Fish

Foto: M. Roggo

Fish are found in virtually all of Switzerland's watercourses. For various reasons, they are appropriate bioindicators of the state of streams and rivers: as fish are relatively long-lived, they can reveal the effects of long-term changes in environmental factors. Their differing habitat requirements make them useful indicators of morphological and hydrological conditions. The mobility and migrations of fish species enable conclusions concerning the passability and interconnectedness of watercourses. In addition, fish are easy to identify in the field, and their ecology is well known.

With the method employed under the Modular Stepwise Procedure, fish are caught by means of electrofishing in a section of the watercourse that is to be assessed, and the species are identified. Fish length is measured, and any deformities or abnormalities are recorded. The survey data provide information on the species composition, age distribution and reproductive capacity of fish populations.

This assessment takes into consideration natural differences in the occurrence of fish species. In the relatively steep, small streams and rivers that make up a large proportion of Switzerland's watercourses, the brown trout is the commonest naturally occurring species and is thus considered as the dominant species of the trout region. Further downstream, in larger, slower-flowing and progressively warmer waters, this gives way to the grayling region, then the barbel region and finally the bream region.

The assessment includes the following parameters:

• Fish region and potential spectrum of species
• Population structure of the dominant fish species
• Natural reproduction of the dominant fish species
• Relative density of brown trout
• Deformities and abnormalities
• Relative frequency of fish species

The module "Fish Level I" was published in 2004 by the Federal Office for Environment, Forests and Landscape FOEFL. (In German and French, "Vollzug Umwelt, Mitteilungen zum Gewässerschutz Nr. 44")

Download Module "Fish Level I" (German / French)