Holstenkoski Dam, Finland

The dam removal and the rapid restoration of the Holstenkoski will facilitate fish migration upstream by up to 43 kilometers, enhancing the ecological condition of the Aneriojoki River.


The river Aneriojoki has a native and genetically specified Brown trout (Salmo trutta) population. One of the main targets is to improve the spawning possibilities of the native trout population and other fish species and to improve fish migration between spawning and feeding areas in The Baltic Sea.  

 River Kiskonjoki-Perniönjoki watershed has been selected as one of the Finnish watersheds to be specially protected by the Finnish Ministry of the Environment (1992) and by the Nordic Council (1990). River Anerionjoki and Holstenkoski area belongs to the Natura 2000 network. Protected species by EU habitat directive Otter (Lutra lutra) and Dipper (Cinclus cinclus) live in the river Aneriojoki. Many valuable and protected headwaters, brooks, and riparian forests are also upstream of the Holstenkoski Dam. The river Varesjoki above the Holstenkoski dam is classified as having good ecological status (WFD). 

At a glance

Project typeDemolition
Project statusLive
Demolition DateMarch 2023
Km opened upover 40 km
Focal speciesnative trout population (Salmo trutta, endangered at inland waters of Southern Finland) and salmon (Salmo salar, vulnerable at Southern Finland)

Project context and opportunity

After the partial dam removal (12 metres of the 45-metre wide dam) and restoration, Holstenkoski will be an important example site for educational purposes. The Holstenkoski project could support upcoming political decision-making for biodiversity and ecosystem restoration projects and other future dam removals and river restorations. The Holstenkoski dam removal is a regionally unique project because similar removals have not been made in southwest Finland rivers, which are important for threatened migratory fishes. 

Project aims

The Holstenkoski dam removal will benefit fishing tourism in the Baltic Sea due to increasing sea trout smolt production. The dam removal area will also become a local nature attraction site. The ecological status of the Anerionjoki River will be enhanced after dam removal and rapid restoration. Dam removal will improve significantly, especially migratory fishes (ie. trout and salmon, both threatened species in Southern Finland), spawning possibilities and strengthening the genetic mixing of fish populations.