Evaluation of River Restoration Measures: Interstitial Habitat Variables and Salmon Hatching Rates as Indicators of Success

Sammanfattning: River ecosystems are some of the most complex ecosystems on Earth, characterized by a highly heterogenous environment which is able to sustain a disproportionally high level of biodiversity in relation to the area of which they occupy. However, anthropogenic activities are encroaching on the highly diverse riverine landscape, threatening freshwater biodiversity. Species with complex life cycles involving migrations between marine and freshwater environments are disproportionally threatened. One such species is the Atlantic salmon, which now is at historically low population levels. Restoration efforts are being made to restore degraded river ecosystems and strengthen salmonid populations. However, the response from salmonids and other organisms used as indicators for restoration success are inconclusive or lacking. Additionally, evaluation of restoration success lacks standardized procedures and tools for monitoring. Habitat variables have been shown to be robust indicators when assessing restoration success. This study attempts to asses restoration success using an accessible and user-friendly tool which can be used for measurements of both biotic and abiotic variables. Salmon eggs were planted in the riverbed at potential spawning grounds, from where hyporheic water was extracted to measure physicochemical habitat variables which are critical to salmon embryos. The results indicate that the functionality of available spawning habitats in the study area is adequate in both restored and non-restored sections. Habitat variables displayed spatial variation but were not more favorable at restored sites compared to non-restored sites. Similarly, egg hatching rate varied spatially but not between restored and non-restored sites. The apparent lack of response to the restoration efforts in the study area is likely due to a combination of factors such as biased site selection, limited sampling period and a lack of comparable pre-restoration values. This highlights the need of defined restoration goals, a robust monitoring plan and criteria for evaluation of restoration success. Nevertheless, the method used in this study can be applied to measure biotic and abiotic variables and may add to the toolbox for assessing restoration success. This will hopefully contribute to new insights for monitoring and evaluation of restoration success in other projects.