Trout return to previously acidic Šumava lake

Laka lake, photo: Wikimol, CC BY 2.5

The Laka lake in the Šumava mountain range was severely damaged by acid rain in the twentieth century. Now the quality of the water is improving again and several dozen trout have been detected in the lake. A science team is conducting research to find out how they got there.

Less than three hectares in size, Laka is the smallest lake in the Šumava Mountains. However, it is also the highest situated and now holds another title - it is the first lake in Šumava to have trout return to its waters.

The discovery has excited hydro-biologists from the Czech Academy of Sciences, because up until relatively recently, Šumava lakes were so acidic that no fish species could survive in them. However, the water composition in the area has been changing since 1989 and ten years ago a trout was spotted in the stream leading out of the Laka lake.

Hydrobiologist Petr Blabolil from the Biology Centre of the Czech Academy of Sciences told Czech Radio that the return of trout to Laka is the result of several factors.

“It is thanks to constraints on the surrounding industry as well as due to improvements in waste collection technologies. In the past, the Laka was especially affected by exhaust gasses that got into the air and then lowered the Ph level of the lake, making it more acidic.

Illustrative photo: Pe-Jo,  Wikimedia Commons,  CC BY 3.0

“The local beavers also played their part, because they built dams on the tributary streams which caused the lake to raise its surface level and form spillways. It is through these that we believe the trout got into the lake.”

He has collected several samples of fins from the fish which are now being analysed by his colleague Kateřina Soukalová.

"When we look at the trout’s scales, we can see yearly increments comparable to the annual growth rings on the stump of a tree. With the help of these markings we can then determine how old the fish is.”

Estimating the age of the fish living in the lake is only part of the research, says Petr Blabolil. The main focus will be on genetics.

“We want to find out whether the trout population in the Šumava area is different from that in other parts of the country. If so, it would suggest that they are indigenous. If not, it means they were somehow brought here.”

The science team plans to visit the lake again during the spring of next year and search for fish larvae. If found, it will be proof that trout is not only back in the lake, but also capable of reproducing there.

Authors: Thomas McEnchroe , Jitka Cibulová Vokatá
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