Published as: Verhelst, P.; Buysse, D.; Reubens, J.; Pauwels, I.; Aelterman, B.; Van Hoey, S.; Goethals, P.; Coeck, J.; Moens, T.; Mouton, A. 2018. Downstream migration of European eel (Anguilla anguilla) in an anthropogenically regulated freshwater system: implications for management. Fisheries Research 199: 252 -262.For the supplemental material and appendices, we refer to the online version of the article: https://doi. Downstream migration in a polder area AbstractConnectivity between freshwater habitats and marine areas is heavily obstructed by anthropogenic structures (e.g. weirs, pumping stations, sluices...), leading to a high pressure on diadromous fish populations. A better understanding of fish migration behaviour in relation to these barriers is needed to take proper mitigation actions. We investigated the impact of migration barriers on downstream migrating European eel by tracking 50 acoustically tagged eels between July 2012 and March 2015 in a Belgian polder area. The study area was selected due to the presence of a wide range of migration barriers, such as two pumping stations, a weir and tidal sluices. These structures regulate the water level, resulting in discontinuous flow conditions. The results showed that migration was primarily nocturnal and discharge appeared to be the main trigger for migration in the polder. We also observed substantial delays and exploratory behaviour near barriers. Delays can have a serious impact on eels since their energy resources are limited for a successful trans-Atlantic migration. In addition, delays and exploratory behaviour can also increase predation and disease risk. The obtained knowledge can contribute to efficient management such as improved fish passage and guidance solutions.

Acoustic telemetry is a commonly applied method to investigate the ecology of marine animals and provides a scientific basis for management and conservation. Crucial insight in animal behaviour and ecosystem functioning and dynamics is gained through acoustic receiver networks that are established in many different environments around the globe. The main limitation to this technique is the ability of the receivers to detect the signals from tagged animals present in the nearby area. To interpret acoustic data correctly, understanding influencing factors on the detection probability is critical. Therefore, range test studies are an essential part of acoustic telemetry research. Here, we investigated whether specific environmental factors (i.e. wind, currents, waves, background noise, receiver tilt and azimuth) influence the receiver detection probability for a permanent acoustic receiver network in Belgium. Noise and wind speed in relation to distance, the interaction of receiver tilt and azimuth and current speed were the most influential variables affecting the detection probability in this environment. The study indicated that there is high detection probability up to a distance of circa 200 m. A new setup, making use of features that render valuable information for data analysis and interpretation, was tested and revealed general applicability.