This taxon represents one of four closely-related sympatric trout morphs (see below) that are endemic to Lake Ohrid. These are understood to have evolved via a process of adaptive radiation which led to subsequent reproductive isolation through spatiotemporal differences in their spawning ecology. Since the mid-20^th century, they have been variously regarded as conspecific with the wide-ranging Brown Trout (Salmo trutta), a single polymorphic taxon, or distinct species/subspecies in their own right. For Red Listing purposes, they are currently treated separately following Fricke (2024), and thus comprise the species Salmo letnica (Karaman 1924), S. balcanicus (Karaman 1927), S. lumi Poljakov 1958 and S. aphelios Kottelat 1997.

Although no recent morphological analysis exists, these taxa can reportedly be separated by meristic and morphometric characters (Sell & Spirkovski 2004). Moreover, the discrete genetic status of S. letnica and S. lumi has been confirmed by nuclear DNA studies (Pustovrh et al. 2014; Segherloo et al. 2021). However, while analyses based on mitochondrial DNA have revealed subdivision between S. letnica and S. aphelios (Sell & Spirkovski 2004; Geiger et al. 2014), they could not be separated by nuclear DNA (Sušnik et al. 2007). Unfortunately, S. balcanicus has not been included in any recent analysis and may already be extinct (Kottelat & Freyhof 2007). The distinctly divergent Salmo ohridanus is also endemic to Lake Ohrid, but is understood to represent a considerably older genetic lineage.

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 & 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).

Global and European regional assessment: Critically Endangered (CR (PE))
EU 27 regional assessment: Not Recorded

The Struga Trout has a restricted range (extent of occurrence (EOO) c. 0-386 km², area of occupancy (AOO) c. 0-360 km²), which meets the thresholds for the Critically Endangered category under Criterion B1 (EOO < 100 km²) and Criterion B2 (AOO < 10 km²). It is present at one location where the quality of habitat is estimated to be declining.

It has not been unequivocally observed in the wild since around the 1960s and might today be extinct. However, no dedicated field surveys appear to have been carried out and there is a possibility that it has been overlooked by commercial fisheries due to the presence of similar-looking sympatric congeners.

Therefore, this species is assessed as Critically Endangered (Possibly Extinct).

This species is endemic to Lake Ohrid (sq. Liqeni i Ohrit; mk. Охридско Езеро), located at the boundary between eastern Albania and southwestern Republic of North Macedonia.

This species' population size and trend have not been quantified, and there are no baseline data. There are no confirmed records since the latter half of the 20^th century, although it is unclear whether dedicated surveys have been carried out.

Oligotrophic and oligomictic Lake Ohrid occupies a steep-sided graben and is of tectonic origin. Its surface area is 358 km², and it is the deepest lake in the Balkan region, with a mean depth of 164 metres and maximum depth of 289 metres. The lake remains oxygenated even in the profundal zone, but is stratified by salinity below depths of 150 metres and only mixes completely about once per decade. With an estimated minimum age of c. 4 million years, Ohrid is probably the oldest extant lake in Europe.

It is fed primarily by two major karstic surface springs at its southern end plus a series of sub-lacustrine (underwater) springs which are located along the eastern and, to a lesser extent, western shores. All such inflows on the eastern and southern sides originate in the adjacent Lake Prespa (sq. Liqeni i Prespës; mk. Преспанско Езеро; el. λίμνη Πρέσπα) basin, which is located at a slightly higher altitude and drains to Ohrid via subsurface karstic conduits. Four small rivers and numerous intermittent streams also influence the overall water level to a minor extent in spring or during periods of heavy rainfall. Ohrid is drained by the Black Drin (sq. Drini i Zi; mk. Црн Дрим) River, which flows towards the Adriatic Sea after exiting the lake at its northern tip, and it loses significant volumes of water (c. 40%) via evaporation. Due to its relatively limited recharge and discharge regimes compared to its overall volume, it is characterised by a long retention time of c. 83.6 years. The lake's water balance and watershed were artificially enlarged during the 1960s when the River Sateska, which formerly flowed into the Black Drin, was diverted into the lake to increase its potential for hydropower generation.

The lake contains exceptionally clear water with a Secchi depth of 7-16 metres in the pelagic zone. It is rich in macrophytes, among which the littoral vegetation includes a discontinuous strip of Common Reed (Phragmites australis) extending around 1.5 metres into the water, dense stands of angiosperms (particularly Potamogeton perfoliatus) to depths of c. four metres, and a largely continuous dense belt of stonewort (Chara spp.) at depths of four metres to c. 11 metres, extending to c. 18.5 metres in places.

The lake’s low-nutrient, high-oxygen environment supports an exceptional biodiversity consisting of at least 1,200 taxa, of which more than 200 are endemic. It is thus among the most species-rich inland water bodies on Earth when surface area is taken into account. The lake is also considered to be a hotspot of endemic salmonid diversity, with up to five recognised species (see 'Taxonomic Notes').

The Struga Trout's biology and ecology are largely unknown. The annual reproductive period is understood to extend from December to January, when spawning putatively takes place on sandy substrata in the littoral zone at the northern end of the lake. The major breeding sites were reportedly located in the Black Drin River, but these were flooded due to the construction of the Globochica hydroelectric dam during the 1960s. The installation of a weir on the outlet of Lake Ohrid at Struga may also have impeded access to the river in one or both directions.

The Lake Ohrid ecosystem is currently threatened by a series of factors, but this species' decline is believed to have been driven by dam construction on the Black Drin River (see 'Habitat and Ecology').

Lake Ohrid supports a long-established commercial fishery, with Ohrid Trout (Salmo letnica), Ohrid Belvica (Salmo ohridanus), Skadar Bleak (Alburnus scoranza), European Eel (Anguilla anguilla) and the non-native Eurasian Carp (Cyprinus domestic strain) comprising the most important species. The Struga Trout was apparently harvested during the 20^th century, but no explicit data are available.

A hatchery-based supportive breeding and stocking programme for the Ohrid Trout has been established at the Hydrobiological Institute Ohrid in the Republic of North Macedonia since 1935, and the Struga Trout was apparently reared at a separate hatchery during the 1960s.

The North Macedonian portion of Lake Ohrid has been included in UNESCO's list of World Heritage sites since 1979. A bilateral agreement between Albania and North Macedonia resulted in establishment of the Ohrid Watershed Management Committee in 2004 and the Ohrid-Prespa Transboundary Biosphere Reserve in 2014. At the national scale, a number of additional protected sites have been created around the lake, including the Pogradec Aquatic and Terrestrial Protected Landscape and Drilon National Monument in Albania, and the Lake Ohrid Monument of Nature and Galičica National Park in North Macedonia. In April 2021, Lake Ohrid was added to the Ramsar Convention on Wetlands of International Importance (site 2449).

Efforts to reinforce the Ohrid Trout fishery through supportive rearing and restocking have taken place in most years since 1935, but there exist only vague references to this species' inclusion in the programme.

Dedicated field surves are required to clarify whether this species' remains extant.