Study area
The study area is concentrated in northern Fennoscandia, with lakes situated in Norway, Finland and Russia. All lakes are located north of the Arctic Circle, towards the northern end of the distribution of cool-water fish species (> 68˚N) (See Supplementary information, Fig. S3, for map of the study lakes and area). To assess the relative importance of Eurasian perch, we compiled data from 11 lakes sampled on multiple occasions over the last 30 years. The eleven study lakes were sampled between 2 and 26 times, with time series ranging from 8 to 32 years (Appendix Table S1).