Brown Trout (Salmo trutta) Migration Through Block Ramps on the Porębianka Stream: The Role of Low-Flow Channels

The aim of the study was to assess hydrodynamic conditions in the low-flow channels of the block ramps with regard to the migration potential of brown trout (Salmo trutta). The research was conducted on the Porębianka Stream, where a cascade of 25 block ramps had been constructed in the outlet reach to stabilise the watercourse’s longitudinal profile. The analysis was performed for all 25 block ramps, but the detailed presentation of the results was only shown for four of them (7, 13, 18 and 19). Field measurements were performed under four flow conditions, defined as low, medium–low, medium and high. The analysis included three parameters that determine the potential passability of migration: water depth, flow velocity, and the water-level drop between the stilling basin and the downstream reach. Flow velocities were assessed against threshold values that depend on the length of the section the fish must negotiate. The results showed that the mere presence of a low-flow channel concentrating the discharge does not guarantee the maintenance of ecological continuity in the watercourse. Under low-flow conditions, the main limiting factor was insufficient water depth. In contrast, at higher discharges, the dominant constraints were exceedances of permissible flow velocities and the absence of a continuous corridor of reduced velocity. The most favourable migration conditions were identified in the low-flow channel of block ramp nos. 7 and 19, from medium–low to medium flow and from low to medium–low flow conditions, respectively. Block ramps nos. 13 and 18 acted as complete barriers under all of the analysed water discharges. The assessment of the entire cascade demonstrated that when flow velocity, water depth and water level drop were considered simultaneously, only four of the low-flow channels (nos. 6, 7, 17 and 19) could be classified as passable or conditionally passable. The obtained results indicate the need for a comprehensive assessment of block ramps, including several flow conditions and the continuity of a potential migration trajectory. The modernisation of the most problematic channels should aim to increase water depth, reduce excessive flow velocities, reduce local water-level drops, and introduce a more diverse flow structure that resembles natural hydraulic conditions.