The two cyprinid species Abramis brama and Blicca bjoerkna appear superficially similar and often hybridise across much of their overlapping distributions. Many fish identified in the field in Projet Lac as Blicca bjoerkna, were revealed by genetic barcoding to be Abramis brama in their mitochondrial genome. In the other direction, two individuals identified as A. brama were B. bjoerkna in their barcode. A. brama grows to a larger size of 70 cm compared to maximum standard length of 33 cm in B. bjoerkna [9]. The species can also usually be distinguished based on the number of lateral line scales (51 – 60 in A. brama versus 43 – 46 in B. bjoerkna) and branched rays in the anal fin (23 – 30.5 inA. brama versus 19 – 23.5 in B. bjoerkna [9]), but all these meristic traits are difficult to evaluate in juveniles and small subadults. Both species were only recorded in the northern perialpine lakes in Projet Lac. A. brama was generally more common, recorded in 15 lakes, compared to nine lakes for B. bjoerkna. Indeed, B. bjoerkna was not recorded in any lakes that did not also have A. brama. A. brama was recorded in particularly high numbers in Geneva (220 fish) and Lower Constance (69 fish). B. bjoerkna, on the other hand, was particularly abundant in Bret (234 fish) and Upper Constance (143 fish). Both species were similarly abundant in Lake Morat (38 A. brama and 37 B. bjoerkna). In future monitoring efforts great attention has to be paid to carefully distinguish these species. Genetic work to establish the extent of genetic mixing between the species in Switzerland and the perialpine region would also be worthwhile. Both species are also known to hybridize with Rutilus and we recorded several Abramis brama x Rutilus rutilus hybrids. Monitoring of this in the future would help understand the drivers and consequences of hybridization among cyprinid species.

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 119, DOI: 10.5281/zenodo.5779569