Some recent studies suggest that this taxon represents a translocated subpopulation of the Black Sea Trout (Salmo labrax), which is native to the Danube River system (Marić 2019; Mrdak & Milošević 2023). However, the results of molecular analyses indicate that the Nikšić Trout is native, has been isolated for a considerable period of time, and belongs to a different mitochondrial lineage than trouts inhabiting the middle and lower Zeta River (Pustovrh et al. 2014; Simonović et al. 2017).

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).

European regional assessment: Endangered (EN)
EU 27 regional assessment: Not Recorded

The Nikšić Trout has a restricted range (extent of occurrence (EOO) c. 642 km²), which meets the threshold for the Endangered category under Criterion B1 (EOO < 5,000 km²). It is present at one location where the extent and quality of habitat are estimated to be declining.

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

This species was described from Nikšić polje in the upper Zeta River system, Montenegro, to which it is putatively treated as endemic for the purposes of this assessment (see 'Taxonomic Notes').

This species is likely to have declined since the mid-20^th century (see 'Threats'), but the current population size and trend have not been quantified.

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

The Nikšić area is traversed by the upper course of the Zeta River and represents a typical karstic polje, comprising a depressed flat plain surrounded by limestone ridges. The river is formed by a series of spring-fed tributaries and submerges into a large ponor at the southeastern extremity of the polje, where it descends rapidly through subterranean karstic conduits before emerging at the Glava Zete (Zeta source) springs. The upper reaches in Nikšić polje are thus isolated from the remainder of the Zeta system.

The upper Zeta undergoes seasonal fluctuations in discharge, and under natural conditions parts of the surrounding polje would be inundated between early winter and early summer. However, these processes have been largely restricted by human development (see 'Threats').

This species' biology and ecology have not been studied, but they are probably comparable to that of other stream-dwelling members of the Brown Trout complex, which tend to be visual predators feeding opportunistically on benthic and drifting invertebrates. Larger individuals typically occupy deeper pools and glides and are territorial, selecting stream positions in dominance hierarchies based on maximising their energy intake.

The timing of its annual reproductive period is unknown, but it is likely to be characterised by nuptial individuals undertaking short 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.

The hydrological regime of the upper Zeta River has been extensively modified since the late 1950s in order to facilitate operation of a large hydropower plant. Accumulation dams have been constructed on each of the river's three main tributaries, and their discharge is regulated through artificial canals. The third tributary is polluted by runoff from a series of bauxite mines located within its catchment. The lowermost kilometres of the upper Zeta itself have also been converted into a canal, and its water is directed into an intake structure before being transported to the hydropower plant via a tunnel and pipeline system. The canalised section of the river is polluted due to widespread disposal of household garbage and other waste. The ponor into which the upper Zeta formerly submerged has been isolated by a circular dam and no longer receives water, while seasonal flooding in the polje has been significantly reduced.

Non-native or domestic strains of Brown Trout (Salmo trutta) and other congeners have been widely introduced throughout most of the Western Balkans region for the creation or restocking of recreational fisheries. This has led to widespread introgressive hybridisation and a loss of genetic diversity in many native species and subpopulations. Non-native individuals have apparently been stocked in Nikšić polje, but it is currently unclear whether such hybridisation has occurred.

This species is probably targeted by recreational anglers and may be fished for human consumption at the local scale, but no specific details are available.

This species does not occur within any protected areas, and no conservation measures are understood to be in place.

A deeper understanding of its present demographics (population size, trend and the extent of any hybridisation with non-native trouts), life history and response to the ongoing threats would undoubtedly prove beneficial in the development of any future management plan.

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.