Switzerland’s rivers harbour a unique biodiversity. From 2013 to 2018 – in order to assess this diversity in more detail for the first time – scientists from the Fish Ecology & Evolution department systematically collected fish samples (in September and October in each case) from hundreds of rivers and streams. The project, known as “Progetto Fiumi”, is funded by the Federal Office for the Environment and coordinated by Jakob Brodersen. “More than half of the fish caught were trout,” he says. “And we found them in almost all habitats – from small mountain streams at an altitude of over 2000 metres above sea level right down to the lowlands.” Altogether, fish were sampled at 308 sites across all types of river and stream throughout Switzerland.

Trout have long been known to exhibit considerable diversity: to date, five species have been distinguished, which developed in complete isolation from each other in the major river systems – Atlantic trout (Salmo trutta) in tributaries of the Rhine, Rhône trout (Salmo rhodanensis), marble trout (Salmo marmoratus) and Northern Italian brook trout (Salmo cenerinus) in rivers draining to the Adriatic, and Black Sea trout (Salmo labrax) in the Inn. In the early twentieth century, Swiss fish biologists also drew attention to different forms of trout around the lakes of the Engadine.

New findings on trout diversity

In systematically recording the multiple variations between and within species, Brodersen’s group found forms that differ in appearance, behaviour and in some cases also genetic composition. For example, studies in the catchment of Lake Lucerne showed that trout in streams with constant flows show greater individual specialisation in prey selection than trout in mountain torrents with highly variable flows. In Graubünden, the researchers compared lowland river trout – living in relatively favourable environmental conditions – with their relatives in the inhospitable upper reaches of mountain torrents. Contrary to expectations, however, the “Alpine trout” were not smaller at first spawning but in fact significantly larger than the lowland trout – an important consideration when minimum catch sizes are specified. In the canton of Vaud, authorities and fishery inspectors drew the researchers’ attention to a stream where spotted trout live alongside unspotted specimens. Genetic studies revealed that the spotted fish are closely related to other river trout living nearby, but not to their unspotted neighbours, with which they do not generally mate.