Abstract
Brown trout of non-native lineages (e.g. Atlantic) have been stocked into Croatian streams and rivers primarily to meet angling demand, resulting in the presence of admixed populations at many locations. The clear, easily distinguishable morphological differences between the brown trout of the introduced Atlantic (AT) and indigenous Danubian (DA) lineages, as well as their hybrids (HY), were unknown. Therefore, the aim of this study was to identify classical morphometric and meristic traits that differentiate them. Brown trout were collected by electrofishing in ten streams from three mountain areas (Gorski Kotar, Mt. Žumberak and Mt. Papuk) during 2017 and 2018. Additionally, 10 specimens belonging to the AT lineage were obtained from a fish farm in continental Croatia. In total, 90 fish specimens were analysed: 17, 22 and 51 specimens of AT, DA and HY, respectively. The results revealed the most prominent differences in the head features of the AT compared to the DA lineage and their HY. Discriminant analysis demonstrated that eye diameter and postorbital length were significantly smaller, whereas interorbital width and head height were significantly larger in the AT lineage. These findings suggest the need for a detailed head morphometry analysis to identify distinctive morphological traits that differentiate brown trout lineages and detect their hybrids.