Statistical analysis
The proportion of perch in the fish community was calculated as the
number of perch caught relative to the total number of fish caught in
the littoral zone of the different lakes. To estimate how the proportion
of perch in the catches changed over time we used a Linear Mixed Effect
model (LME) with location (lakes) as random effect, using the
nlme-package in R. The response variable was log(x+1) transformed.
Relative density of fish in terms of Catch-Per-Unit-Effort (CPUE) was
readily available for the two main study systems, Lake Skrukkebukta and
Lake Vaggatem. The CPUE was calculated as the number of fish caught per
100 m2 per night (or 12 hours). To investigate the
change in relative density over time, we estimated separate linear
models for the two lakes, with log(CPUE+1) as the response and year as
the predictor. To estimate the relationship between relative density and
water temperature, we again used CPUE on a natural logarithmic scale
with annual mean water temperature as predictor in a linear regression
model. We transformed the predictor to a weighted-moving-average over
the last two years with more weighting given to the latest year, to
better reflect any long-term effects of temperature on relative density
of perch. To investigate recruitment in the perch population, we used
the relative proportion of one-year-old individuals within the perch
population as a measure of the number of individuals surviving the first
winter, which is suggested to be dependent on water temperature (Dahlke
et al. 2020), but also food availability (or density of competitors) and
density of predators their first growth season. Relative proportion of
one-year-old individuals is calculated as the proportion of one-year-old
perch relative to the total CPUE of the perch populations. We used
linear regression with summer water temperature the preceding year on a
natural logarithmic scale to predict the relative proportion of
one-year-old individuals in the perch populations. In the statistical
analyses, we combined both lakes as the number of sampling points were
too low to treat them separately.