Although the systematics of Eurasian and North African brown trouts (Salmo spp.) remain somewhat disputed, this taxon is well-established as a genetically and morphologically distinct species that is clearly differentiated from members of the Brown Trout (Salmo trutta) complex (Whiteley et al. 2019). It has previously been classified in a number of different genera, most recently Salmothymus (Snoj et al. 2002).

The four identified subpopulations (see 'Population') have previously been described as individual subspecies based on morphological characters, but molecular analyses have suggested that they should be treated as a single taxon (Snoj et al. 2002; Pustovrh et al. 2014). However, the Vrljika River subpopulation appears to be particularly divergent (Snoj et al. 2008).

Global and European regional assessment: Vulnerable (VU)
EU 27 regional assessment: Vulnerable (VU)

The Softmouth Trout has a restricted range (area of occupancy (AOO) c. 460 km², EU 27 member states 52 km², extent of occurrence (EOO) c. 19064 km², EU 27 member states 3,426 km²), which meets the threshold for the Endangered category under Criterion B2 (AOO < 500 km²), and the extent and quality of its habitat are estimated to be declining. However, it occurs at nine locations, meeting the threshold for Vulnerable rather than Endangered, therefore this species is assessed as Vulnerable under Criterion B (B1ab(iii)+2ab(iii)) both globally and for the EU 27 member states.

This species is native to the Krka, Jadro, Neretva and Morača rivers in the Dinaric Karst system of Croatia, Bosnia and Herzegovina and Montenegro.

In the Krka catchment, it was confined to the river's headwaters upstream of the Bilušića buk waterfall, but may now be extinct at this location. It is similarly restricted to upper reaches of the Jadro, a naturally short river which flows for around 4.5 km before discharging into the Adriatic Sea near the city of Split. It is present in the Neretva main stem and a number of tributaries, including some sinking right-bank affluents that are connected to the river by subsurface karstic conduits, e.g., the Lištica and Vrljika (upper Trebižat River) catchments. The Morača River is the principal affluent of Lake Skadar, but the Softmouth Trout does not occur in the lake itself and is restricted to the Zeta and possibly the Cijevna (sq. Cem) tributary systems which flow into the lower Morača on the Zeta Plain.

Outside of this range, an introduced subpopulation inhabits the short (c. 5 km) Žrnovnica river in Croatia, to which it was translocated from the adjacent Jadro during the 1970s. This subpopulation is included in the present assessment since the intent of the introduction was purportedly to reduce the extinction risk of the taxon, the occupied site is geographically close to the natural range, viable offspring have been produced, and at least five years have passed since the introduction took place.

This species' current population size and trend have not been quantified, and the number of subpopulations is uncertain due to anthropogenic habitat fragmentation in the Neretva River system (see 'Threats'). Individuals inhabiting isolated affluents such as the Lištica are also likely to qualify as belonging to distinct subpopulations based on the Red List definition of the term, since there is unlikely to be any demographic or genetic exchange with those in the Neretva main stem.

Its overall abundance is understood to have declined significantly since the mid-20^th century. In particular, it has not been reported from the Krka River since the late 2000s, and has suffered a range contraction of c. 85% in the Neretva River system since the 1950s (see 'Threats'). Historic records demonstrate that it was present throughout the Neretva River main stem prior to the mid-20^th century, but is now fragmented into a series of remnant subpopulations. It has almost completely disappeared from the middle part of the Neretva watershed, and field observations suggest that it only remains somewhat abundant upstream of the Jablanica accumulation lake and downstream of Mostar city, including the Buna and Bunica rivers, with a small subpopulation in the upper Bregava River. The status of the Lištica, Trebižat (including its tributaries the Studenčica and Vrioštica rivers), and lower Neretva river subpopulations is less clear, but in the river's main stem it may now only be present between the cities of Mostar and Capljina, and plausible threats have been identified at all three locations (see 'Threats').

It was present at three sites in the Zeta River during the 2000s, but no individuals were recorded during surveys carried out in 2019.

The Jadro, Vrljika and Žrnovnica subpopulations are all understood to be relatively stable due to protective measures (see 'Conservation'), although these three sites combined total fewer than 20 km of linear river length.

Individuals inhabiting different river systems exhibit genetic differentiation, with the Vrljika River subpopulation particularly divergent from those inhabiting other parts of the Neretva River system. The Vrljika is also unique in that it is the only known location where the Softmouth Trout is not naturally-introgressed with the West Balkan Trout (Salmo farioides). In addition, individuals from the Žrnovnica, Jadro and Vrljika rivers demonstrate considerably lower intra-subpopulation genetic diversity than the Neretva and Morača subpopulations. The Jadro (and thus the Žrnovnica) subpopulation is also fixed for a Brown Trout mitochondrial DNA haplotype of the Adriatic lineage, which presumably derives from an ancient hybridisation event. These findings have implications for this species' conservation management (see 'Conservation').

This species exhibits a preference for relatively wide reaches of spring-fed karstic river channels containing cool, well-oxygenated water. It is regularly observed in deep glides, pools or runs close to ponors or spring outflows, where it tends to forage in small groups. All of its habitats undergo seasonal fluctuations in discharge, although in some cases these regimes have been hampered by human development (see 'Threats').

Some of its habitats in the sinking Lištica and Trebižat tributary systems flow through flat-floored karstic depressions known as polje. A number of sinking rivers and springs rise around their boundaries and flow across their floors before submerging into ponors. Under natural conditions a portion of each polje would flood to form a temporal wetland for several months per year, but these processes mostly no longer occur (see 'Threats').

The Jadro and Žrnovnica emerge from coastal ponors and flow for 4-5 kilometres before emptying into the Adriatic Sea. Their downstream reaches pass through urban areas where they are channelised, and the resident softmouth trout subpopulations are thus confined to the comparatively well-preserved upper reaches. The perennial discharge of the formerly temporal Žrnovnica River is maintained by the presence of a dam on the adjacent Cetina River.

This species is also understood to enter artificial accumulation lakes in the Neretva watershed, but the extent to which these represent suitable habitat has not been investigated. At least one small subpopulation is reportedly present between the Salakovac and Grabovca hydroelectric dams on the Neretva River main stem.

Adult individuals are occasionally encountered by fishers operating in the Adriatic Sea close to the mouth of the Neretva River. Most of these unusual reports pertain to spring, and are assumed to be the result of fishes being washed downstream during seasonal flood events.

This species exhibits a number of morphological peculiarities, most prominently an elongated muzzle and small, fleshy, subterminal mouthparts. It feeds almost exclusively on benthic macroinvertebrates such as Amphipoda, Diptera, Ephemoptera and Trichoptera.

The maximum lifespan is at least 8 years, with sexual maturity attained at age 3-4+ and males typically maturing one year earlier than females. The annual reproductive period extends from March to May, and its timing may serve to maintain at least partial reproductive isolation from sympatric salmonids, which tend to breed during the winter. Its spawning behaviour is characterised by nuptial individuals aggregating at specific sites comprising well-washed beds of coarse, stony substrata. Individual females create shallow depressions (redds) in the substrate, into which the gametes are deposited. Females also display a series of behavioural anomalies compared with congeneric species, including repeated abandonment of the redd, the release of multiple batches of eggs at short intervals, and failure to conceal the eggs post-spawning. Relative fecundity is c. 3,200 eggs per kilogramme of body weight.

Natural hybridisation with the sympatric West Balkan Trout (Salmo farioides) is reported to occur throughout most of this species range, with the exception of the Vrljika River.

This species is plausibly threatened by introgressive hybridisation and resource competition 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. The non-native individuals produced in hatcheries today are of mixed origin, but are typically derived from the Atlantic and/or Danubian Brown Trout mitochondrial lineages. Non-native Rainbow Trout (Oncorhynchus mykiss) and Brook Trout (Salvelinus fontinalis) are also stocked at many locations, e.g., the upper Krka and Neretva rivers, where they may compete for resources or act as vectors for various pathogens. In addition, the non-native European Grayling (Thymallus thymallus) is particularly abundant at sites occupied by Softmouth Trout in the upper Neretva River and might represent a particular threat since it exhibits comparable habitat and dietary preferences. However, the presumptive negative impact of these species on Softmouth Trout abundance and genetic intregity has not been extensively-investigated.

A number of other non-native fish species that are documented to exert negative effects on native ichthyofauna through predation, resource competition, habitat degradation or transmission of pathogens are established or stocked in different parts of the Softmouth Trout's range, including Eurasian Pikeperch (Sander lucioperca), Northrn Pike (Esox lucius), Pumpkinseed (Lepomis gibbosus), Largemouth Bass (Micropterus salmoides), Eurasian Ruffe (Gymnocephalus cernua), Prussian Carp (Carassius gibelio), Common Carp (Cyprinus domestic strain), Topmouth Gudgeon (Pseudorasbora parva) and Brown Bullhead (Ameiurus nebulosus).

Unlicensed and unregulated harvesting is also believed to have driven this species' decline. These activities often takes place during the annual reproductive period, when fisheries are closed for conservation purposes. Adult individuals are typically targeted when they aggregate to spawn, and harvesting methods include the use of traps, electrofishing devices and even explosives.

The cause of this species' purported extirpation from the upper Krka River is unclear, but insufficiently-treated domestic wastewater is discharged directly into the river's main stem from the town of Knin. Contaminants from local industry are also released into artificial pools along its right bank and may enter the river through the surrounding karst, or via overspill when water levels are high. This has resulted in a series of substantial pollution events characterised by elevated concentrations of hydrocarbons, heavy metals and other contaminants. A trout farm located upstream of Knin represents a localised source of  organic pollution and pathogens, since it utilises a flow-through system exploiting the river's water. Northern Pike and Rainbow Trout are present in the river.

The Vrljika River flows along the southern boundary of the extensive Imotsko polje after rising from a spring linked to the temporal Prološko Blato wetland. The first few hundred metres of the river have been canalised, while the construction of accumulation dams on the affluent Ričice River has interfered with the complex natural hydrology of Imotsko polje. Water is currently redirected from the Ričice to Prološko Blato via an artificial canal, and the lake's water level is partially controlled by a retention dam and canal system. These measures have reduced the Vrljika's discharge, and the river also receives diffuse and point-source agricultural and domestic pollution from nearby villages. Northern Pike and other non-native fish species are present in the river.

The Lištica is a short sinking river which flows mostly through the seasonally-flooded Mostarsko polje. In 1947, a flood-control tunnel was installed to direct water through the limestone hill beneath which the river naturally drains from Mostarsko polje towards the Neretva, and during the 1950s a network of drainage channels was excavated in the polje itself. The combined effects of these engineering works plus unregulated water abstraction has lowered local groundwater levels by 1.5-2.5 metres. A hydroelectric power plant supplied by water from the lower Lištica became operational in 2010, and included the construction of a storage reservoir and concrete canal. A further artificial channel was added in 2016, and a second reservoir in 2018. These activities have resulted in complete degradation of the lower reaches of the river. Extraction of riverine gravel and sand for urban development has driven further habitat degradation in the western portion of the polje, while unauthorised road building and fly-tipping during low-water periods have also resulted in the sometimes-intentional blocking or burying of ponors. Untreated domestic sewage was discharged directly into local groundwater for decades, and it is unclear if proposed wastewater facilities have been installed, while local agriculture represents a further source of diffuse organic pollution. Rainbow Trout and Brook Trout are regularly stocked in the river.

The construction of four large hydroelectric dams along the middle section of the Neretva River main stem, upstream of Mostar, between the 1950s and 1980s has altered natural flow and sedimentation regimes, fragmented Softmouth Trout subpopulations, and markedly reduced the extent of habitat for all life stages. Hydroelectric dams also create unnatural fluctuations in discharge and water temperature (hydropeaking and thermopeaking) which bring about artificial dewatering of downstream river stretches, the loss of stable nursery habitat for juveniles and stranding of individual fish. Furthermore, the combined effect of hydropeaking, dam flushing operations and changes in land use is likely to have increased accumulation of fine sediments and negatively impacted foraging and reproductive success at some locations. The proposed construction of around 70 additional hydroelectric projects in the upper reaches of the river, plus the ongoing "Upper Horizons" expansion scheme together represent a plausible future threat to the entire Neretva ecosystem. The latter project aims to artificially redirect water from poljes draining to the Buna and Bregava rivers to an accumulation reservoir in the adjacent Trebišnjica River through a cascading series of canals, tunnels and hydroelectric stations. Once complete, the scheme is projected to significantly reduce discharge in the lower Neretva and its left-bank tributaries, with a potentially catastrophic outcome for the local Softmouth Trout subpopulations.

In the Bregava River, construction of a hydroelectric power plant in 2016 has resulted in water being diverted from the upper reaches to an open canal, from which it is returned to the river about 2 km downstream. A significant portion of the stretch occupied by Softmouth Trout has thus been extensively dewatered, leading to higher water temperatures and a significant reduction in habitat quality.

The extent and quality of habitat in the Neretva catchment has been further diminished by bank stabilisation, channelisation and other efforts to enhance flood protection or exploit water resources for irrigated agriculture. The industrial extraction of riverine gravel and sand for urban development has taken place since the 1970s, causing the river bed to become deeper in places and hampering the transport of heavy sediments. In the river's delta, only three of its original twelve branches remain and most former marshes and lakes have been reclaimed for agriculture since the 1960s. The change in sediment regime driven by extraction and the upstream dams has led to increased coastal erosion and a progressive intrusion of seawater into the area. Diffuse and point source agricultural, domestic and industrial pollution has resulted in eutrophication or discharge of toxic substances at some locations.
In Montenegro, the Zeta River is increasingly polluted due to discharge of agricultural, domestic and industrial contaminants from the surrounding Bjelopavlići Valley and upstream Nikšić polje, while a dam constructed at the village of Slap has fragmented the resident Softmouth Trout subpopulation. The proposed construction of hydroelectric dams on the Morača River and its tributaries represents the most plausible future threat.

The ongoing trend towards more extensive periods of drought caused by climate change in the Mediterranean region could lead to some habitats becoming unsuitable for Softmouth Trout due to reduced discharge, excessive water temperatures or changes in benthic invertebrate communities.

This species is utilised in recreational fisheries throughout its range. It is also harvested for consumption at the local scale, and is sometimes served in restaurants.

The precise level of offtake is unclear since much of the harvesting takes place on an unauthorised basis, including within no-kill fishing areas and official closed seasons (see 'Threats' and 'Conservation').

Supportive breeding and restocking efforts are underway in at least one hatchery facility in Bosnia and Herzegovina, using broodstock originating from the Buna River. Fingerlings are subsequently stocked into the Neretva River to reinforce the local recreational fishery, but the frequency and intensity of these activities are unclear. It appears that broodstock is maintained in captivity, which can lead to artificial selection issues if not managed accordingly.

This species is included (as Salmothymus obtusirostris) in Annex II of the European Union Habitats Directive.

It is nationally-protected in Croatia, Bosnia and Herzegovina and Montenegro. It is currently assessed as Critically Endangered for the National Red List of Croatia and Endangered in for the National Red List of Montenegro.

It is present inside the boundaries of various protected areas, some of which are included in the European Union's Natura 2000 network. Furthermore, the Jadro and Vrljika rivers comprise the only Ichthyological Special Reserves in Croatia, and are dedicated to this species with all fishing activities permanently prohibited.

At least one project dedicated to artificial propagation and stock enhancement is underway in Bosnia and Herzegovina, but it is unclear to what extent these are based on conservation management principles (see 'Use and Trade').

Initial attempts at semi-captive rearing have took place in Croatia during the 2010s, with eggs artificially-fertilised and young fish grown in an outdoor pond for several months before being released into the Vrljika River. Efforts have also been made to reduce the abundance of Northern Pike in the Vrljika.

Some harvest management regulations are in place, but there is an urgent need to improve monitoring and enforcement at the majority of sites. Harvesting pressure could be reduced and recruitment favoured by the introduction of strict ‘no-kill’ policies and/or increases in the minimum take size limit, and such regulations should ideally be combined with the abandonment of stocking activities for non-native salmonids. Implementation of these actions would require a coordinated approach including the support of local angling associations and fisheries authorities, ideally through the creation of national management plans. Such actions are particularly required in Bosnia and Herzegovina and Montenegro.

The proposed hydropower developments in the Neretva and Trebišnjica rivers 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' demographics (population size and trend), life history (particularly the locations of key spawning sites and nursery zones), and the impact of non-native species (especially European Grayling and Brown Trout) would undoubtedly prove beneficial in the development of any structured conservation plan.