The proportion of perch is increasing in high latitude lakes
The proportion of perch in the littoral zone of the sampled lakes substantially increased over the study period (Fig. 2a). All lakes with more than two sampling points showed an overall increase in proportion of perch with time. The overall mean trend reveals that the proportion of perch increased exponentially over time (p<0.01, mar-R2=0.356, Table S10), from under 10% in the early 1990s to above 70% in most lakes during the last decade, however with large variation between lakes (Fig. 2a). The relative density (CPUE) data in Lake Skrukkebukta and Lake Vaggatem shows a similar trend as the overall proportion data (Fig. 2b), with a substantial increase in relative density of perch in the littoral zone of both lakes (Skrukkebukta: t=8.014 on 22 d.f., p<0.01, adj-R2= 0.73. Vaggatem: t=4.042 on 24 d.f., p<0.01, adj-R2= 0.38, Appendix Table S11 & S12). This was related to an increase in water temperature where relative density of perch increased with annual mean water temperature (Weighted-Moving-Average over the last two years) in both Lake Skrukkebukta and Lake Vaggatem (Fig. 2c). In Lake Skrukkebukta the relative density has increased with 1.83 ln-CPUE·°C-1of temperature increase (t=3.788 on 20 d.f., p<0.01, adj-R2=0.389, Appendix Table S13), while in Lake Vaggatem, the increase was weaker, but significant with an increase of 0.89 ln-CPUE·°C-1 of temperature increase (t=3.788 on 20 d.f., p=0.048, adj-R2=0.141, Appendix Table S14).