This assessment currently includes individuals belonging to the nominal taxa Salmo visovacensis Taler 1950 and Salmo zrmanjaensis Karaman 1938. These were revalidated in 2021, but will not be included in the Red List until a copy of the relevant reference can be sourced.

At the broader scale, there is currently no general 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 and 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 and 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 and 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).

Global and European regional assessment: Near Threatened (NT)
EU 27 regional assessment: Near Threatened (NT)

The West Balkan Trout does not approach the range thresholds for Vulnerable under Criterion B1 (extent of occurrence (EOO) < 20,000 km²) or D2, and Criterion B2 is precluded by its uncertain area of occupancy (AOO). The population size is believed to exceed 10,000 mature individuals, and hence does not approach the thresholds for Criteria C or D. There exists no quantitative analysis which would permit application of Criterion E.

Although no explicit population trend data exists, it is possible that a suspected ongoing reduction based on field observations, declining habitat quality and the effects of introduced taxa may approach or meet the threshold for Vulnerable under Criterion A2 (≥ 30% over the past 18 years = three generations). As a result of this data uncertainty, Least Concern and Vulnerable are equally plausible Red List categories for the present assessment, and this species is assessed as Near Threatened both globally and for the EU 27 member states.

This species is native to the eastern Adriatic and Ionian sea basins, where its range extends southward from the Zrmanja River in Croatia to the Alfeiós (el. Αλφειός) River in the western Peloponnese, Greece.

It is understood to have been stocked outside of this range in some parts of Greece and the Republic of Macedonia, e.g., the Vardar River system.

This species' population size is unknown, but it is understood to exceed the minimum threshold for Red List criteria (< 10,000 mature individuals). The current population trend has not been quantified, and the number of subpopulations is unclear.

Overall abundance is understood to have declined significantly since the late 20^th century, and this pattern is suspected to be ongoing in parts of its range, based on field observations, declining habitat quality and the effects of non-native taxa (see 'Threats').

In terms of genetic structure, this species is included in the Adriatic mitochondrial lineage within the Brown Trout (Salmo trutta) complex (see 'Taxonomic Notes').

This species inhabits perennial, low nutrient headwater rivers and tributary streams which contain cool, well-oxygenated water and are often characterised by seasonal fluctuations in discharge. Substrata in such habitats tend to comprise a mixture of exposed bedrock, boulders, rocks, cobbles and gravel, with refuges in the form of overhanging riparian vegetation, undercut banks and woody structures such as branches, roots or fallen trees. Larger individuals occupy deeper pools and glides and are territorial, selecting stream positions in dominance hierarchies based on maximising their energy intake. In contrast, juveniles and subadults are often observed in riffles and runs. It has been reported to enter and is perhaps resident in Lake Skadar, Albania and Montenegro, and has occasionally been reported to enter artificial accumulation lakes.

It is a visual predator which feeds on benthic and drifting invertebrates, e.g., Ephemeroptera, Diptera, Plecoptera, Trichoptera, while larger individuals also consume amphibian larvae and smaller fishes.

Its lifespan is at least 10 years, and adults mature at age 2-3+. The annual reproductive period extends from November to March, with the precise timing dependent on location. It is characterised by nuptial individuals undertaking upstream migrations to specific spawning sites comprising well-washed gravel beds in shallow, fast-flowing reaches. After arriving at these sites, individual females create shallow depressions (redds) in the substrate, into which the gametes are deposited. The presence of unclogged, well-oxygenated interstitial spaces within each redd is considered to be crucial for successful incubation and early development.

This species is threatened by introgressive hybridisation with non-native Brown Trout (Salmo domestic strain), which has been introduced throughout much of its range for the development of recreational fisheries. The frequency of these activities increased dramatically during the 20^th century, leading to millions of alevins and fingerlings being stocked per year. The non-native individuals produced in hatcheries today are of mixed origin, but are typically derived from the Atlantic and Danubian Brown Trout lineages (see 'Taxonomic Notes').

This species is further threatened by river regulation and other forms of habitat degradation, which have resulted in widespread loss of the heterogeneous, interconnected fluvial habitats required to complete its life cycle. In particular, the construction of dams, weirs and other barriers has altered natural flow and sedimentation regimes, blocked access to spawning sites, fragmented subpopulations, and generally reduced the extent of suitable habitat for all life stages. The quality of habitat has been further diminished by bank stabilisation, channelisation and other efforts to enhance flood protection or exploit water for human development.

Hydroelectric dams have created regular fluctuations in discharge and water temperature (hydropeaking and thermopeaking) which cause dewatering of spawning sites, the loss of stable nursery habitat for juveniles, and downstream displacement and stranding of individual fish. The combined effect of hydropeaking, dam flushing operations, changes in land use, and the removal of riparian vegetation has also increased accumulation of fine sediments at spawning sites, thus impairing the hatching and survival rates of eggs and larvae.

The industrial extraction of riverine gravel and other sediments for urban development is likely to have further reduced the extent and quality of available spawning sites.

Diffuse and point source agricultural, domestic and industrial pollution has resulted in eutrophication or discharge of toxic substances at some locations.

Rising water temperatures due to climate change represent a plausible threat, since they may interfere with food availability, lifespan, and the timing of reproductive processes.

This species is a key component of recreational fisheries throughout its range, where fly-fishing for both native and non-native trouts is a growing industry.

It is also fished for food at the local scale in some areas.

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

It is nationally-protected in some countries within its range, and is present within the boundaries of numerous protected areas of which some are included in the European Union's Natura 2000 network.

It has been widely recommended that the conservation management of European trouts must be considered independent of their systematic classification, due to a lack of consensus regarding their taxonomy plus the existence of notable microgeographic genetic and phenotypic diversity (see 'Taxonomic Notes'). Each subpopulation should therefore be assessed individually, taking into account its evolutionary and genetic significance coupled with the ongoing population trend and threats, to result in a priority ranking permitting the effective allocation of conservation resources through the development of site-specific, catchment-scale management plans. Sympatric morphological forms should also be managed separately, depending on their respective habitat preferences, diets and life histories. The abundance trends of many subpopulations remain unknown, and their individual assessments should ideally form the basis of future research efforts in order to ensure appropriate prioritisation. In practice, such efforts will be most efficiently coordinated at local, national or regional scales.

A deeper understanding of this species' demographics (population size and trend), life history and response to the identified threats would undoubtedly prove beneficial in the design of any future management plan.