Can a restocking event with European (glass) eels cause early changes in local biological communities and its ecological status?

Early settlement and growth of stocked European glass eels in a fragmented watercourse

“…Furthermore, eels showed a preference for boulder areas with crevices, likely used for concealment. The preference for this type of cover may be related to protection against currents and predators, as well as food availability, as these boulder areas have richer and more diverse macroinvertebrate communities (Félix et al., 2020). This is consistent with Domingos et al.…”

“…Hence, total length was site dependent but had no relation with the environmental factors. Prey‐type availability, in terms of nutritional value, could also contribute to different growth rates, but these specific stocked areas (Sites 1 to 3) have no spatial differences regarding their macroinvertebrate assemblages (Félix et al., 2020). Thus, these results suggest other factors may be involved, either through unmeasured parameters or intrinsic biological traits of eels.…”

“…According to accessibility and stretch characteristics, four sites located upstream and downstream of the major weirs (Figure 1) were selected to conduct the study. All sites presented homogenous environmental and biological characteristics (Félix et al., 2020). Site 1 sets the downstream limit of the experimental river stretch and Site 4 its upstream endpoint.…”

“…But the contribution of stocking to increase spawning biomass has been demonstrated (Brämick et al., 2015), so the issue may lie elsewhere. Only recently, new areas have been set for release and follow‐up sites, allowing the success of this practice to be determined, both in terms of growth conditions and survival (Nzau Matondo et al., 2020; Nzau Matondo et al., 2019; Ovidio et al., 2015), and in terms of ecological impact on other biological communities (Félix et al., 2020). Apart from the historical difficulty on assessing stocking success, when considering the total area of the species’ distribution, stocking programmes are carried out at a very small scale and, in addition, other factors such as fishing and obstructions to migration contribute to the decline of the species.…”

An Evaluation of Restocking Practice and Demographic Stock Assessment Methods for Cryptic Juvenile European Eel in Upland Rivers

“…In their new freshwater environments, the stocked young eels are surviving, dispersing, growing and maturing into silver eels that are displaying similar seaward migration behaviour to the naturally recruited wild eels. Therefore, they are probably able to successfully contribute to the spawning stocks [2,27,[29][30][31][32][33][34][35][36][37]. Such encouraging outcomes are signs of significant progress in the knowledge restocking practice; thus, there is a great hope for inland waters where the eel stocks are declining [38].…”