Every living thing leaves behind tiny traces of its genetic material, for example in the form of dead skin cells or excrement. If one now takes water samples and decodes the environmental DNA (also known as eDNA) therein, one knows which species cavort in which waters. One thus discovers rare species that literally fall through the net during normal testing. To be sure, this concept of eDNA has long been known. But: “Until now one could determine from eDNA whether a species is present or not. But how this species is distributed in the whole ecosystem was unknown,” says Luca Carraro. He is now an Eawag researcher, after finishing his doctorate in environmental hydrology this summer at the EPFL.

Together with researchers at Eawag, the ETH and the EPFL, he has succeeded in reconstructing the extensive presence of species from selective eDNA measurements. For the first time, Carraro and his colleagues have used the method to determine the biomass distribution of a bryozoan (Fredericella sultana) and its parasite (Tetracapsuloides bryosalmonae) in the Wigger – a prealpine Swiss river. The parasite causes the dreaded fish disease PKD, which carries off thousands of trout every year in Switzerland.