Rutilus rutilus

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 : 120

Rutilus rutilus

has been introduced into several southern perialpine lakes and was recorded by Projet Lac in lakes Lugano, Maggiore, Varese, Como and Mezzola. R. rutilus strongly numerically dominated the native Rutilus spp in lakes Lugano and Maggiore, and this northern species was the only Rutilus species recorded in Lake Varese. Interestingly, the introduced R. rutilus was less abundant than the two native Rutilus species in Lakes Como and Mezzola, and it was not recorded in lakes Idro, Iseo and Garda.

The presence of native triotto (R. aula) was recorded in all southern perialpine lakes, except Lugano and Varese. R. aula was particularly abundant in lakes Garda and Iseo, and was the only Rutilus species recorded in Lakes Garda, Iseo and Idro. In Lake Lugano, on the other hand, the only trace of R. aula among the many non-native R. rutilus were three individuals with meristic traits of R. aula (9.5 branched rays in dorsal and anal fins, versus 10.5 in R. rutilus ), that though otherwise looked more like R. rutilus . R. aula was very rare in Lake Maggiore, with only four individuals phenotypically identified as this species (versus more than one thousand R. rutilus ). Three of these fish were confirmed as R. aula by mitochondrial barcoding (the fourth was not barcoded). On the other hand, one fish from Maggiore that phenotypically matched R. rutilus (out of four barcoded R. rutilus ), had the mitochondrial barcode of R. aula. This suggests that hybridisation and backcrossing of hybrids between native R. aula and invasive R. rutilus is occurring.