Environmental factors influencing downstream migration of European eel (Anguilla anguilla L.) in a free-flowing river system

Background To support the recovery of the critically endangered European eel ( Anguilla anguilla ), understanding downstream migration triggers is essential. Unlike well-studied regulated systems, this study investigates migration drivers in a free-flowing river system to provide a needed reference framework. Methods Using acoustic telemetry we tracked the movement of 39 silver eels starting in the upper reach of the River Grote Nete (Belgium), a free-flowing tributary of the Scheldt basin. Generalized linear mixed models were applied to analyse environmental factors influencing migration onset and pauses. Chi-squared tests were used to investigate cyclic migration patterns (circadian, circatidal and circalunar patterns). Results In the non-tidal zone, river discharge was the primary trigger. Migration onset coincided with discharge peaks exceeding seasonal averages and migration pauses were significantly correlated with reduced discharge. Next to this, eels prioritized migration during the first part of the night, however these patterns became less clear in the tidal zone. Here, their activity was largely driven by the tidal cycle, as such inducing selective tidal stream transport. We also observed a clear preference towards migration during new Moon. Conclusion Our findings underscore the critical role of natural flow dynamics and light regimes in regulating eel migration. As removing water regulating structures is not always realistic, water management strategies at these structures could target periods of high discharge, low lunar brightness and the first half of the night, to maximize eel escapement.