This study was conducted in three streams in the upper part of the catchment of the Viskan River, situated in southwest Sweden (57°40′318′′N, 12°59′300′′E; Appendix S1, Fig. S1). Brook trout was introduced to Sweden in 1892 (Aas et al. 2018), with the first electrofishing reports indicating a self-reproducing brook trout population in our study system dating to 1985 (SERS, 2013). While brook trout have occurred in European streams for over a century, its impact on the native species is still significant and it is recognized by many European countries including Sweden as an invasive alien species (e.g., Strand et al. 2018). Populations of brook trout are established in the upstream sections of the sampled streams. Native brown trout occupy the whole system, and thus each stream contains an allopatric (brown trout only) and a sympatric (brown and brook trout co-occurring) section. The brown trout populations in the study stream are landlocked with no reported lake or seawards migration. Abundance of brown and brook trout in the studied streams ranges from 0.2 – 1 individual per meter of the stream length. At sympatric sites, brook trout is as abundant or more abundant than brown trout (Závorka et al. 2017). Other fish species occurring in the study system are brook lamprey Lamperta planeri, common minnow Phoxinus phoxinus, and occasionally northern pike Esox lucius.