Fast Colonization of Wild Brown Trout in Nature‐Like Compensation Channel

ABSTRACT Salmonids and other aquatic migrating species are strongly affected by river damming, but the declining salmonid populations have been traditionally compensated by fish stocking. Nature‐like compensation channels are a new way to mitigate diversity loss in lotic environment. In this study, a newly constructed side‐channel in Finland called Imatra City Brook (ICB) and its brown trout ( Salmo trutta ) population were studied. ICB was built partly to enhance brown trout reproduction and to compensate the loss of riffle habitat caused by the Imatrankoski hydropower plant. Stages in brown trout life cycle were compared with those in several natural streams and rivers in southern Finland. In a 6‐year study from 2016 to 2022, estimated yearly egg density was 184–511 eggs/100 m 2 in ICB. A 0‐year‐old parr density was 10–126 parr/100 m 2 . Yearly egg‐to‐parr survival was 0.03–0.25 and survival of 0 to 1 year‐old parr was 0.24–0.64. Egg density in the ICB was average compared with reference sites. Parr density was significantly higher in ICB than in the reference streams. Accordingly, ICB appears to serve its compensation function by providing adequate spawning and early‐life stage rearing habitat for native brown trout that is equivalent or even surpasses production metrics observed in nearby natural stream channels. Therefore, constructed compensation channels are a great option to rehabilitate habitat for the benefit of native salmonids.