Biology and Ecology of the European Eel as Revealed by an Original Sampling Technique Performed in a Deep and Large Riverine Ecosystem

Few studies have documented the biology, demography, and ecology of eels in deep and large riverine ecosystems, which nevertheless contain important growing areas for this endangered species. Using an original eel sampling technique, this 6-year study, started in 2018, aims to characterise the bioecology and demography parameters of eels in a deep and wide riverine ecosystem, the Meuse in Belgium. It was performed over a 125.8 km stretch and included four sampling sites. This technique, never previously used in the Meuse, trapped the eels in fyke nets when they swam back upstream, swimming against the current, after an avoidance reaction generated by the screens of hydropower facilities. Results revealed a high performance in catching eels as well as their sympatric biodiversity. The inter-site growth of eels was good. Yellow eels were mostly caught in the spring and summer under the influence of rising water temperatures, and silver eels were caught during autumn in October–December when flow and turbidity were high. A down-to-upstream decreasing demographic gradient was observed, but this pattern was not found for entry rate, catchability, survival, and immigration and emigration nets. Survival was low, and net emigration was high at a site experiencing noise pollution and hydraulic disturbance due to the work of expanding the ship lock to facilitate the passage of large boats. Effective strategies for the local eel stock and habitat management have been provided for deep and large riverine ecosystems.