Fish sampling and individual data
The total number of sampled fish across the 11 lakes was nearly 60 000,
of which 12 000 were perch, and the littoral catches from this data were
used to describe the development in proportion of perch in the 11 lakes
(Appendix Table S1, the fish community of the individual lakes are
described in the Supplementary information). Fish were collected using
multimesh gill-nets or gill net series in all three sampled habitats
(i.e. littoral, pelagic, profundal) of the lakes, but only fish from the
littoral zone were included in the analyses because the sampling in the
other habitats was more scattered in both time and space (See
supplementary information for details on the fish sampling procedure).
We used individual data from the two lakes most frequently and
intensively sampled (Lake Vaggatem and Lake Skrukkebukta), to assess
changes in life history parameters over time in the perch populations.
These two lakes were selected because of their long time series
(~1991-2020) with mostly annual sampling, including
daily data on water temperature, and detailed individual data collected
from year 2003 to 2020. A total of 2960 individual perch from Lake
Vaggatem (n=38-608 per year) and Lake Skrukkebukta (n=30-130) were
sampled. The individual fish sampling procedure and summary information
on individual- and population level data are described in the
supplementary information Appendix 1 (see Tables S2 & S3).