Sampling and study area
Ånnsjön is located in Central Sweden (63.261212°N, 12.567719°E) at an elevation of 526m (Figure 1). Most of the approximately 57 km2 lake is relatively shallow (below 2 m deep), but the deepest point is 39.5m (34). Minnows are the most common fish species and the fish community is further composed by brown troutSalmo trutta , Arctic charr Salvelinus alpinus , lake troutSalvelinus namaycush , and brook charr Salvelinus fontinalis (34). In Southern Europe, P. phoxinus belongs to a species complex (23, 35, 36). However, minnows inhabiting Norway and Sweden belong to one species only (37). In August 2018, minnows were caught from three lake locations (L1, L2, L3; Figure 1) using gill nets (1 m x 10 m with 6 mm mesh size) for up to 12 hours overnight. Furthermore, minnows were collected from three different slow flowing tributaries: downstream Stor Klockbäcken (location S1), downstream Sjöviksbäcken (location S2), and downstream Kvarnbäcken (location S3) (Figure 1). In the streams, minnows were caught using an electrofishing approach and killed with an overdose of benzocaine. Fish were frozen to -20°C and transported to the lab at Uppsala University.
In total, 279 minnows were analyzed, 158 from the lake locations (L1: 52, L2: 52, L3: 54) and 121 in the streams (S1: 50, S2: 50, S3:21). In the lab, fish were thawed and subsequently individual length (to the nearest mm) and weight (to the nearest 0.01 g) were measured. For geometric morphometric analyses, a photograph was taken on the left side of the fish with fins stretched out. The entire gut was collected and kept frozen at -20 °C for subsequent gut content analyses.