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 aconsequence of sand walk and adjustments of streams, is one of the majorobstacles. Another important factor is obstacles in connection with migration.A very important obstacle during migration is the passage of weirs which denythe fish access to important habitats, both when it comes to downstream andupstream migration. The weirs’ negative effect on the population of migratingfish is well-documented (Aarestrup et al. 2003; Aarestrup et al.2006a, b, c; Baktoft et al. 2007). It has lead to a number of modelreflections on the impact on the fish stock if the weirs are removed (Olesen& Aarestrup 2006). However this model has not been validated. The possibilityof such a validation now exists in the River Villestrup, where the originalmodel was developed. In this comprehensive restoration project, the plan is toremove all weirs in the main stream. This gives a unique chance to test thesize 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 ofdefined years before and after the removal of the dams. The restoration projectwas in 2011. The run has been estimated in 2008 and 2009 beforeweirs were removed and again in 2015-2017 after the completion of therestoration project and allowing for juvenile cohorts to develop. The estimatedsmolt run before the removals was around 5000 smolt migrating into MariagerFiord. In 2015 the smolt run was estimated to app. 20000 smolts. The projectprovides us with valuable information on the potential for optimizing the fishstocks without releases.
Theproject is running concurrently with project 38258 “The marine life andsurvival of sea trout” and with the EU funded project 39301: “Expertise inmarine and aquatic ecology and genomics for sustainable management of fish andshellfish in Skagerrak-Kattegat-Øresund (MarGen)”. MarGen is an interregionalmanagement project with a specific aim to increase scientific and managementcompetencies of marine resources in Kattegat/Skagerrak including understandingof fish migration.
This project is funded by the Danish Rod and Net Fishing License Funds.
|Period||01/01/2008 → …|
- Research area: Freshwater Fisheries and Ecology