Long‐term fish assemblage responses to instream restoration in boreal streams: insights from a 24‐year BACI study

Freshwater ecosystems are increasingly threatened by human activities and climate change, emphasizing the need for effective habitat restoration supported by long-term monitoring.We examined fish species responses to instream restoration in six boreal streams in northern Finland over a 24-year period (1999–2022) using a Before–After–Control–Impact (BACI) design, including 3 years before and 21 years after restoration. Each stream contained three reaches: one restored with large woody debris and stones (LWD + s), one with added stones only and one left as a control. Using generalized linear mixed models, we assessed the effects of restoration type, water temperature and discharge on total fish density and biomass as well as on key species, brown trout (Salmo trutta), burbot (Lota lota) and European bullhead (Cottus gobio).Overall fish abundance and biomass declined over time, including in control reaches, suggesting a shifting baseline of ecological conditions. However, LWD + s-restored reaches showed weaker long-term declines in fish density and biomass than control reaches, indicating that wood additions buffered fish populations against long-term environmental deterioration. Brown trout showed more positive long-term responses in reaches containing LWD + s, whereas burbot responses were more strongly associated with stone additions, reflecting species-specific habitat preferences. European bullhead was not affected by restoration. Rising water temperatures and variable flows were associated with reduced fish densities, consistent with regional climatic trends.Our results demonstrate that while local restoration can mitigate habitat degradation, its success is constrained by broader hydrological and thermal variability. Effective river management in boreal regions may require integrating catchment-level water retention measures with instream habitat enhancement, supported by sustained long-term BACI-based monitoring to capture delayed ecological responses.