There is currently no broad consensus regarding the systematic classification of most 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. 201, 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 (Maddison 1997, 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 and 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: Vulnerable (VU)
EU 27 regional assessment: Vulnerable (VU)

The Marble 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 understood to exceed 10,000 mature individuals, and thus 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 suspected that a continuing reduction in abundance based on field observations, declining habitat quality and the effects of introduced taxa could approach or meet the thresholds for Vulnerable or Endangered under Criterion A (≥ 30% or ≥ 50% over the past 15 years = three generations). As a result of this data uncertainty, Near Threatened and Endangered are equally plausible Red List categories for the present assessment, and this species is assessed as Vulnerable both globally and for the EU 27 Member States.

This species has a somewhat discontinuous distribution in rivers draining to the Adriatic Sea. In the northern part of the Adriatic basin, its range extends from the Po River and its present and historical orographic left-bank tributaries in Italy and Switzerland eastward to the Rižana River in Slovenia. It is restricted to systems draining the southern slopes of the Alps, and is thus absent from most right-bank tributaries of the Po. In the southeastern Adriatic, it is present in the Neretva River and Lake Skadar watersheds but is absent from some sinking affluents that are isolated by karstic landscape features, e.g., the Trebižat and Trebišnjica rivers in the Neretva River watershed and Nikšić Polje in the upper Zeta River catchment.

It has been introduced to several European river systems outside of this range in the past, but is not understood to have become established in any of them.

This species' population size is unknown, but it clearly exceeds the minimum threshold for Red List criteria (< 10,000 mature individuals). The current population trend has not been quantified, but an ongoing reduction which may have exceeded 30% within the past three generations is suspected as a result of a continued decline in habitat quality and the effects of non-native species. The number of subpopulations is unclear.

It is understood to have declined significantly during the 20^th century, when a series of documented site-scale extirpations occurred in Italy, Switzerland and Slovenia, e.g., the Ticino River (Po River system) and Predelica River (Soča River system). It has been replaced by non-native trout (see below) in the upper reaches of most headwater rivers in Italy, but typically remains present in downstream stretches. This pattern is suspected to be ongoing in most parts of its range, based on field observations, declining habitat quality and the effects of introduced taxa (see 'Threats').

In terms of genetic structure, this species comprises the Marble Trout mitochondrial lineage within the Brown Trout (Salmo trutta) complex (see 'Taxonomic Notes'). This grouping is itself subdivided into two distinct lineages which occupy rivers draining to northern and southeastern parts of the Adriatic Sea basin, respectively. At the finer scale, notable genetic divergence has been detected both within and between individual river systems.

Most subpopulations in the Soča River catchment and that inhabiting the Isarco River in the upper Adige catchment, Italy, demonstrate putatively natural introgression with the Danubian Brown Trout lineage, which may have reached these systems through historic river capture events.

Genetic admixture with non-native, domestic Brown Trout lineages has occurred throughout most of this species' range over the past two centuries, and the only confirmed non-hybrid (hereafter "pure") subpopulations in the northern Adriatic basin occupy a series of headwater tributaries in the upper Soča River, Slovenia, which collectively total c. 15 km of linear river length (see 'Habitat and Ecology'). An altitudinal upstream-downstream cline of introgression has been identified, with pure subpopulations confined to the upper reaches and an increasing tendency towards non-native genetic profiles in the lower part of the Soča system. Elsewhere in Slovenia, subpopulations inhabiting the Reka and Rižana river systems are also significantly introgressed with non-native Brown Trout.

No pure subpopulations are understood to remain in northern Italy, where the extent of introgression does not demonstrate a predictable altitudinal pattern. This variance is probably related to site-scale discrepancies in fisheries management (see 'Conservation'), plus the original standing stock at each location. For example, exceptionally high levels of introgression observed in parts of the upper Adige River could be attributable to naturally low Marble Trout abundance prior to stocking with non-native Brown Trout.

Genetic analyses suggest that complete panmixia has not yet been reached at some locations in Italy or Slovenia, where a number of pure individuals remain despite the high abundance of hybrids. This pattern could be the result of partial reproductive isolation (see 'Habitat and Ecology'), and/or site-scale differences in the relative abundance of introduced and remnant native stocks. Since the 1990s, conservation management efforts have led to an increasing abundance of pure individuals at some locations, especially in the upper Soča and Adige rivers (see 'Conservation').

The demographic trends and distribution patterns of subpopulations inhabiting the southeastern Adriatic basin are comparatively undocumented. The results of recent field surveys are indicative of extremely low abundance in both the Neretva River and Lake Skadar catchments, and commercial landings in the latter have crashed since the mid-20^th century (see 'Use and Trade'). These presumptive declines may be related to factors other than introgression with non-native taxa (see 'Threats'), and additional research is required in order to verify the conservation status of Marble Trout in these systems.

This species inhabits a range of fluvial habitats, but is most commonly associated with relatively lotic middle-to-lower reaches of submontane river and stream channels at altitudes below 1,500 m asl. These environments are typified by cool, well-oxygenated, low-nutrient water with a temperature range of 2-15°C and seasonal fluctuations in discharge. Some stretches flow swiftly through steep-sided canyons, ravines or forested valleys, and substrata tend to comprise a mixture of exposed bedrock, boulders, rocks, cobbles and gravel, with refuges formed by woody structures such as branches, roots or fallen trees.

In the Soča River, the seven remaining pure subpopulations occupy rhithral streams located in karstic uplands, where they are isolated by impassable waterfalls. Individuals inhabiting these locations are characterised by their relatively small adult size and abridged lifespan.

Some Marble Trout subpopulations are partially-resident in oligotrophic lakes such as the perialpine Maggiore, Como, Iseo and Garda basins in the Po River system, Italy and Switzerland. Elsewhere, lacustrine subpopulations may inhabit Hutovo Blato wetland and some artificial accumulation lakes in the Neretva River catchment, Bosnia and Herzegovina, and Lake Skadar in Montenegro and Albania. In the Adige River system, this species has colonised some artificial accumulation lakes.

In fluvial habitats, 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.

This species is a visual predator which feeds on benthic and drifting invertebrates, e.g., Ephemeroptera, Diptera, Plecoptera, Trichoptera, while older age classes demonstrate a pronounced tendency towards piscivory. Cannibalism has often been observed and might play a role in subpopulation dynamics by influencing recruitment and size distributions.

The maximum recorded age is 20+ and individuals mature at age 2-3+. The annual reproductive period comprises a spell of 2-3 weeks between November and January, with the precise timing dependent on environmental factors such as altitude and water temperature. This species is iteroparous and potamodromous, and nuptial individuals display homing behaviour to specific upstream spawning sites comprising well-washed gravel beds in shallow (typically 40-80 cm depth) reaches with moderate flow which are often located in tributaries. After arriving at these sites, individual females create 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. Alevins typically emerge between March and June, and immediately begin to feed close to the spawning site. They remain in marginal habitats during the initial growth phase, and larger individuals tend to position themselves closer to the water current. Juveniles (parr) usually disperse downstream in riffles or runs, and often aggregate in the vicinity of cover such as undercut banks or submerged wood.

It is believed that the timing of the reproductive period facilitates partial ecological segregation from non-native Brown Trout, which tend to spawn slightly earlier. It has not yet been determined whether hybrid individuals demonstrate any heterotic traits.

As observed in other salmonids, this species exhibits a remarkable ontogenetic plasticity in response to conspecific abundance and environmental factors during the initial growth period. Although it is Europe's second-largest salmonid and able to exceed 1 metre in length and 25 kilogrammes in weight, individuals inhabiting some locations, e.g., Slovenian rhithral streams, never weigh more than a few hundred grammes. Moreover, the length of age 1+ individuals is significantly correlated with their eventual size, and larger-bodied females produce a greater number of eggs. These may represent density-dependent regulatory processes that serves to moderate recruitment to counterbalance high local abundance or low resource availability. Density-dependent growth and recruitment are also likely to favour subpopulation resilience in the wake of environmental catastrophes such as severe flood events.

This species is primarily threatened by introgressive hybridisation with non-native Brown Trout (Salmo domestic strain), which has been widely introduced throughout most of its range for the creation or restocking of recreational fisheries. These activities may have taken place for several centuries at some locations, especially in Italy, but underwent a dramatic expansion with the development of aquaculture techniques during the late 19^th century. The non-native individuals produced in hatcheries today are of mixed origin, but are typically derived from the Atlantic Brown Trout mitochondrial lineage (see 'Taxonomic Notes'). The negative impact of stocking with non-native individuals is widely-documented in the northern Adriatic basin, but has not been extensively-investigated in the Neretva River or Lake Skadar catchments.

This species' decline has also been driven 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, increased stranding rates, 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. Furthermore, the increase in habitat homogeneity associated with these modifications is understood to reduce reproductive isolation between native and introduced trouts.

Hydroelectric dams have created unnatural 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, while unstable flow and temperature regimes may also increase hybridisation rates. 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 middle reaches of the Neretva River system have been particularly impacted by hydropower projects, and the proposed construction of around 70 additional schemes in the upper part of the catchment plus the ongoing "Upper Horizons" expansion scheme in the lower basin together represent a plausible threat to the entire ecosystem.

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. In the Neretva River, removal of sediments from the main stem has deepened the river bed and hampered the transport of heavy sediments.

This species is also understood to be threatened by diffuse and point source agricultural, domestic and industrial pollution, which has resulted in eutrophication or discharge of toxic substances at some locations. For example, the lower reaches of the Morača River, which provides more than 60% of Lake Skadar's water, are polluted due to long-term discharge of agricultural and industrial contaminants plus insufficiently-treated municipal wastewater from the city of Podgorica and other urban centres. As a result, the lake has become increasingly eutrophic since the 1970s, which has led to bottom-up changes in food web structure. Most perialpine lakes in the Po River system also suffered from anthropogenic eutrophication during the 20^th century.

Periodic major floods and associated debris flows driven by stochastic environmental processes comprise the principal threats to the pure subpopulations inhabiting small upland streams in Slovenia. In 2000, one of these was extirpated following a landslide caused by tectonic activity in the Predelica River watershed.

Northern Adriatic subpopulations are plausibly threatened by increasing abundance of the piscivorous Great Cormorant (Phalacrocorax carbo), particularly during the winter.

Rising water temperatures due to climate change represent a plausible future threat, since they may interfere with food availability, lifespan, and the timing of reproductive processes, with the latter potentially leading to greater overlap with non-native Brown Trout.

This species is a key component of recreational fisheries throughout the majority of its range, being popular with anglers due to its unique colour pattern and large adult size.

In Slovenia, fly-fishing for Marble Trout on the Soča River and its tributaries is a major tourist activity, and the local industry has been valued at > €2 million annually.

In Italy, both angling and stocking are viewed as lucrative activities, since they involve the building and maintenance of hatcheries and trading of reared stocks, plus the sale of fishing licenses, association memberships, competition entry fees, and dedicated tourism offerings.

Restocking of recreational fisheries through supportive breeding is widespread in both countries, but somewhat contrasting methodology is applied (see 'Conservation'). In Italy, stocking is also carried out as a compensatory measure to offset environmental damages caused by hydropower projects, and energy firms routinely purchase live individuals from private hatcheries.

This species was reportedly harvested by commercial fishers operating on Lake Skadar until the 1980s, with annual landings of up to five tonnes during the mid-20^th century. However, it is currently considered to be scarce in the lake and is barely mentioned in recent fisheries reports.

This species is listed in Annex II of the European Union Habitats Directive, and is nationally-protected in most countries within its range.

It was assessed as Critically Endangered under Criterion A3e (projected future population decline of ≥ 80% within 10 years or three generations, based on the effects of introduced taxa) for the latest (2022) iteration of the Italian Red List of Vertebrates. However, the same category and criteria were applied during the previous (2013) assessment, and it is unclear whether the rate of decline between these two assessments has been quantified. It is currently assessed as Critically Endangered in Switzerland and Croatia, and Endangered in Slovenia and Montenegro.

It is present within the boundaries of numerous protected areas, including several national parks plus multiple sites included in the European Union’s Natura 2000 network and/or the Ramsar Convention on Wetlands of International Importance.

An extensive series of conservation actions have been implemented in Slovenia since 1993, particularly in the upper Soča River where a long-term Marble Trout rehabilitation project continues to be administered by the Tolmin Angling Association (sl. Ribiška družina Tolmin/RDT) alongside the French research institute Tour du Valat. Direct activities within the framework of this project were initiated in 1996, and included the prohibition of rearing and stocking non-native Brown Trout in the Soča watershed plus the development of an ex situ supportive breeding and restocking programme. The primary goal of this project continues to be the eradication of non-native trout alleles from the Soča system through long-term stocking of non-introgressed, hatchery-reared Marble Trout individuals. This activity was initially dependent on the stripping of gametes from broodstock obtained from the pure subpopulations located upstream (see 'Population'), but currently utilises individuals with pure genetic and phenotypic signatures selected from the hybridised subpopulations. This switch was considered favourable due to the contrasting ecological conditions encountered by the pure subpopulations compared with those further downstream. The majority of stocking has consisted of early life stages, with age 0+ fingerlings released annually since 1999 and eyed eggs since 2007. The RDT has built a dedicated hatchery and two fish farms, where around 400,000 Marble Trout are produced each year. At least 17 locations in the upper Soča catchment are monitored on a regular basis. In addition, the minimum take size limit for recreational angling has been increased on several occasions and is currently 60 cm, while an annual closed fishing season extends from October 1^st to March 31^st. The minimum size limit for Brown Trout and hybrid individuals has been maintained at 26 cm in order to uphold the rate of removal, and fly-fishing is not permitted on river stretches inhabited by pure subpopulations.

Numerous scientific studies have been published as a result of this project, and molecular analyses have demonstrated that the overall strategy has been broadly effective, with the proportion of non-native trout alleles present in the river falling by c. 2% per year. The stocking activities have mainly been carried out within the boundaries of a 145-km stretch of the Soča main stem and its major tributaries, and it has been recommended that they should be extended to minor affluents which currently lack active management, since subpopulations inhabiting some of them are highly introgressed and could represent an ongoing source of genetic contamination.

In order to preserve the native gene pool, a number of previously fishless streams have also been stocked through the translocation of individuals from pure subpopulations in the upper Soča system. Some of these efforts were ultimately unsuccessful due to severe flood events, which have also created demographic bottlenecks and reduced genetic diversity.

Stocking programmes are established elsewhere in Slovenia, including the Reka and Rižana river systems, but have been applied on a less consistent basis. For example, in the Vipava river, a principal tributary of the lower Soča, the local angling club rears its own broodstock which are selected on the basis of the resident Marble Trout phenotype but not verified by genetic screening. This project is occasionally supplemented with pure stock from the upper Soča.

Other activities carried out in the Soča watershed include scaring of Great Cormorants during the winter and an annual campaign to clear garbage and other domestic waste from the river.

A number of conservation efforts co-financed by the European Union's LIFE programme have taken place in Italy. For example, activities undertaken within the BIOAQUAE project (LIFE11 BIO/IT/000020) from 2012-2017 included the establishment of a Marble Trout hatchery and the release of individuals at four locations in the Gran Paradiso National Park. From 2016-2022, the IdroLIFE project (LIFE15 NAT/IT/000823) implemented actions in the Piedmont region including an ex situ breeding programme, restocking, alien species removal and construction of fish passages restoring habitat connectivity along more than 40 km of the Toce River and a stretch of the San Bernardino River from Lake Maggiore to the lower part of the Val Grande National Park. The Marble Trout is one of two taxa targeted by the GrayMarble project (LIFE20 NAT/IT/001341), which aspires to improve its conservation status in the Dora Baltea River, a tributary of the Po River in Valle d'Aosta, northwestern Italy. Stated aims of the project, which is scheduled to run from 2021-2026, include ex situ propagation, restoration of connectivity to a section of the river, eradication of non-native fish species and hybrid lineages, and restocking with pure individuals. Other projects which have included Marble Trout stocking include Salmo Ticino (LIFE00 NAT/IT/007268), SILIFFE (LIFE14 NAT/IT/000809) and SHARESALMO (transboundary project between the region of Lombardy, Italy and Canton of Ticino, Switzerland, financed by the Interreg programme).

However, Marble Trout conservation efforts in Italy are generally uncoordinated, and no area-wide management plan is in place. The stocking of non-native trout is currently prohibited under European and national legislation, and the origin of all broodstock should be verified by genetic-screening, but these rules are rarely enforced. Stocking activities are largely carried out by private fishing associations and/or local fisheries authorities using both native and non-native stocks, leading to inconsistent rehabilitation measures for Marble Trout. Moreover, the selection of individuals for supportive breeding programmes is largely based on morphological characters and lacks procedures designed to maintain genetic variability. These methods have created artificial selection processes, which have been further exacerbated by the trade of individuals between hatcheries, the use of captive, semi-domesticated broodstocks for the production of gametes and the translocation of individuals to different river systems. Such approaches drive genetic drift, accelerate introgression through the rearing of hybrids, reduce the scale of genetic differentiation between locally-adapted subpopulations, and lead to a loss of hereditary behaviours such as spawning site fidelity. In numerous cases, current stocking practises thus actively threaten the integrity of wild gene pools. Furthermore, the survival rate of hatchery-reared individuals appears to be very low, e.g., only around 1.3% of those occupying the Toce River (Po River system) are of hatchery origin despite the annual release of c. 800,000 juveniles.

A notable exception is the multi-stakeholder MARMOGEN project, which has been active in the upper Adige River catchment, Autonomous Province of Bolzano, since 2016. Supportive breeding and restocking has been ongoing in the area since the 1950s, and this collaborative programme has resulted in fine-scale genetic screening of wild stocks and improved genetic control in hatcheries. By 2020, more than 4,500 Marble Trout individuals sampled from 75 sites had been sampled, and the results showed that the density of non-hybrid individuals was relatively low (c. 14/ha in lowland habitats). Captive rearing methods were improved through the maintenance of individuals in semi-natural conditions and the introduction of non-processed diets, broodstock was selected only from verified pure individuals, restocking was limited to early life stages, Brown Trout stocking was prohibited, and a strict no-kill fishing policy was introduced for Marble Trout.

Since the 2010s, the production of Brown Trout belonging to the putatively native Mediterranean genetic lineage has been increasingly advocated as a means to circumvent regulations related to the stocking of non-native freshwater fishes. Such individuals are marketed as "Mediterranean Trout" but in reality comprise hybrids deriving from multiple locations in central and southern Italy, which are often introgressed with Atlantic and domestic lineages. Moreover, stocking often takes place in areas where the Mediterranean lineage is itself not native, hence this approach does not mitigate the spread of non-native alleles. Scientific analyses of these practices are currently lacking, but records provided by angling associations in the Lombardy Region suggest that "Mediterranean" individuals have been stocked in tributaries of the Po River known to be inhabited by Marble Trout.

Established angling regulations may also be inadequate. For example, in the Piedmont region of northwestern Italy there is a daily bag limit of two individuals (maximum ten per year, including hybrids) with a minimum take size of 40 cm. However, many individuals are still immature at this length and are thus removed from the gene pool before they are able to spawn.

A series of measures to improve Marble Trout conservation measures in Italy have been advocated by multiple researchers. Since no pure subpopulations have been identified, the use of high-resolution genotyping to identify potential broodstock, strengthen supportive breeding activities and define meaningful management units at the evolutionary microgeographic scale is strongly recommended. In addition, gametes should be obtained from wild individuals rather than captive broodstock and analysed rapidly to ensure their viability. Restocking with eyed eggs rather than hatchery-reared juveniles should also be explored. The creation of natural nurseries, as trialled on the Ischielle River in Trentino from 2013-2016, could prevent the domestication of offspring and increase their rate of survival in the wild. Improved restocking techniques should ideally be combined with habitat restoration efforts in areas where the latter is a realistic target.

Angling pressure on Marble Trout could be reduced and recruitment favoured by the introduction of a strict ‘no-kill’ policy and/or an increase in the minimum take size limit, and such regulations should ideally be combined with the abandonment of stocking activities and withdrawal of bag limits for non-native Brown Trout. Implementation of these actions would require a coordinated approach including the support of local angling associations and fisheries authorities, ideally through the creation of a national management plan.

In Bosnia and Herzegovina, the proposed hydropower developments in the Neretva River system have been opposed by local communities and several NGOs, leading to the publication of various news articles, reports, research projects and dissemination events, e.g., Neretva Science Week (July 2022)

A deeper understanding of this species' present distribution, abundance, population dynamics, life history and response to the ongoing threats in the southeastern Adriatic basin is required.