Current-loving fish species, many of which are on the "Red List" of endangered species, are particularly severely affected. "Brown trout, grayling and Danube salmon are demanding fish species that require oxygen-rich water and spawn in coarse gravel areas. As typical residents of the upper reaches of rivers, they are unable to find suitable habitats in dammed areas," explains Juergen Geist, Professor of Aquatic Systems Biology at TUM. "These river sections are often dominated instead by bream, chub, and even carp generalist species that are actually adapted to stagnant waters. The ecological impoverishment of rivers is particularly dramatic when series of dams prevent the sufficient interlinking of different habitat types," says Geist. According to the Bayerische Landesamt fr Umwelt (Bavarian Environmental Agency) there are more than 10,000 weirs in Bavaria's rivers alone.
According to the researchers, the main reason for the species decline is not the impermeability of the barrier to migrating fish species. Instead, the deciding factor is the chemical and physical alteration of the river itself which leads to a reduction of biodiversity. If the current is decelerated or interrupted, the flow rate upstream of the weir declines, and this is accompanied by increased water depth. In all of the surveyed dammed areas, the TUM scientists also recorded major differences in the oxygen content and temperature of the water and the sediment in the river bed, which hinders reproduction in current-loving fish species. The situation is compounded by differences in the structure of the sediments. The particles in the sediment found downstream of weirs are twice as large on avera
|Contact: Undine Ziller|
Technische Universitaet Muenchen