There is currently no broad consensus regarding the systematic classification of Eurasian and North African brown trouts, an assemblage comprising all representatives of the genus Salmo except the well-differentiated Atlantic Salmon (Salmo salar), Marble Trout (Salmo marmoratus), Softmouth Trout (Salmo obtusirostris) and Ohrid Belvica (Salmo ohridanus). While numerous, often range-restricted, members of this grouping have been described based largely on their ecological and morphological diversity, this variability is not consistently reflected by phylogenetic and phylogeographic evidence (Sanz 2018; Whiteley et al. 2019; Segherloo et al. 2021).

Despite a relatively recent diversification history spanning the period 0.5-2.5 Mya, brown trouts exhibit marked ecological and phenotypic variability throughout their large native range, which extends eastward from Europe and Northwest Africa to Russia and the Aral Sea basin. They occupy a wide range of habitats, from mountain streams and larger rivers to lakes and estuaries. Individual subpopulations can exhibit sedentary, anadromous or potamodromous life history strategies. Some freshwater systems are inhabited by multiple sympatric forms which differ in traits associated with foraging and reproductive ecology, and are sometimes referred to as "morphs", "ecomorphs" or "ecotypes" (Klemetsen et al. 2003; Kottelat & Freyhof 2007; Ferguson et al. 2019; Segherloo et al. 2021).

Some authorities have viewed this combination of factors to be representative of high species diversity and recognised around 50 nominal taxa, a number of which have been described this century (Kottelat & Freyhof 2007; Snoj et al. 2011; Sanz 2018). Alternatively, their systematics have been viewed from a phylogenetic and phylogeographic perspective based largely on mitochondrial DNA (mtDNA) analyses, with all subpopulations treated as a single polymorphic taxon customarily referred to as the “Brown Trout (Salmo trutta) complex” (Sanz 2018; Whiteley et al. 2019; Segherloo et al. 2021).

The latter approach led to brown trout diversity being defined by ten mtDNA lineages or sublineages corresponding to extensive catchments (the Danube, Atlantic, Mediterranean and Adriatic basins), specific geographic areas (the Balkan Peninsula and North Africa), individual watersheds (the Dades, Duero and Tigris rivers) and a distinctive phenotype (Marble Trout). Subsequent studies revealed that the distribution of some of these mtDNA lineages extends beyond their defined boundaries, e.g., the Adriatic lineage occurs from the Iberian Peninsula to the Republic of Türkiye, and the Marble Trout lineage is present in areas where no marbled phenotype exists, such as Corsica, central Italy, Albania and Greece (Bernatchez et al. 1992; Apostolidis et al. 1997; Bernatchez 2001; Suárez et al. 2001; Cortey & García-Marín 2004; Sušnik et al. 2005, 2007; Splendiani et al. 2006; Martínez et al. 2007; Snoj et al. 2009, 2011; Tougard et al. 2018; Schöffmann et al. 2022).

However, several studies have revealed the presence of mosaic distributions of mtDNA haplogroups among wild brown trout populations, plus mitochondrial-nuclear phylogenetic discordance in reconstructions made with both mitochondrial and nuclear trees (Snoj et al. 2009; Pustovhr et al. 2014; Leucadey et al. 2018; Splendiani et al. 2020). This suggests the presence of incomplete lineage sorting or asymmetric introgressive hybridization, which are common phenomena in rapidly diverging lineages and indicate that mtDNA genealogies might be generally unsuitable for defining phylogenetic relationships between brown trout taxa (Pustovhr et al. 2011, 2014). In the case of brown trouts, naturally intricate patterns of diversification and secondary contact shaped by repeated glaciations during the Pleistocene have been additionally complicated by widespread anthropogenic translocation and introgressive hybridisation since the Middle Ages (Largiadèr & Scholl 1996; Sanz et al. 2006; Lerceteau-Köhle et al. 2013). The combined use of multiple nuclear (nDNA, e.g., microsatellites, nuclear genes) and mitochondrial markers has already provided better insight into this complex scenario, resulting in progress towards a deeper understanding of evolutionary relationships at particular geographic scales or among subsets of putative taxa (Snoj et al. 2002, 2010, 2011; Sušnik et al. 2006, 2007; Berrebi et al. 2013, 2019; Gratton et al. 2014; Marić et al. 2017).

An integrative taxonomic approach combining morphological and ecological data with next generation sequencing of nDNA to identify genomic clusters may represent the most promising option for resolving brown trout systematics (Guinand et al. 2021; Segherloo et al. 2021). However, no comprehensive morphological or nDNA analyses have yet been completed, and it is plausible that the elaborate genetic and phenotypic diversity demonstrated by these fishes may never be adequately captured by a single accepted taxonomic system (Whiteley et al. 2019).

Pending a definitive outcome to the above, the Red List broadly follows the nomenclature provided by Fricke et al. (2024).

The Louros Trout has a restricted range (area of occupancy (AOO) c. 56 km², extent of occurrence (EOO) c. 407 km²), within which one subpopulation occurs at a single location where the quality of habitat is estimated to be declining.

It is plausibly threatened by introduction of non-native fish species, extensive modification of the Louros River channel, overharvesting by recreational anglers and water abstraction for irrigated agriculture.

This species is therefore assessed as Endangered under Criterion B (B1ab(iii)+2ab(iii)).

This species is endemic to the Louros (el. Λούρος) River system in Epirus, northwestern Greece.

It is currently restricted to c. 28 kilometres of linear river length, extending upstream from a small hydropower dam at the village of Agios Georgios to the vicinity of Chani Tereovou and Vouliasta.

It is more abundant in the upper reaches of this range, comprising a stretch of c. 10 kilometres.

This species' current population trend has not been quantified, but abundance within the single subpopulation has been estimated at c. 1,750 individuals.

This rheophilic species mostly occurs in the headwaters of the karstic, spring-fed Louros River, where discharge is stable throughout the year and habitats comprise cool, fast-flowing water with substrata of unclogged gravel, cobbles and boulders.

It is less abundant further downstream, where the river flows through a small agricultural valley before reaching the artificial dam lake at Agios Georgios (see 'Threats'). Crops grown in the area include citrus and other fruits, corn, olives and cotton, while pig, poultry and occasionally cattle farming is widespread.

The non-native Rainbow Trout (Oncorhynchus mykiss) has been reared in the Louros catchment since the 1960s and there are currently 22 hatcheries which directly exploit the river's water. These together comprise a surface area of 34,120 square metres and produce c. 520-550 tonnes of fish per year (see 'Threats').

This species' life history has not been studied in depth, but the maximum lifespan is at least five years. The annual reproductive period takes place in winter, during which sexually mature adults migrate short distances to spawning sites mostly located downstream of the springs at Vouliasta (sometimes referred to as "Terovo Springs").

The diet is likely to comprise terrestrial insects plus epibenthic invertebrates such as Amphipoda, Ephemeroptera, Plecoptera and Trichoptera. The Louros River is noteable for the richness and density of its macrobenthic community, and the relatively deep body shape of the Louros Trout might be indicative of a sedentary, territorial lifestyle evolved to exploit this resource.

This species is plausibly threatened by introduction of the non-native Rainbow Trout, which is reared in commercial aquaculture facilities located immediately adjacent to the upper Louros and has been stocked in the river. The two species are likely to compete for similar resources, and larger Rainbow Trout individuals may prey on subadult and juvenile Louros Trout. There is also a risk that pathogens and parasites from these hatcheries could enter the Louros system.

Extensive modification of the river channel took place in the upper Louros, downstream of Terovo Springs, during November 2022. In particular, a series of embankments were created by depositing boulders and material excavated from the riverbed and banks along its margins, with both riparian and aquatic vegetation removed, and a 10-kilometre stretch of the river was temporarily dewatered. These activities have not only significantly reduced the extent and quality of suitable habitat in this species' primary spawning area, but were carried out at the onset of its reproductive period. The major negative impacts are likely to include increased water temperatures and sedimentation at spawning sites, both of which may hamper recruitment and reduce prey availability.

Overharvesting by recreational anglers is believed to constitute a significant threat since catch and release is not mandatory, although no precise data are available.

The hydroelectric dam at Agios Georgios was built in 1963 and has no fishway, thus limiting the Louros Trout's downstream dispersal.

Upstream of the dam the Louros runs through a valley which has been largely converted to irrigated agriculture, and livestock farming also takes place in the area. Water is abstracted directly from the river, but it is unclear whether this has driven any significant reduction in discharge. It is plausible that water quality within and downstream of this stretch is negatively affected by diffuse pollution from nutrients and agroindustrial toxins, and/or that increased soil erosion and siltation is leading to a reduction in suitable foraging and spawning habitat due to gravel beds becoming clogged.

This species is targeted by recreational anglers, but is not otherwise used commercially.

This species is included (originally as Salmo macrostigma) in Annex II of the European Union Habitat Directive.

No conservation actions are currently in place, but a National Action Plan for Greek trouts published in 2019 recommended the creation of a new protected area covering this species' entire range.
Official protection of the upper Louros River, including regulations preventing the further release of non-native aquatic taxa plus creation of a no-kill angling zone therefore represent the primary conservation goals.

It is recommended that future research should initially focus on establishing this species' population trend and its response to the identified threats.