Contributions of short-term flow regulation patterns to trout habitats in a boreal river

Physical habitat simulation modelling was used to evaluate the effects of flow regulation on the Siikajoki, a boreal river in Central Finland. In the modelling, the “natural flow” regime, the “implemented regulation” pattern and two alternative patterns of regulation were compared by using brown trout (Salmo trutta) as a test fish and by analysing example periods in winter and in summer. The availability of suitable habitats for brown trout (Weighted Usable Area, WUA) was clearly dependent on the flow rate. The fewest usable habitats were available for the size class “fry” (< 15 cm). We found only minor differences between the compared regulation patterns in terms of riverine habitats. A side channel in the test area proved to be good “buffer area” against flow changes, highlighting the importance of habitat diversity. In a sensitivity analysis of habitat modelling, modifications of the depth preference curve seemed to have a major influence on the WUA for young brown trout.