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

Abstract

One of the actions that has been implemented to support the recovery of the panmictic population of European eel is stocking of waterbodies where natural recruitment is low or null. However, growth conditions of the stocked eels can vary greatly. This circumstance emphasises the importance to determine ideal habitat conditions to contribute to the success of stocking actions and, consequently, to increase the production of silver eels. This study aims to evaluate the early settlement and growth of stocked glass eels in the upper reaches of a fragmented river. Stocking was carried out, in 2014, at three sites of an inland tributary of the Mondego river basin (Central Portugal), and its monitoring was conducted during the following two years, until 2016, along with the collection of environmental and hydromorphological parameters. The results showed a successful dispersion throughout the study area. Growth varied spatially, although environmental parameters have not clearly explained this variation, but overall with high growth rates and a positive allometric growth in this early stage of stocking, suggesting a good condition of the stocked individuals. This study showed that these upper reaches of fragmented watercourses, a shared feature amongst most European rivers that are currently inaccessible for natural recruitment, may be suitable habitats for eel stocking.

What are the best upland river characteristics for glass eel restocking practice?

“…We have shown the occurrence of restocked eels in a wide diversity of upland riverine ecosystems where they adapted to translocation from an estuary and grew satisfyingly after being restocked. Undoubtedly, our results demonstrate that with regard to the drastic decline in the process of the upstream natural colonisation of wild eels, glass eel restocking practice in upland rivers can be deemed an original solution as well as a source of hope for countries further from the sea to enhance their local stocks and probably over the long run to increase silver eel escapement rates through a greater contribution of upland freshwaters to the production of restocked-origin silver eels (Ovidio et al, 2015;Nzau Matondo et al, 2019, 2020Félix et al, 2020aFélix et al, , 2020b. As the European eel consists of a single panmictic population that is generally unstructured in meta-population throughout its entire range, there is no genetic argument against translocation of eels within its distribution area or between river basins for restocking purposes (van Ginneken and Maes, 2005;Palm et al, 2009;Nzau Matondo et al, 2020).…”

“…The observed link between river width and eel density, although weak, raises interest in experimenting on larger inland rivers, which may have important growing areas for eels, especially when the above-mentioned characteristics are fulfilled. Such characteristics provide optimal living conditions like greater protection against currents and predators through better eel burrowing and abundant food resources for this cryptic species, which exhibits a highly sedentary lifestyle (Cucherousset et al, 2010;Ovidio et al, 2015;Nzau Matondo et al, 2019;Félix et al, 2020aFélix et al, , 2020b. River A (Berwinne) is an eutrophic alkaline freshwater body with high riverbed roughness (large stones and blocks predominating) and a diversified flow facies with dominant pools and riffles, which are factors favourable to greater eel density and species diversity of aquatic fauna due to higher primary production (Keithan and Lowe, 1985;Boulenger et al, 2014).…”

“…Higher eel growth rates have been reported in individuals dwelling in riffle-type habitats because benthic community productivity is higher in riffles than in run-and pool-type habitats (Whitledge and Rabeni, 2000;Boulenger et al, 2014). The larger granulometry of riverbed substrates has been found to be positively correlated with eel density and the size of the areas used by eels as it provides better protection against currents and predators and offers abundant food resources through the occurrence of more diverse communities of macroinvertebrates (Nzau Matondo et al, 2019;Félix et al, 2020aFélix et al, , 2020b. Temperatures in riverine ecosystems also affect the growth of eels through food availability and eel activity: eel growth and activity generally stop below 8°C (Nyman, 1972;Sadler, 1979;Baras et al, 1998;Morin et al, 1999;Ovidio et al, 2013;Itakura et al, 2017).…”

“…Although restocking is an important conservation measure for the enhancement of local eel stocks and over the long run most likely for the achievement of the silver eel escapement target in the Eel Recovery Plan of Europe, it is still poorly understood. Indeed, owing to difficulties in the implementation of efficient monitoring due to eels' low recapture efficiency, many studies on restocking practice have been performed within the same catchment area at sites located along a longitudinal gradient and during short time periods (Desprez et al, 2013;Kullmann and Thiel, 2018;Félix et al, 2020aFélix et al, , 2020b. In particular, there remains a paucity of information on the possible effects of the typological diversity provided by multiple catchment areas of watercourses within the same river basin on eel yields after restocking.…”

“…In particular, there remains a paucity of information on the possible effects of the typological diversity provided by multiple catchment areas of watercourses within the same river basin on eel yields after restocking. Given the plasticity of the species to colonise and adapt to a wide range of aquatic ecosystems, with encouraging outcomes from restocking in inland freshwaters recently reported (Ovidio et al, 2015;Nzau Matondo et al, 2019, 2020Félix et al, 2020aFélix et al, , 2020bDelrez et al, 2021), this practice sounds interesting to investigate. In the Belgian Meuse River, the number of ascending yellow eels entering from the North Sea via the Dutch Meuse has dropped by about 3.6% per year from 1992 to 2020; in 2020 the number represented 0.6% of the historical level recorded in 1992 (Nzau Ovidio, 2016, 2018;Nzau Matondo et al, 2019, 2020.…”