In regulated rivers, canalization and reduced water discharge may lead to loss or impairment of salmonid spawning areas, and thereby negative effects on stock recruitment. We discuss the possibility of mitigating such effects through establishing new spawning areas by addition of gravel. We monitored the occurrence of nests in seven gravel deposition sites in five different Norwegian rivers. The total area covered by each of the depositions spanned from 25 m 2 to 300 m 2 . The areas were examined for nests during a 2-5 year period following the addition of gravel, that is, a total of 27 spawning periods. The fish were found to spawn at all localities and years with the exception of two occasions when gravel was lost during floods. Spawning success measured as egg survival was generally high (>80%). These results show that the additions met the criteria for successful spawning. Nests of Atlantic salmon were found to be aggregated in the area with the highest water current. However, successful nests were also found in areas with added gravel where the water current was much reduced. This unusual selection of spawning site most likely reflects the lack of other, more favourable spawning areas. The results therefore suggest that gravel additions can be successful even if the only available areas are suboptimal with respect to water flow and water depth. Three of the seven localities were unsuitable as the gravel was partly or totally displaced downstream during floods. At two of these localities, nests were probably lost because the floods occurred after the spawning season. This shows that large floods can be a major drawback. Monitoring of the spawning success and displacement of gravel is therefore essential to evaluate the method. Subsequently, stable areas, favoured by the spawners, can be expanded by supplementary addition of gravel. In sum, the results show that the careful addition of gravel areas can be used to provide suitable spawning locations for salmonids in regulated rivers.

In the late 1970s, the construction of weirs in Norwegian regulated river systems for aesthetic reasons was common. However, today, the focus of river restoration has shifted towards improving biological functionality and biodiversity. In the present study, two weirs, originally built to create a stable water level, were removed on a residual flow reach in a Norwegian regulated river as a measure to restore river connectivity and to re-establish the local population of Atlantic salmon. The removal design was based on hydraulic modelling, and biological monitoring was implemented before and after the weir removal to evaluate the biological response to weir removal. The results demonstrated that salmon spawning sites were recreated in the old bed substratum and were occupied immediately the first season after weir removal, when water velocities increased to more suitable levels for spawning. Accordingly, mortality of Atlantic salmon eggs was reduced and the densities of juveniles showed a marked increase after weir removal. Conversely, pike and cyprinids in the reach were found in the samples before weir removal but not after removal, indicating that the desired shift in fish community in response to habitat alteration was obtained. Furthermore, enumeration of migrating adult salmon at a fishway upstream of the study reach showed that the migration peak, on average, was 1 month earlier in the 3 years after removal as compared with the 5 years before removal. Finally, the use of hydraulic modelling represented a useful method for designing physical habitat adjustments and assessing their influence on fish biology. The model results also supported a rapid process in planning and execution of construction works. Figure 4. Calculated water velocities before (a) and after (b) complete weir removal at 15 m 3 s −1 discharge on the lower reach. Arrow indicates inflow and outflow directions. H.-P. FJELDSTAD ET AL.Figure 5. Calculated water velocities and depths before and after weir removal at 15 m 3 s −1 (a and b) and 3 m 3 s −1 (c and d) on the upper reach.

River regulation alters flow and sediment regime, habitat availability, and ultimately the ecological functioning of rivers. Various restoration and mitigation measures have been developed to improve ecological function, and among them is the addition of gravel to enhance the reproduction of gravel bed spawning fishes. However, information on long-term efficiency, costs, and maintenance needs of gravel additions are scarce. Here, we study the functioning of gravel additions at eight sites in three rivers in western Norway for up to 18 years. Gravel was added between 2002 and 2010 to enhance spawning of Atlantic salmon and anadromous Brown trout. We monitored changes in the size of the gravel-covered areas, sediment composition, interstitial oxygen, egg survival, and juvenile fish densities. Additionally, we report monetary costs, identify potential maintenance needs, and calculate a cost-benefit ratio. Juvenile densities of Atlantic salmon and Brown trout increased significantly after the gravel augmentations. After 10–18 years, the median egg survival was still high (> 90%) and sediment conditions were still suitable for salmonid fish reproduction. The areas were, however, shrinking across time (median area reduction 26%), mostly caused by scouring of gravel in the steep, supply-limited, and partly regulated rivers. The average construction costs of spawning gravel augmentations were 11.2 NOK (1.12 €) m−2 year−1. Compared to similar measures elsewhere, the measures have had a long life span (up to 18 years) at relatively low costs. Gravel augmentation was concluded to be a successful management measure that contributed to significantly increased Atlantic salmon and Brown trout reproduction.