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Background
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mahtivirta_iso The work of our company is based on the experience gained in biological surveys of northern rivers in a natural state. We have travelled during the past 20 years thousands of kilometres along salmon and trout rivers with the aid of inflatable boats or by foot. We have seen the continuum of different habitats from the headwater brooks downstream to the fjords in the rivers Tenojoki and Näätämöjoki - some of the best salmon rivers in the whole Europe. Hundreds of different kinds of areas sampled by electrofishing have taught us to recognize the subtle relation between many environmental variables and the habitat selection by the fish themselves.

The huge water system of the River Tuloma has also provided an important opportunity for us to learn about salmonid habitats in the northern rivers. Power plant construction and the river impoundment have caused a big change in the fish species composition there from the 1960s. The spawning migration of the Atlantic salmon has been prohibited since that and as a result water areas upstream the dam are nowadays empty of Atlantic salmon. The original lake trout populations have in turn started to colonize the empty areas. The River Tuloma water system maintains today one of the northernmost original migrating trout stock in the whole Europe.

All these experiences have helped us to conceive the complex, continually changing interactions between the physical environment, the river biota and the people. Fish stocks exist always under the regulative power of these interactions. This complex of factors must be noticed when assessing the status of animal communities in any water systems. Understanding the interdependencies below the river surface is in our opinion always a prerequisite for a successful restoration work.