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).

Global and European regional assessment: Endangered (EN)
EU 27 regional assessment: Endangered (EN)

The Prespa Trout has a restricted range (extent of occurrence (EOO) c. 1,384 km², area of occupancy (AOO) c. 100 km²), which meets the thresholds for the Endangered category under Criterion B1 (EOO < 5,000 km²) and Criterion B2 (AOO < 500 km²). It is present at four locations where the quality of habitat and number of mature individuals are estimated to be declining.

Therefore, this species is assessed as Endangered under Criterion B (B1ab(iii,v)+2ab(iii,v)).

This species is endemic to the Lake Prespa (sq. Liqeni i Prespës; mk. Преспанско Езеро; el. λίμνη Πρέσπα) transboundary basin, which is shared by Albania, the Republic of North Macedonia and Greece.

It is restricted to a series of four small affluent rivers draining northern and eastern parts of the catchment, comprising the Agios Germanos River in Greece plus the Brajcinska, Kranska, and Golema rivers in North Macedonia (see 'Habitat and Ecology'). The Agios Germanos River formerly discharged into Lesser Prespa, but was diverted to Greater Prespa in 1936.

This species' current population size and trend have not been explicitly quantified, and an accurate appraisal might be hampered by intra- and interannual fluctuations in the number of mature individuals (see 'Habitat and Ecology').

However, its population size is understood to have reduced significantly since the late 20^th century, most clearly demonstrated by its reported extirpation from several sites, e.g., the Golema River main stem, and a noticeable decrease in the frequency of older, large-sized individuals. Data collated for the Brajcinska River, Republic of North Macedonia, demonstrates a declining abundance trend over the period 2006-2020.

Molecular analyses indicate that the four identified subpopulations are genetically divergent, with implications for their management (see 'Conservation'). In terms of its overall genetic structure, this species is included in the Adriatic mitochondrial lineage within the Brown Trout (Salmo trutta) complex (see 'Taxonomic Notes'). It has been isolated from trout subpopulations inhabiting adjacent watersheds for a considerable period of time, and is not introgressed with any non-native taxon.

The Prespa Lakes basin lies at 850 m AMSL and is a hotspot for endemic biodiversity, including freshwater fishes and migratory birds. It comprises two relatively shallow interlinked lakes; the larger Greater Prespa (el. Μεγάλη Πρέσπα; mk. Преспанско Езеро; sq. Liqeni i Prespës së Madhe) and Lesser Prespa (el. Μικρή Πρέσπα; mk. Мало Преспанско Езеро; sq. Prespa e Vogël), which are separated by a narrow strip of alluvial land. The catchment is fed solely by precipitation and snowmelt. It partially drains to the adjacent Lake Ohrid (sq. Liqeni i Ohrit; mk. Охридско Езеро), and by extension the Drin (sq. Drini; mk. Дрим) River system, via subsurface karstic conduits.

This species' fluvial habitats are characterised by seasonal variations in discharge with low flow during the summer, when the lower courses are partially dewatered. At some locations, e.g., the Agio Germanos River, significant interannual fluctuations in abundance have been observed during particularly dry years.

It is confined to cool, well-oxygenated upstream (mostly > 1,000 m AMSL) reaches with a relatively high gradient, perennial flow and heterogenous bed morphology comprising riffles, runs and deeper pools, and substrata of exposed bedrock, rocks, pebbles and gravel. Refuges comprise undercut banks and woody structures such as branches, roots or fallen trees, and extensive shade is provided by the surrounding riparian forest. Larger individuals tend to 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 relatively gregarious and are often observed in riffles and runs.

There are a few lacustrine records from Greater Prespa, although these may refer to non-native introduced trout. On the other hand, it is plausible that some individuals are occasionally flushed into the lake during flood events.

This species 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 life history has not been extensively-studied, but there is evidence that individuals mature relatively early, which is an adaptation observed repeatedly in salmonids inhabiting unstable environments. The annual reproductive period is believed to occur during winter, and is likely to involve nuptial individuals migrating short migrations to specific spawning sites comprising well-washed gravel beds in shallow, fast-flowing reaches. In related species, 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 habitat alteration due to abstraction of water for agricultural and domestic supply. The installation of small accumulation dams, weirs and unregulated intakes throughout much of its range has caused the permanent dewatering of some tributaries, reduced connectivity within subpopulations, hindered seasonal movements and driven a general decline in the extent of suitable habitat for all life stages. The lower reaches of the main river channels often run completely dry for several months per year.

In addition, escalating bank erosion due to channel modification, livestock grazing, logging and the removal of riparian vegetation has caused accumulation of fine sediments at some spawning sites, plausibly impairing the hatching and survival rates of eggs and larvae. Degradation of the riparian zone may also lead to excessively warm water temperatures and interfere with the abundance of macroinvertebrates and other prey. In general, the loss of natural vegetation has been greatest in the Golema and Agio Germanos catchments.

Diffuse and point source agricultural, domestic and industrial pollution has resulted in eutrophication or discharge of toxic substances at some locations, particularly in the lower reaches of the Agio Germanos and Brajcinska rivers. Sources include cultivated and grazed land, poultry farming, small-scale food, textile, metal, wood, ceramic and chemical factories, illegal landfills, fly-tipping and discharge of untreated domestic wastewater.

In 2014, several small hydropower projects were approved for construction within the Brajcinska and Kranska watersheds. These have further reduced the extent and quality of habitat through dewatering of adjacent river stretches alongside an increase in habitat fragmentation, including the creation of impassable artificial landscape features.

The removal of mature individuals by anglers and poachers may also be driving reduced abundance and reproductive success, since no fishing regulations are in place (see 'Use and Trade').

Recent field surveys in the Republic of North Macedonia have demonstrated that the physical condition of many individuals is representative of inadequate nutrition. This is plausibly related to reduced accessibility to suitable downstream nursery habitats, since environmental factors during the initial growth period are often correlated with the eventual body size of adult trout.

Climate change represents a further plausible threat, with significant decreases in discharge, winter precipitation and snowfall observed throughout the Prespa catchment since the 1960s. Water abstraction and climate change have been identified as the central drivers of the diminishing water level in Greater Prespa, which has receded by at least 8 metres since the mid-20^th century.

This species has traditionally been targeted by local fishers and recreational anglers, but all forms of fishing are currently prohibited in Greece and at some locations in the Republic of North Macedonia (see 'Conservation').

At sites where harvesting remains legal, no daily bag limits, minimum take sizes or licensing systems are in place and the current level of offtake is unclear. Poaching is understood to be an issue in protected areas.

This species is included (originally as Salmo macrostigma) in Annex II of the European Union Habitat Directive. It is nationally-protected in the Republic of North Macedonia.

Some of its habitats are located within the boundaries of the Prespa Park, which was formalised in 2017 and represents the first transboundary protected area in the Balkan Peninsula. All of the occupied sites in Greece are also covered by the Prespa National Park, while in the Republic of North Macedonia most locations in the Brajcinska River system are inside Pelister National Park. Fishing for Prespa Trout is prohibited in both of these areas.

In the Agios Germanos River catchment, establishment of fish farms or hydropower plants, fly-tipping, disposal or processing of any kind of waste, discharge of untreated waste water, excavation of sand or gravel, concentrations of grazing animals, unlicensed cutting of trees and forests, and inappropriate utilisation and burning of pastures are all forbidden. The environmental organisation 'Society for the Protection of Prespa' currently coordinates monitoring efforts on a tri-annual basis. Water wuality in the lower part of the system may have improved following installation of new wastewater treatment facilities during the mid-2010s.

A European Union co-funded LIFE+ Information and Communication Project entitled 'Fish, Fisheries and European Policy in the Prespa Basin (LIFE09 INF/GR/000319) aiming to "promote the conservation of the threatened endemic and rare fish fauna of the area and the implementation of sustainable fishery practices" took place from 2010-2013.

A number of additional management proposals have been put forward at various times since 2002. These included a species action plan published in 2010, preparation of which comprised a collaborative effort by the Society for the Protection of Prespa, Society for the Investigation and Conservation of Biodiversity and the Sustainable Development of Natural Ecosystems (BIOECO) and French research institute Tour du Valat. However, none of the recommended conservation and monitoring measures were subsequently implemented, and in 2020 the development of a new conservation plan for the Republic of North Macedonia was initiated with financial support from the Prespa Ohrid Nature Trust and Critical Ecosystem Partnership Fund. The draft plan was published in 2022, and includes a detailed series of objectives which aim to improve habitat and water quality, reduce fishing pressure, enhance understanding of the Prespa Trout's biology and ecology, establish long-term monitoring and ex situ reproduction programmes, and raise public awareness.