Enhancing Salmonid Reproduction in a Natural River System: A Case Study of the Ina River (Baltic Sea Catchment)

Salmonid fish only reproduce in habitats that meet specific environmental requirements, including appropriate gravel–cobble substrate, suitable flow velocity, and adequate oxygenation. Long-term drainage practices and river channel regulation have led to substantial alterations of river systems, particularly affecting bed structure. The aim of this study was to assess habitat conditions in the Ina river catchment and to restore spawning grounds for salmon and sea trout through the construction of artificial redds, as well as to evaluate the effectiveness of these measures over subsequent years. The number of fish nests recorded prior to the implementation of the restoration project in 2011 was significantly lower (3 ± 1, mean ± SD) compared to post-restoration periods in 2013 (23 ± 11) and 2015 (21 ± 14). Spawning nests were predominantly located in areas characterized by high flow velocity and elevated water conductivity, hardness, and alkalinity. During the spawning migrations in 2013–2015, a total of 4593 individuals were recorded using a fish scanner. Despite a gradual decline in water levels from pre-restoration to post-restoration periods, the number of nests remained consistently high. The results indicate that ongoing environmental and climatic changes necessitate continued efforts to improve spawning conditions for anadromous salmonids. Currently (2024–2025), the potential for natural reproduction in the Ina River catchment remains comparable to the study period (89 redds), largely determined by the availability of gravel habitats and river discharge enabling upstream migration.