## 1 Introduction  
Intentional introductions of alien fish species, or translocations of fish populations belonging in the same species, genus or family, to different ecosystems, have been taken place in Greek inland waters for nearly one hundred years, aiming to the enhancement of the ichthyofauna of several local drainages [1]. In many cases, however, these actions have been proved not only unnecessary, but also harmful for the indigenous freshwater fish populations [2]. Such incidents, unfortunately, include the numerous, still debatable, _Salmo_ taxa described in Greece, which exhibit considerably high levels of phenotypic and genetic variation [3]. As a result of these stocking activities, the non-indigenous populations or species have hybridized with the local ones putting in danger their unique genetic profile [4]. The Nestos (Mesta) River in north-eastern Greece represents such a case, where the local populations of trout, inhabiting the tributaries of the river, have been affected by stocking activities conducted many times in the past. Specifically, in the mid-1970s, 20,000 _Salmo trutta_ fingerlings of domestic strains, originating from a fish farm in Acheloos River (central Greece), were released in the lower parts of Nestos' tributaries, Arkoudorema and Diavolorema [1; 5]. Subsequent translocations, some of which remaining unrecorded, have also been conducted during the next decades in other streams of Nestos, in Greece or Bulgaria, with trout from the same river or other southern Balkan's drainages. For instance, in 2011 in Arkoudorema River, the local Hunting Club with the assistance of the Forestry Agency released several thousands of trout fingerlings (Department of Fisheries, Regional Unit of Drama, Administrative Region of Eastern Macedonia and Thrace, Greece-personal communication), while unconfirmed information from fishermen, report also brown trout translocations from former Czechoslovakia to Bulgaria, in the second half of the 20 th century without any additional information [6]. It must be noted that Nestos River together with its tributaries in the area, in the last decade, have been included in the protected area of the National Park of Rodopi Mountains, Administrative Region of Eastern Macedonia and Thrace, Greece. We stress that the taxonomic status of the brown trout from Nestos River remains a matter of debate [7]. [PERSON] et al. [8], based on morphological characters, reported the trout from Nestos River as _Salmo macedonicus_. On the other hand, according to the morphological analyses of [PERSON] and [PERSON] [9] and [PERSON] et al. [10], _Salmo macedonicus_ is a distinct lineage distributed in the northern part of Axios River, North Macedonia. However, this taxonomic status is not supported by mtDNA data [11]. It is notable that [PERSON] et al. [11] did not confirm the presence of two species as suggested by several authors but support the existence of two different allopatric morphotypes (_S. trutta macedonicus_ and _S. trutta pelagonicus_), which evolved separately. Similarly, although a previous mtDNA sequence analysis clustered some native Nestos haplotypes in a distinct clade, the classification of brown trout from Nestos River as a valid species was not supported [5]. On the other hand, the taxonomic status of the trout from Acheloos River is also debatable as according to [PERSON] and [PERSON] [9] it is classified within the _S. farioides_ complex, whereas molecular genetic analyses did not support its taxonomic distinctiveness [4]. In a previous study conducted approximately 25 years ago on trout samples from the Nestos River, [PERSON] et al. [12] applying the mitochondrial DNA (mtDNA) PCR-RFLP methodology, found that 17 out of the 22 individuals examined had the same genotype with the prevalent genotype of the samples originating from Acheloos. Similarly, in a posterior mtDNA sequence analysis of these samples, [PERSON] et al. [5], found that five out of the nine examined individuals from Nestos, had the same haplotype with the fish collected from Acheloos.