Eurasian perch (Perca fluviatilis) is negatively affected by lake browning in southern Sweden

Climate change and human land use leads to browning of waters with decreased visual conditions. This impacts the fish community via changes in foraging ability and food chain efficiency. There are indications that brownification leads to decreased body size of the Eurasian perch (Perca fluviatilis) and that roach (Rutilus rutilus) are favoured over perch in brown lakes since roach is less dependent on its eyesight when foraging. The aim with this project is to examine how browning impacts the fish community in lakes in the south of Sweden and if browning reduces the size of perch in relation to their age. Data on water colour and length-at-age for 1+ and 5+ perch as well as the number of perch and roach captured was collected from 20 lakes. The data was examined using linear regression and paired samples T-test in SPSS. The results show that browning does reduce the body size of perch. Perch is smaller in relation to their age in brown lakes. There was however no evidence indicating that roach is favoured over perch in brown lakes, but there was a difference between the number of 1+ and 5+ perch caught in relation to roach. The perch: roach ration decreased for older fish (from 1+ to 5+) indicating that there is a difference in survival rate. Survival rate of perch was however not dependent on water colour or TOC. This shows that climate change and increased browning could result in changes of the fish community and size structure of fish.