Assessment of the viable effects of structural reconstruction of habitats in a mountain stream: A long-term study

The Carpathians are biodiversity hotspots in Europe. However, transformations resulting from human activity have affected stream catchments where forest management is carried out. Numerous anti-rubble weirs and other low-head barriers disrupt the continuity of many streams. New requirements for human interference in stream channels were formulated. Measures are urgently needed to restore the ecological continuity and mosaicism of semi-natural habitats in mountain streams. Recently, small streams have been restored in mountainous areas. To assess the viable effects of stream restoration, analyses of in-stream habitats and fish communities, which are excellent bioindicators, were performed. This study aimed to assess the effects of structural reconstruction of habitats in a mountain stream (Eastern Carpathians, Poland). The analyses were based on a long-term assessment of fish responses to barrier removal and habitat changes following stream restoration measures. The research was conducted from 2009 to 2022. Our study indicates numerous positive and viable effects of using boulder ramps as technical solutions for restoring small mountain streams. Restored ecological continuity and in-stream habitats provide fish with adequate feeding grounds, spawning grounds, and fry-growth places. On the other hand, we indicated that the assessment of the effects of mountain stream restoration should be carried out over a long-term period, in three stages after the completion of work: early, medium, and late. This may allow for the most reliable assessment of the effects of restoration measures and the early detection of the need for corrections. It is also necessary to monitor possible changes that may occur in new hydrotechnical objects, such as boulder ramps, due to natural hydro-morphodynamic processes in the stream bed.