"The trout can only withstand very short periods of time with water temperatures above about 22 degrees," says Serena Donadi, one of the researchers behind the study.
Donadi and her research colleagues at the Swedish University of Agricultural Sciences (SLU) have analysed 30 years of data on the occurrence of young trout in 174 Swedish watercourses. In rivers and streams in southern Sweden, a negative trend was noted.
"There is a risk that some of these stocks will be lost in the long term if no measures are taken," says Serena Donadi.
Increasing in northern Sweden
But at the same time as trout are decreasing in southern Sweden, the study also shows that populations are increasing in northern Sweden. It is likely that temperature increases there have led to the water ending up closer to 15 degrees – which is the water temperature in which the trout grow best.
"The temperature rise there benefits the trout to a certain limit before it swings back and becomes negative again," says Joacim Näslund, one of the researchers.
To give trout better conditions in southern Sweden, you can work with shading and plant trees along field edges.
In the video, Joacim Näslund answers the question why it is a problem that trout populations are declining in the south.