Altering of streams in order to use the power of the water was initiated more than a couple hundred years ago. Clearing for e.g. timber floating by removing boulders, stones and damming parts of the stream for mills and splash dams resulted in dramatic decreased habitat heterogeneity, affecting fish and other organism negatively.
Habitat restoration is an ecosystem action for improvement of the basic for all species – the biotope. This means that the habitat heterogeneity will be increased resulting in more different microhabitats, shelter, places for food, etc. The action will also restore the internal dynamic, i.e. temporal and spatial changes, for example formation of “new” areas of bottom substrate resulting in spawning areas for fish and microhabitat for invertebrates.
The stream sections that has been restored were very uniform in terms of physical habitat due to channelization and stream clearance to facilitate timber floating. Restoration in this case meant reintroducing boulders, dead wood, etc., thus generally increasing habitat heterogeneity and complexity. As a consequence, fluvial processes has been be altered. More diverse stream water velocities has lead to improved conditions for a more diverse bottom substrate. For instance, gravel sections with higher water velocities has occured and it have less silt. The fine organic/inorganic particles have been shown to be detrimental for invertebrates, e.g. juvenile mussels, and the development of fish, e.g. brown trout, eggs. Furthermore, a heterogenic habitat will strengthen the fish and invertebrate populations leading to increased ecosystem resilience.
About 30 streams has been restored in the project. Most of the targeted streams has been restored using excavators, and few of the smaller tributaries by hand and winches.
In Life- Triple lakes we have reintroduced freshwater pearl mussels (M. margaritifera) in streams with extinct or very weak populations of pearl mussels. This action was necessary to re-establish mussel populations since they had gone extinct in most tributaries. Known records of mussels are registered in the late 19th and early 20th century. The disappearance is probably due to stream clearing, damming linked to floating and hydro-power plants and/or pearl fishing. M. margaritifera are filter feeders, thus reducing transport of organic matter to the project lakes. Therefore dense populations of M. margaritifera will improve water quality conditions in the lakes. Two different methods have been used to reintroduce M.margaritifera:
- Moving adult mussels from nearby dense populations. The relocated mussels was concentrated (placed together) in dense groups, which increases the chances for successful reproduction. This measure should also increase the degree of infection of the host fish brown trout. The County Administrative Board has performed a pilot test in the project area while introducing mussels to a tributary of Lake Näkten. This test has been very successful so far since survival of transferred mussels is very high.
- Artificial infection. Pregnant mussels and host fish, Brow trout, collected in streams were brought together in a common container, mussel larvae infected the fish and was then re-stocked in the identified tributaries.
Only local types of mussels and Brown trout was used in the artificial infection.