SummaryThis paper summarizes the objectives, activities initiated and approaches used in developing Atlantic salmon stock rebuilding programmes in three large river systems in northern Finland in order to identify the priorities for research and management for further restoration and rehabilitation steps. Until recently, these historically renowned major salmon rivers have been harnessed for hydropower for 40-60 years without provisions for fish passage. Hydropower companies are obliged to compensate for the losses caused by dam construction by annual fish releases, including juveniles of Atlantic salmon and migratory brown trout. Returning fish are harvested in the designated Ôterminal fishery areasÕ at river mouths. Recently, activities aimed at restoring salmonid stocks have started in all these rivers. First fishways at the dams closest to the river mouths were constructed on two rivers, and preliminary projects were completed to review the preconditions for restoration, options for bypassing the dams, and to study the performance of these fishways. New projects incorporating further detailed fishway planning are underway. The various projects have proved successful in bringing together authorities, hydropower companies, local organizations, and expertise from various institutions for a joint effort to tackle these multifaceted and multidisciplinary problems.

Stream channel morphology and hydraulic conditions were measured before and after channel modification and boulder structure placements in a channelized boreal river to determine whether more favourable rearing habitat for brown trout, Salmo trutta L., was created. The assessment was performed using physical habitat simulation (PHABSIM) procedures based on summer and winter habitat preferences of brown trout for depth, velocity and substrate. The results showed that the availability of potential physical trout habitat can be increased in the study river at simulated low and moderate flow conditions by reconstruction of the river bed and placing instream boulder structures. The resulting diversity of depth and velocity conditions created a spatially more complex microhabitat structure. Improved habitat conditions were able to sustain a larger trout population. Hydraulic habitat models, like the PHABSIM framework, seem to be a suitable procedure to evaluate the benefits of physical habitat enhancement.

Summary This paper describes a case study to rehabilitate habitat for adult European grayling (Thymallus thymallus L.) in a large river reservoir in northern Finland. A channelled river reach was restored by building small islands and reefs as well as cobble and boulder structures for grayling. The total area of the restored stretch was 1.0 ha. The physical habitat was mapped using an echosounder, Doppler device, tachometer and scuba diving, and modelled with a 2D hydraulic model. The mean water velocity in the modelled stream section was 0.28 m s−1 during 110 m3 s−1 flow and 0.43 m s−1 during 300 m3 s−1 flow. Twelve adult grayling, tagged with transmitters, were released into the area and tracked for a maximum period of 30 days. The grayling largely stayed in the restored area and tended to avoid the unchanged channel of the river. The range of daily movement was from stationary to 2700 m per day. The adult grayling preferred water velocities between 0.20 and 0.45 m s−1, water depths between 0.20 and 1.55 m and coarse substrate. The study provides a small part of the information needed in habitat restoration for grayling.

Many successful invasions have taken place in systemswhere harmful disturbance has changed habitat conditions. However, less attention has been paid to the role of habitat restoration, which modifies habitats and thus also has the potential to facilitate invasions. We examined whether in-stream habitat restorations have the potential to either facilitate or resist invasion by two widely introduced non-native stream salmonids, Salvelinus fontinalis Mitchill and Oncorhynchus mykiss Walbaum, in Finland. A physical habitat simulation system was used to calculate whether the habitat area for the target species increased or decreased following the restorations. For comparison, we also reported results for four native stream fish species. The simulations showed that the restored streams provided the highest amount of usable habitat area for the native species, particularly for Salmo salar L. and Gottus gobio L. However, it was interesting to note that the restorations significantly increased habitat quality for the two nonnative species, especially at low flows. Nevertheless, the non-native species had the lowest amount of usable habitat area overall. The modeling results indicated that not only habitat destruction but also habitat restoration could contribute to the spread of non-native species. Fisheries and wildlife managers should be aware of the possibility, when restoring habitats in order to preserve native ecosystems, that non-native species could manage to gain a foothold in restored habitats and use them as population sources for further spread. Knowing the widespread negative effect of non-native species, this risk should not be underestimated.

Studies on the effectiveness of in-stream restoration have generally reported increased habitat heterogeneity, but biological responses have been more variable. One hypothesis states that the restored habitat structure does not persist through time, resulting in fading biological responses. We studied the durability of in-stream restoration in northern and central Finland by assessing short-term (0-1 years) and long-term (≥10 years) changes in habitat structure after restoration. In 2010, we repeated the field surveys first conducted in the 1990s in 27 stream reaches. We also made similar habitat measurements in ten near-pristine sites. Restoration caused significant changes in the stream habitat that either remained unaltered or were reinforced through time, with several of the restored sites resembling closely the near-pristine reference sites 10-20 years post-restoration. Cover of aquatic mosses initially decreased sharply but recovered close to near-pristine level within about 15 years. However, substrate variability still remained somewhat lower in the restored than in near-pristine streams. Individual restoration structures had changed little over time. The most evident failures were the disappearance and entrenchment of gravel beds. Restoration of our study sites had shifted the sites to a trajectory towards more natural habitat conditions, and our results do not support the hypothesis that gradual destruction of the restored habitat might partly explain weak biological responses. From the perspective of channel evolution, the restorations were still fairly recent, and only long-term monitoring will tell if the sites will continue on the trajectory to hydromorphological and ecological recovery.