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Eutrophication of coastal areas as a consequence of the agricultural use of fertilizers is a widespread problem. The development of artificial lakes and constructed wetlands in nutrient‐rich rivers is a widely used management tool in the fight to decrease eutrophication. Juvenile salmonids that have to negotiate these lakes during their downstream migration to the sea are commonly subjected to high mortality due to increased predation pressure and delayed passage. In this study, we double tagged 39 brown trout smolts with passive integrated transponder and radio tags to gain further insight into predation rates and migration patterns during their passage of an artificial lake in a Danish lowland stream in the spring of 2016. Thirty‐four of the tagged smolts, caught and released upstream, entered the lake, of which 22 (65%) successfully exited the lake. Four smolts (12%) returned upstream to the river. Three smolts were predated in the lake by two northern pike (Esox lucius). Three tags were recovered from the lake bottom, and two disappeared out of the study area after a last detection in the lake. Tracking the smolts manually and by automatic listening stations showed highly erratic movement patterns during lake passage. Further, we observed long delays of up to 27 days after the smolts reached the river mouth and before they entered the sea, potentially due to low sea water temperature or due to the stocking of a large amount of hatchery‐reared brown trout smolts. The results are discussed in the context of abiotic and biotic factors, which differed considerably in the year 2016 compared with previous years.

Artificial lake development is often used as a management tool to reduce nutrient runoff to coastal waters. Denmark has restored more than 10 000 ha of wetlands and lakes in the last 14 years as a consequence of ‘Action Plans for the Aquatic Environment’, which aim to meet the demands of the European Union's Water Framework Directive. Juvenile, seaward migrating salmonids are highly affected by impounded waterbodies, as they are subjected to extraordinary high mortalities due to predation and altered habitat. From 2005 to 2015, survival and migration patterns of wild brown trout (Salmo trutta) smolts were investigated by using radio, acoustic and Passive Integrated Transponder telemetry both before and after the development of an artificial lake in a small Danish lowland stream. In 2005 and 2006, before the lake developed, survival was estimated to be 100% in the river stretch where the lake later developed. In 2007 and in the period between 2009 and 2015, mean yearly survival decreased to 26%. Mean time for passing the area increased significantly after the development of the lake from 0.42 to 5.95 days. Generalized additive models were used to model the probability of a successful passage. Water temperature and discharge were key environmental factors affecting survival of the smolts during the passage of the lake. Furthermore, smolt survival was negatively correlated with condition factor. This elevated level of smolt mortality may seriously compromise self-sustaining anadromous salmonid populations when artificial lakes are developed in connection with rivers. Copyright © 2016 John Wiley & Sons, Ltd.