; Silberschneider et al ., 2004), whereas elver substrate choice in the absence versus presence of predator cues has to our understanding hitherto not been experimentally examined. If habitats differ in protection from predation, perceived predator presence could affect habitat choice.…”

“…Interestingly and conveniently, this fraction should be advantageous also for salmonid spawning and benthic organisms ( e.g ., Giller & Malmqvist, 1998; Januschke et al ., 2014; Palm et al ., 2007), suggesting synergies between restoration incentives for multiple species. Coarse 8–16 mm gravel may also be considered in habitat restoration programmes for elvers, as it was only marginally significantly less favoured than coarse gravel (Table 1 and Figure 1), and as diversity in stream and river substrate‐size composition should benefit a broader range of species (Degerman et al ., 2019). Moreover, as the μ / g ratio may change over ontogeny (Werner & Gilliam, 1984), habitat choice by yellow eel of different sizes should also be experimentally addressed to secure habitat availability and eventual recruitment of silver eel.…”

“…and, specifically, show that the choice of coarse gravel persists also in the presence of chemical cues from a piscivorous fish. River substrate composition is an important predictor of elver occurrence (Degerman et al, 2019;Silberschneider et al, 2004), likely due to elver active habitat choice considering, for example, foraging opportunity, preferred environmental conditions and refuge from predation. Habitat choice should be an optimization of the mortality risk (μ)-growth opportunity ( g) trade-off, and if different habitats provide different foraging and shelter opportunities, habitat choice would change with the μ/g ratio (Brönmark et al, 2008;Werner & Gilliam, 1984).…”