The restoration of ecological continuity is one of the central tasks of water management. By constructing weirs and locks, numerous waterways have been blocked over the centuries in order to be able to drive mill cycles or to operate shipping. Artificially built transverse structures lead to the fact that most of the German water bodies are hardly, or no longer, permeable to aquatic organisms. Fish migration within the widely ramified waterways becomes mostly impossible. Since the amendment of the Federal Water Act in 2010, the Federal Waterways and Shipping Administration (WSV) has been obliged to maintain or restore the ecological continuity of its weirs on federal waterways, as long as it is necessary to achieve the objectives of the Water Framework Directive.
The WSV also responds to this legal obligation at the Lahn river: Action A5 is designed to support all activities aimed at restoring the ecological continuity of the Lahn by providing basic data and the contracting of preliminary studies and expert opinions. These preparatory measures are intended to speed up the planning processes and to improve the communication between all stakeholders. The concrete planning and construction of new fish ways for the Federal Waterway Lahn are supported by the LiLa project, but carried out in other departments of the WSV.
The weir Lahnstein (Lahn-km 135,960) is the first migration barrier from the Rhine to the Lahn river and the highest weir in the river system. Since 1957, the basin fish ladder at Lahnstein and the two eel pipes in the abutment piers have a very limited functionality, and the construction of a new fish-climbing system is a still ongoing process, which faces many problems due to difficult local conditions. Therefore, the Waterways an Shipping Office Mosel-Saar-Lahn (WSA MSL) analysed an additional way of fish migration management, using the locks to sluice fishes. The so-called "fish locking management" was introduced in December 2016 by a pilot operation at the lock Lahnstein. By means of an adapted control of the lock gates and contactors, migrating fish were directed into the lock chamber and released to the upstream river or downstream into the Rhine. As a result, it is possible to create an additional option for migration of aquatic organisms which cannot find or pass through the existing fish way between the weir and the hydropower plant. This measure (Action C 1) serves as an interim solution for improving the ecological continuity, but is not a substitute for the construction of both a new upstream fish way and downstream fish migration facilities. There are several monitoring campagnes during the whole LIFE IP (for example in spring and autumn 2018/2019) to monitore and optimize the fish locking management in Lahnstein. If necessary, structural adjustments for the optimization of the fish migration through the lock Lahnstein shall be carried out. After gaining enough results, the fish locking management can also be transferred to other locks of the Lahn or even to other rivers.