IMIS

European eel Anguilla anguilla (L.) populations have declined dramatically during the last decades and are now among the most critically endangered fish populations in Europe. The limitation of upstream migration of glass eels is considered to be one of the key factors reducing eel populations. The migration of eels in their juvenile stage (glass eels) was studied, and options to improve eel passage at a tidal barrier complex at the mouth of the Yser River, Flanders, Belgium, were assessed. Glass eels were sampled during tidal rise with stow nets and lift nets to analyse their distribution over the study area, while a fyke net was used to evaluate the impact of limited barrier opening on glass eel migration. Support-vector machine-based analysis of the lift net data indicated that migrating glass eels are attracted by the fresh water flow leaking from the barriers, whereas other variables such as the sampling location only had a weak influence on the glass eel density. Limited barrier opening during tidal rise appeared to be a cost-efficient and effective mitigation option to improve upstream glass eel migration without significant intrusion of seawater. Adjusted barrier management could often be implemented and applied on numerous tidal barriers. Therefore the results of this paper are of interest to a wide range of river managers and stakeholders and may contribute to the conservation of many eel populations.