What happens to glass eels after restocking in upland rivers? A long‐term study on their dispersal and behavioural traits

Abstract

1. The European eel Anguilla anguilla is a critically endangered fish species as a result of human activities and climate change in river and oceanic ecosystems. Restocking using glass eels in continental freshwater areas is a potential conservation measure for enhancing local eel stocks and for conserving the species in aquatic habitats, where it may otherwise disappear. However, little is known about the fate of these restocked individuals and the early ecological behaviour of the young eels translocated in rivers. 2.A portable radio-frequency identification (RFID) telemetry system and 12-mm tags were used to track restocked eels for a duration of 4 years. The aim was to understand the early movement, behavioural traits, dispersal, and habitat use of elvers after restocking performed in 2013 with glass eels in a shallow riverine environment.3. From the 241 tagged eels (total length, Q 50 = 152 mm), 85% were detected in 1968 positions during a period of 4 years, beginning in 2014. Clear seasonality in eel activity was observed, with higher mobility in summer when the water temperature was high (above 12°C). Dispersal was slowed by numerous artificial obstacles and the high carrying capacity of habitats. There was a negative relationship between the body size of eels at tagging and their mobility. Five behavioural categories of mobility patterns were identified: ascending, descending, oscillating with an upstream trend, oscillating with a downstream trend, and stationary. The first four categories depleted with time, in favour of stationary individuals that displayed a highly sedentary lifestyle.4. This study provides new knowledge of the long-term dispersal behaviour of restocked eels and the influence of seasons, barriers, and habitats on their colonization strategy changing with time. The results contribute to a better understanding of the issue of uncommon restocking practices in upland rivers.

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

“…In 2019, the level was 1.2% of the historical number recorded in 1992 [20]. These declines of eels in the Belgian Meuse River have been accompanied with an increase in body size and a loss of upstream colonisation behaviour linked to a density dependence mechanism, and thus the reason for these declines is clearly the riverine recruitment failure from the North Sea [2,13,24]. In this context, the upper habitats of inland waters in the Belgian Meuse River are continually emptying themselves of their eels because of the progressive departure of the oldest individuals at the silver eel stage and the shutdown of the natural immigration of wild eels from the North Sea.…”

“…The European eel (Anguilla anguilla L.) is a critically endangered fish species because of human activities and climate change in continental and oceanic ecosystems [1,2]. Habitat loss, fragmentation by barriers along the freshwater systems, turbine mortality, overfishing, parasite dissemination, pollution and changes in oceanic currents caused the decline of the eel population.…”

“…Elvers become yellow eels, the highly sedentary period of the life cycle [14]. Yellow eels remain in freshwaters for 3-15 years, until they are ready to migrate out of rivers and estuaries as silver eels, a pre-pubertal stage [2,15]. Silver eels achieve sexual maturation as they swim actively to the Sargasso Sea spawning grounds where they reproduce and die [3,[16][17][18][19].…”

“…These critical eel stock levels led to the application of conservation measures for stock recovery and management plans for eel fisheries [25]. Among the conservation measures for inland waters that are distant from the sea, restocking is the only solution that enhances the local stocks and is probably the best long-term plan to meet the silver eel escapement target in the Eel Recovery Plan of the European Union [2,26,27]. As the success of artificial reproduction in captivity has not yet been achieved, restocking is totally dependent on wild-caught glass eels and elvers that were translocated from estuarine environments to riverine habitats with low or no natural immigration [2,28].…”

“…Among the conservation measures for inland waters that are distant from the sea, restocking is the only solution that enhances the local stocks and is probably the best long-term plan to meet the silver eel escapement target in the Eel Recovery Plan of the European Union [2,26,27]. As the success of artificial reproduction in captivity has not yet been achieved, restocking is totally dependent on wild-caught glass eels and elvers that were translocated from estuarine environments to riverine habitats with low or no natural immigration [2,28]. 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.…”