Pigo (Rutilus pigus), the other Rutilus species native to the southern lakes, was recorded in fewer lakes than R. aula (Como, Mezzola, and Maggiore), and was everywhere less abundant than R. aula. R. pigus was recorded in highest numbers in lakes Como and Mezzola where R. rutilus had not become abundant (yet). In Lake Maggiore, only six juvenile fish phenotypically resembling R. pigus were recorded.Three of these were confirmed as R. pigus by genetic barcoding, while two had the mitochondrial barcode of R. rutilus and one had that of R. aula (Figure 34). In Lake Lugano, no fish phenotypically matched R. pigus, however one adult fish that was phenotypically mostly R. rutilus had the mitochondrial barcode and also the lateral line scale count of R. pigus (Figure 34). Several other fish also had meristic traits of R. pigus (>43/44 lateral line scales), yet otherwise resembled R. rutilus. The mismatch between general appearance, meristic traits and mitochondrial barcode again indicates hybridisation and backcrossing between R. pigus and R. rutilus, and in Lake Maggiore also between the two native species R. aula and R. pigus. To our knowledge, this is the first genetic data that show that the dramatic decline of native R. pigus and R. aula in Lakes Lugano and Maggiore was associated with introgression into the abundant invasive northern R. rutilus (as predicted in [92]), but also with introgression between the two native species. It is possibly that the breakdown of reproductive isolation between the native species was mediated by the hyperabundant invasive species. Such complex species interactions deserve further attention by researchers. Another important line of research for the conservation of the native southern Rutilus species would address the reasons for the dominance of the invasive Rutilus rutilus in Lakes Lugano, Maggiore and Varese, but not in Lakes Como and Mezzola. There was no evidence for translocation of either of the southern Rutilus species to the lakes in the Rhine or Rhone catchments (several thousand phenotypes inspected, 70 fish barcoded).

Published as part of Alexander, Timothy & Seehausen, Ole, 2021, Diversity, distribution and community composition of fish in perialpine lakes – " Projet Lac " synthesis report, Eawag: Swiss Federal Institute of Aquatic Science and Technology on page 120, DOI: 10.5281/zenodo.5779569