Study sites and fish sampling
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; Supporting Information 1; Table 1). 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 .
Experimental fish were collected by electrofishing (LR-20B; Smith-Root, Vancouver, WA, USA) carried out over a 500 m stretch at each study site (Supporting Information S1) between May 11 and June 11, 2019 (Table 1). All sampled brown trout were measured (fork length i.e. from the tip of the snout to the end of the central caudal fin ray) and a small clip of the pelvic fin (~0.5 cm2) was stored on ice and later in laboratory kept in freezer at -80 °C, awaiting stable isotope analysis. Aquatic macroinvertebrates for fatty acid and bulk stable isotope analysis were collected using kick sampling and terrestrial macroinvertebrates were collected by hand-picking and dragging the kick net over the canopy surrounding the stream. The macroinvertebrate samples were collected at each sampling site the same day as the fish and were stored alive in an icebox and after determination in the laboratory kept frozen at -80 °C until further processing. Habitat quality was evaluated at each study site on July 10, 2019 using standardized protocol (Jönköpingsmodellen - Halldén et al. 2002, Table 1) and photographs of habitats at each sampling site were taken (Supporting Information S1). Consumption of aquatic macroinvertebrates by brown trout is known to increase with their abundance in the stream (Evangelista et al. 2014; Sánchez‐Hernández & Cobo 2018). Therefore, we performed a quantitative assessment of abundance and biomass of aquatic macroinvertebrates at each sampling site on July 11 and 12, 2019 during sunny stable weather, with no precipitation and low water flow. Aquatic macroinvertebrates were collected by a kick-net (30 x 25 cm, mesh size, 30 seconds of kicking at three spots within each study site representing all available habitats). Macroinvertebrates were counted and oven-dried at 60 °C for 48 hours and total dry biomass of macroinvertebrates at each sampling site was measured to the nearest 0.001 g (Table 1).