Synergistic impacts of sediment generation and hydrotechnical structures related to forestry on stream fish...

The increase in anthropogenic sediment has become the most significant factor responsible for degrading flowing waters, preventing them from attaining the good ecological state of rivers requested by Water Framework Directive. After agriculture and mining, forest exploitation is a major source of mineral suspension. The accumulation of fine sediments may reduce fish spawning effectiveness, inhibit migration, and interfere with feeding. Streams degraded by the influx of anthropogenic sediments may be recolonised provided that ecological continuity of the dendritic river network is maintained. The present study focused on mountain stream fish communities whose catchments have undergone natural and intensified soil erosion. We tested the hypothesis that forestry practices transforming the stream habitats combined with the discontinuities created by hydrotechnical structures have had negative impacts on the entire catchment and adversely altered fish communities. The research was carried out in two catchments: i) in the national park, and ii) outside the national park, in an area with forest exploitation. It was observed that the negative effects of the dramatic increases in mineral suspension loads were synergistically aggravated by the presence of objects disturbing fluvial system continuity. The consequences of the synergy between these two factors were evident in local brown trout populations. Sites formerly inhabited by brown trout lacked downstream barriers during our study. After eliminating this parameter, our analysis revealed that channel silting also reduced brown trout abundance downstream from the barrier. The weir in the Muczny Stream inhibited ichthyofaunal re-establishment in streams prone to enhanced mineral suspension input. Forest mountain streams must be preserved also as cold water refugia, hence our results are of significance for forest management. Based on our results, the complete removal of the barrier in Muczny Stream is planned for 2020, and a substantial reduction in the inflow of mineral suspension to stream channels is strongly recommended.