Recovering and restoring deleted salmonid populations

Abstract

Salmon and trout have been considered as an umbrella species indicating a good status of the ecosystem. They have a notable existence value producing variety of ecosystem services but moreover, indicating healthy rivers, they can be seen as the key species for stream restoration and adapting human induced changes (e.g. climate change). In less than hundred years we have lost the genetic diversity of salmon by hydropower production all over the country and the Baltic Sea region. Besides hydropower salmonid populations have encountered challenges by habitat loss due timber floating, overall human impact on catchment scale deteriorating water quality and effective fisheries which have all been stressing populations.

Basically the loss of salmon catch has been usually compensated by stocking hatchery reared juvenile salmon to be fished during the sea migration. This compensation has been paid for people, not to the lost river ecosystem. Implementing ecological flow to a single hydropower plant may enhance specific situations like extreme flash floods or zero discharges but it merely solves one challenge. For salmonids, if natural reproduction habitats are available in the upper reaches of the river, could a fishway passing the dam be enough - at least for adult upstream migration. Smoltified juveniles do not generally find a safe downstream migration route and turbines and increased predation in low flow sections between hydropower plants are major threats.

When the total river system is used for hydropower production it means a chain of plants. For example the main stream of the river Oulujoki (107 km) with eight hydropower plants leaves the stream without visible drop and natural flowing water. In these kinds of environments the only possibility for salmonid population would be reconstruction of a habitat compensation sites for every dam, especially downstream, to reestablish the population. With this action approximately 20 % of the original salmonid production is estimated to be possible to compensate. A compensation channel could work as a nature-like fishway but moreover it would support a complete riverine habitat for salmon reproduction. Reproduction channels are studied to host higher densities of juveniles than natural rivers (Imatra brook: 150 brown trout juveniles/100 m2) probably due to controlled discharges and possibility to optimize the quality of needed habitat types.

In some cases population compensation in the same river can be seen impossible e.g. by water quality problems or physical limits for construction. In these cases we should be able to raise stream ecosystems to the spotlight and make compensation in a nearby watercourse possible. Until now compensation has been targeted to compensate loss for fishery and to mitigate the losses for local residents. Habitat compensation should contain calculations of lost habitat areas and natural reproduction rates.