Generally, the size of a sea trout population is under the influence of a number of ‘bottlenecks’ in the life cycle. Reduced spawning- and nursery habitat, as a consequence of sand walk and adjustments of streams, is one of the major obstacles. Another important factor is obstacles in connection with migration. A very important obstacle during migration is the passage of weirs which deny the fish access to important habitats, both when it comes to downstream and upstream migration. The weirs’ negative effect on the population of migrating fish is well-documented (Aarestrup et al. 2003; Aarestrup et al. 2006a, b, c; Baktoft et al. 2007). It has lead to a number of model reflections on the impact on the fish stock if the weirs are removed (Olesen & Aarestrup 2006). However this model has not been validated. The possibility of such a validation now exists in the River Villestrup, where the original model was developed. In this comprehensive restoration project, the plan is to remove all weirs in the main stream. This gives a unique chance to test the size in the stock of migrating fish before and after the removal of the weirs.
The study aims at estimating the spring run of smolt and kelts in a number of defined years before and after the removal of the dams. The restoration project was in 2011. The run has been estimated in 2008 and 2009 before weirs were removed and again in 2015-2017 after the completion of the restoration project and allowing for juvenile cohorts to develop. The estimated smolt run before the removals was around 5000 smolt migrating into Mariager Fiord. In 2015 the smolt run was estimated to app. 20000 smolts. The project provides us with valuable information on the potential for optimizing the fish stocks without releases.
The project is running concurrently with project 38258 “The marine life and survival of sea trout” and with the EU funded project 39301: “Expertise in marine and aquatic ecology and genomics for sustainable management of fish and shellfish in Skagerrak-Kattegat-Øresund (MarGen)”. MarGen is an interregional management project with a specific aim to increase scientific and management competencies of marine resources in Kattegat/Skagerrak including understanding of fish migration.
|Period||01/01/2008 → …|
- Research area: Freshwater Fisheries and Ecology