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Fish populations vary naturally but are also affected by anthropogenic factors such as changes in land use, climate, acidification, invasive species and eutrophication. An overall picture is important for understanding the changes that are taking place and their effects. In this study, I have linked several environmental factors, such as turbidity and chlorophyll-a, with the presence of pikeperch and perch in Lake Mjörn. I have also investigated how the size of the perch has changed between the years 2000 and 2018 for the entire population, and specifically within the size class 80 - 120 mm. Gill net surveys from 2000, 2007 and 2018 shows that the median length of pikeperch decreased from 2007 to 2018 by 39.6% and increased in average per net effort by 630% between 2000 and 2018, a clear shift in the population from few and older to more, and a larger proportion of, younger individuals. The turbidity has had a clear downward trend with a decrease of 3.9 cm per year since 1985, with a Secchi depth of less than 4 meters in the last 10 years, which is probably an important factor contributing to the increase of the pikeperch population. At the same time, the chlorophyll-a concentration has shown an upward trend of 0.174 μg/l per year. An observed decrease in macrophytes around the lake over the past 20 years may have caused some increased wind-driven mixing and a greater amount of suspended solute particles. An increased amount of nutrients from the sediment could have explained the increasing amount of phytoplankton, but sampling carried out at the inlet to and the outlet of Lake Mjörn shows that the concentrations of nutrients in the lake have decreased. The median length of perch has decreased by 52.3% from the year 2000 to 2018, and the median length of 2 summer old perch shows a decrease of 17.5% during the same period. The difference in median length between the entire perch population and the 2 summer old individuals was 47% in 2000 but only 8.3% in 2018, which indicates that the population in 2018 largely consisted of 2 - 3 year old individuals.