Space and Time Use of European Eel Restocked in Upland Continental Freshwaters, a Long-Term Telemetry Study

Abstract

There is a lack of information on many biological and ecological aspects of the critically endangered European eel during its growth phase in inland waters, such as when the sedentary life stage begins, mobility according to age and response to habitat alteration. We used mobile radio frequency identification (RFID) telemetry technology to track tagged eels over 6 years after their restocking as glass eels in six typologically different rivers. We also cross-referenced telemetry data with those of several electrofishing monitoring sessions to better understand the mobility and behaviour of eels. The relative abundance (maximum 52 individuals km−1) and detection rate (maximum 28%) of eels were not significantly correlated with the time/age after restocking. Eels were present in all restocked rivers, but their abundance was low and mobility was high in a slightly acidified, oligotrophic river that had experienced a great loss of fish habitat heterogeneity. This loss of habitat heterogeneity was due to flooding events and machinery works in riverbeds to restore the altered riverbanks. Four years after glass eel release, restocked eels became sedentary and moved from shallow to deep microhabitats with riverbeds dominated by blocks as the bottom substrate. After this age, they exhibited high fidelity to the residence site. This study provides new insights concerning the biology and ecology of eels restocked as glass eels in freshwaters, which should lead to improved management plans for the species through the implementation of more effective conservation measures and strategies.

Ontogenetic habitat shift of Anguilla marmorata and A. japonica in the rivers of southern Japan: Implications for habitat use evaluation and conservation

“…These findings are consistent with the results presented here, suggesting that a shift to deeper and more lentic habitats as well as changes in substrate size preference with growth are common phenomena in anguillid eels. Such shifts may occur gradually, considering that Nzau Matondo et al (2023) recently reported that restocked Anguilla anguilla also gradually shifted to deeper habitats as they grow. However, these gradual shifts have not been sufficiently considered when evaluating their habitat use.…”

The Historical Trends and Recent Collapse of European Eel, Anguilla anguilla Fishery in the Neretva Estuary (Eastern Adriatic, Croatia)

“…and a better understanding of the impact of invasive species on the local eel population from recruitment to spawning migration. For the future Eel Management Plan for the Neretva River Basin, which should have an international aspect (Bosnia and Herzegovina as a partner), several important actions should be proposed, such as research on current eel recruitment and potential increase of the glass eel population [28] to promote restoration of abundance in different habitats, a cooperative approach to local eel management, implementation of better fisheries and local market control, and improvement of available eel habitats.…”

Glass Eel Restocking Experiments in Typologically Different Upland Rivers: How Much Have We Learned about the Importance of Recipient Habitats?

“…The study was carried out in Southern Belgium, in the six following upland rivers: Berwinne (A), Gueule (B), Wayai (C), Hoegne (D), Winamplanche (E) and Oxhe (F) (Figure 1). They belong to the Belgian Meuse River and were selected to receive glass eels on the basis of their typological difference on the hydromorphological, physicochemical and trophic levels, as described by our previous research [24,47]. The experimental sites were situated at a distance exceeding 320 km from the North Sea.…”

“…Furthermore, the glass eel stage is absent, and there is a drastic decline, if not total absence, of yellow eels [35,36,[50][51][52]. Rivers A and F are direct tributaries of the Meuse, alkaline, typical of the brown trout Salmo trutta fish zone, have a similar width and exhibit morphodynamic units of runs and rapids alternating with pools [24,47,53,54]. The occurrence of large stones (particle diameter 13-25 cm) and blocks (26-102 cm) is prevalent in eutrophic river A, whereas in oligotrophic river F, large and fine (6.4-12.8 cm) stones were found to be more frequent [54].…”

“…Some scientific works have employed the electrofishing methodology to capture the eels and have reported the success of growth, survival, sex ratio, quality and health of the individuals [21,22,24,28,40,[44][45][46]. Other studies, using the telemetry method, have stated the success of restocked eels in mobility, habitat utilization, resilience after extreme environmental events, ubiquity and sedentary lifestyles [22,47]. Restocking for scientific objectives has been implemented in Belgium/Wallonia, along with persistent surveillance of the reintroduced eels through the utilization of electrofishing and telemetry techniques.…”