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 Fibreno Trout has an extremely restricted range (area of occupancy (AOO) 4 km²), which meets the threshold for the Critically Endangered category under Criterion B2 (AOO < 10 km²). It is present at a single location where the quality of habitat is inferred to be declining.

Therefore, this species is assessed as Critically Endangered under Criterion B (B2ab(iii)).

This species is endemic to the oligotrophic Lake Posta Fibreno basin in the Garigliano River system, Lazio, western Italy.

This species' current population size and trend have not been quantified, and there is a notable lack of recent information regarding its conservation status.

The most recent published studies are based on data and observations from the mid-2000s, at which time anecdotal reports from local fishers suggested that its abundance declined significantly between the 1970s and 1990s (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').

Lake Posta Fibreno is a small oligotrophic lake with an average depth of 2.5 metres and maximum depth of 15 metres. It is fed by a series of karstic springs which maintain a relatively constant flow and rapid renewal time of less than 24 hours, and contains clear, well-oxygenated water with a stable temperature of c. 10-11°C throughout the year. There are abundant beds of aquatic vegetation throughout the affluent springs and in the lake itself, and the Fibreno River represents the only outflow.

This species is largely restricted to the eastern shoreline, where it is reportedly most abundant in the vicinity of sublacustrine springs and within a complex of semi-natural subterranean pools which drain to the lake. It apparently exhibits a preference for areas with rocky refuges and may be more active at night.

Individuals mature at remarkably small body sizes of c. 12 cm in males and c. 14 cm in females. The annual reproductive period extends from December to January, although females with mature gonads have been observed from October to April. Spawning takes place in the underground pools and in the vicinity of small affluent springs, some of which feed the southeastern affluent Torrente Dova. Mature female individuals lay relatively few, large eggs compared with related species, which is a phenomenon observed regularly in stygophilic fishes.

The Fibreno Trout is understood to hybridise with a sympatric native congener (possibly Salmo ghigii) at some spawning sites, although the two taxa belong to well-defined individual gene pools.

Molecular analyses have demonstrated that the Fibreno Trout has not suffered from introgressive hybridisation with non-native Brown Trout (Salmo trutta), which was regularly stocked in Lake Posta Fibreno until the practice was abandoned during the 1970s. This can plausibly be attributed to reproductive isolation driven by its unusual spawning behaviour.

During the mid-2000s, Lake Posta Fibreno basin had reportedly suffered from habitat degradation including a significant reduction in macrophyte coverage, unregulated water abstraction, and sedimentation of an extensive reedbed which borders the southern part of the lake. Pollution due to runoff from adjacent agricultural land and discharge of untreated domestic wastewater was also observed, but the high turnover of the lake was assumed to to maintain water quality through the constant flushing of excess nutrients.

Non-native and potentially threatening species recorded at the time included Coypu (Myocastor coypus), Goldfish (Carassius auratus) and Eastern Mosquitofish (Gambusia holbrooki), among which the former was considered to be damaging both squatic and riparian vegetation. A number of native fish species, including Apennine Roach (Sarmarutilus rubilio) and Italian Chub (Squalius squalus) were also observed in the lake for the first time, the presence of which may plausibly have resulted in increased competition for resources in the system.

The increasing winter presence of Eurasian Coot (Fulica atra) and the piscivorous Great Cormorant (Phalacrocorax carbo) was also noted as a potential threat.

This species was traditionally harvested for human consumption by local fishers, but this practice is understood to have ended during the mid-1990s as a result of declining yields.

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

Lake Posta Fibreno and its surroundings were designated as a protected area (regional nature reserve) in 1983, which has been included in the European Union's Natura 2000 network (site IT6050015) since 2016.

It is unclear whether any direct conservation actions for this species or its habitat have been implemented.

A deeper understanding of this species' present abundance, population dynamics, life history and response to the putative threats would undoubtedly prove beneficial in the design of any future management plan.