This taxon represents one of four closely-related sympatric trout morphs (see below) that are endemic to Lake Ohrid. These are understood to have evolved via a process of adaptive radiation which led to subsequent reproductive isolation through spatiotemporal differences in their spawning ecology. Since the mid-20th century, they have been variously regarded as conspecific with the wide-ranging Brown Trout (Salmo trutta), a single polymorphic taxon, or distinct species/subspecies in their own right. For Red Listing purposes, they are currently treated separately following Fricke (2024), and thus comprise the species Salmo letnica (Karaman 1924), S. balcanicus (Karaman 1927), S. lumi Poljakov 1958 and S. aphelios Kottelat 1997.

Although no recent morphological analysis exists, these taxa can reportedly be separated by meristic and morphometric characters (Sell & Spirkovski 2004). Moreover, the discrete genetic status of S. letnica and S. lumi has been confirmed by nuclear DNA studies (Pustovrh et al. 2014; Segherloo et al. 2021). However, while analyses based on mitochondrial DNA have revealed subdivision between S. letnica and S. aphelios (Sell & Spirkovski 2004; Geiger et al. 2014), they could not be separated by nuclear DNA (Sušnik et al. 2007). Unfortunately, S. balcanicus has not been included in any recent analysis and may already be extinct (Kottelat & Freyhof 2007). The distinctly divergent Salmo ohridanus is also endemic to Lake Ohrid, but is understood to represent a considerably older genetic lineage.

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).

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

The Ohrid Summer Trout has a restricted range (extent of occurrence (EOO) c. 386 km², area of occupancy (AOO) c. 360 km²), which meets the thresholds for the Endangered category under Criteria B1 (EOO < 5,000 km²) and B2 (AOO < 500 km²). It is present at one location where the quality of habitat is estimated to be declining.

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

This species is endemic to Lake Ohrid (sq. Liqeni i Ohrit; mk. Охридско Езеро), located at the boundary between eastern Albania and southwestern Republic of North Macedonia.

This species' current population size and trend have not been quantified, and an accurate appraisal may be complicated since this species does not appear to be separated from the sympatric Ohrid Trout (Salmo letnica) in fisheries data. Anecdotal reports by local fishers indicate that its abundance has declined since the late 20^th century.

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

Oligotrophic and oligomictic Lake Ohrid occupies a steep-sided graben and is of tectonic origin. Its surface area is 358 km², and it is the deepest lake in the Balkan region, with a mean depth of 164 metres and maximum depth of 289 metres. The lake remains oxygenated even in the profundal zone, but is stratified by salinity below depths of 150 metres and only mixes completely about once per decade. With an estimated minimum age of c. 4 million years, Ohrid is probably the oldest extant lake in Europe.

It is fed primarily by two major karstic surface springs at its southern end plus a series of sub-lacustrine (underwater) springs which are located along the eastern and, to a lesser extent, western shores. All such inflows on the eastern and southern sides originate in the adjacent Lake Prespa (sq. Liqeni i Prespës; mk. Преспанско Езеро; el. λίμνη Πρέσπα) basin, which is located at a slightly higher altitude and drains to Ohrid via subsurface karstic conduits. Four small rivers and numerous intermittent streams also influence the overall water level to a minor extent in spring or during periods of heavy rainfall. Ohrid is drained by the Black Drin (sq. Drini i Zi; mk. Црн Дрим) River, which flows towards the Adriatic Sea after exiting the lake at its northern tip, and it loses significant volumes of water (c. 40%) via evaporation. Due to its relatively limited recharge and discharge regimes compared to its overall volume, it is characterised by a long retention time of c. 83.6 years. The lake's water balance and watershed were artificially enlarged during the 1960s when the River Sateska, which formerly flowed into the Black Drin, was diverted into the lake to increase its potential for hydropower generation. Two large hydroelectric dams were also constructed on the Black Drin main stem immediately downstream of the lake around this time.

The lake contains exceptionally clear water with a Secchi depth of 7-16 metres in the pelagic zone. It is rich in macrophytes, among which the littoral vegetation includes a discontinuous strip of Common Reed (Phragmites australis) extending around 1.5 metres into the water, dense stands of angiosperms (particularly Potamogeton perfoliatus) to depths of c. 4 metres, and a largely continuous dense belt of stonewort (Chara spp.) at depths of 4 metres to c. 11 metres, extending to c. 18.5 metres in places.

The lake’s low-nutrient, high-oxygen environment supports an exceptional biodiversity consisting of at least 1,200 taxa, of which more than 200 are endemic. It is thus among the most species-rich inland water bodies on Earth when surface area is taken into account. The lake is also considered to be a hotspot of endemic salmonid diversity, with up to five recognised species (see 'Taxonomic Notes').

The Ohrid Summer Trout's biology and ecology are largely unknown. The annual reproductive period is understood to extend from July to August, when spawning takes place on rocky substrata in the sublittoral zone at depths of 45-60 metres. The spawning sites are reportedly confined to an area associated with sublacustrine springs on the eastern shore of the lake, in a bay extending south from Gradište.

The Lake Ohrid basin has experienced increasing urbanisation (including tourism) and cultivation since the late 1940s. It is thus threatened by the progressive accumulation of anthropogenic nutrients, chemicals and sediment loads from a variety of sources, including untreated domestic sewage, mining and other industry, and agriculture, which are driving increased rates of eutrophication and sedimentation. In the northeastern part of the basin, the Sateska River has become a major source of pollution since it was diverted into the lake. In addition, water abstraction and eutrophication in the Lake Prespa watershed is believed to be responsible for elevated nutrient levels in the southern and eastern affluent springs. These alterations are reflected by increasing phosphorous concentrations in the littoral and pelagic zones, decreasing dissolved oxygen content in the hypolimnion, and reduced water transparency.

A shift to non-endemic algae, macrophyte, diatom, and macroinvertebrate species associated with eutrophication has been observed close to polluted affluents and sewage outflows, but this has not yet resulted in major shifts within the pelagic phytoplankton community. It has been hypothesised that the extensive littoral macrophyte stands and filter-feeding benthic invertebrates may be acting as a phosphorus sink which buffers the pelagic from eutrophication. Signs of narrowing associated with decreased water transparency have already been detected in the stonewort belt, and the coverage by reedbeds has also decreased due to recreational and residential shoreline development. Further degradation of the littoral vegetation could plausibly drive increased nutrient loading in the pelagic and accelerate the rate of eutrophication throughout the lake. Since this species spawns at significant depths in the sublittoral, it appears unlikely that it has been negatively impacted to date.

Increased boat traffic due to tourism, direct abstraction of domestic drinking water and the ongoing capture and pollution of karstic springs are also of concern. Plastics and other solid wastes are regularly washed into the lake because effective landfill sites are limited or do not meet modern standards.

Moreover, under a global warming scenario, the detrimental effects of increased nutrient inputs are projected to intensify. Even a slight increase in the lake's productivity might lead to a complete loss of dissolved oxygen in the profundal zone, while warming water temperatures and increasing eutrophication could interfere with the Ohrid Summer Trout's foraging behaviour and reproductive processes.

At least 7 non-native fish species known to exert detrimental pressures on native freshwater fauna through increased competition, predation, transmission of pathogens or habitat degradation have been introduced to Lake Ohrid. Among these, Eastern Mosquitofish (Gambusia holbrooki) was introduced around the mid-20^th century in order combat water-borne disease. Rainbow Trout (Oncorhynchus mykiss) was recorded regularly from 1974 to 1994 but now appears to be scarce after the Albanian hatchery from which it originated switched production to native Ohrid Trout. Topmouth Gudgeon (Pseudorasbora parva) was also introduced during the 1970s. Prussian Carp (Carassius gibelio) was first reported in 1983 and is now abundant with annual landings exceeding 20 tonnes, while European Bitterling (Rhodeus amarus) and Pumpkinseed (Lepomis gibbosus) have been established since the 1990s. The origin of Common Carp (Cyprinus domestic strain) in the lake is less clear, but it is currently stocked on an annual basis despite the Ohrid basin being located outside of its native range. Most of these taxa predominantly inhabit littoral and sublittoral habitats and could plausibly predate on the eggs and early life stages of native salmonids, although their impact has not yet been investigated and the location of Ohrid Summer Trout nursery zones is unknown.

There is currently no clear evidence that the Ohrid Summer Trout is threatened by overharvesting, but conclusive and unambiguous research is required. Abandoned or discarded fishing gear may represent a secondary threat, since nets placed by unlicensed fishers are typically located at known spawning sites and can continue to trap individuals for several months or even years.

Lake Ohrid supports a long-established commercial fishery, with Ohrid Trout (Salmo letnica), Ohrid Belvica (Salmo ohridanus), Skadar Bleak (Alburnus scoranza), European Eel (Anguilla anguilla) and the non-native Eurasian Carp (Cyprinus domestic strain) comprising the most important species. This species is apparently harvested and well-known to local fishers, but no explicit offtake data are available and it is probably combined with Ohrid Trout. Gill nets are the most widely-used gear by both licensed and unauthorised fishers, but purse seines and long lines are also employed.

During the period 1960-1990, a joint Ohrid fisheries commission managed issues of transboundary concern such as quotas per country, minimum landing sizes, closed seasons and stocking measures. This authority no longer exists, although collaboration continues at political and research levels.

In the Albanian portion of the lake, the local fishery was administrated by a state enterprise employing around 35 fishers until the fall of communism in 1991. This was followed by a period of deregulation during which an estimated 800 authorised and unauthorised fishers operated on the lake (see 'Population'). Since 2002, a dedicated Fisheries Management Organization issues a maximum of 110 licenses at any given time.

Prior to 1994, the Republic of North Macedonia fishery was managed by two state-owned companies, but is today governed by a private firm which employs around 50 fishers. The current concession was granted following a total fishing moratorium, which was in place from 2004-2012 due to fears of overharvesting.

In the Republic of North Macedonia, up to 2,000 recreational licenses were granted per day during the mid-to-late 20^th century, permitting fishers to circumvent the need for a professional license. Although daily bag limits were established for some species, these were routinely ignored and the exploitation of recreational permits resulted in some fishers doubling their annual income in some years. Moreover, recreational landings have never been included in official fisheries statistics.

A hatchery-based supportive breeding and stocking programme for the Ohrid Trout has been established at the Hydrobiological Institute Ohrid in the Republic of North Macedonia since 1935, and Ohrid Summer Trout was apparently included at a later point in time. However, there are no data available and it is unclear whether this species continues to be reared or stocked.

The North Macedonian portion of Lake Ohrid has been included in UNESCO's list of World Heritage sites since 1979. A bilateral agreement between Albania and North Macedonia resulted in establishment of the Ohrid Watershed Management Committee in 2004 and the Ohrid-Prespa Transboundary Biosphere Reserve in 2014. At the national scale, a number of additional protected sites have been created around the lake, including the Pogradec Aquatic and Terrestrial Protected Landscape and Drilon National Monument in Albania, and the Lake Ohrid Monument of Nature and Galičica National Park in North Macedonia. In April 2021, Lake Ohrid was added to the Ramsar Convention on Wetlands of International Importance (site 2449).

Efforts to reinforce the Ohrid Trout fishery through supportive rearing and restocking have taken place in most years since 1935, but the Ohrid Summer Trout stocking regime is far less clear (see 'Use and Trade'). Although it cannot be discounted that unintentional hybridisation between one or more of the sympatric trout taxa has taken place (see 'Taxonomic Notes`), a major impact on this species' genetic structure seems unlikely since broodstock are not maintained in captivity, with adults are collected in the vicinity of their spawning sites and stripped on a single occasion before being sold for consumption or released.

Fisheries management plans have been developed in both Albania and the Republic of Macedonia since the 2000s, but the Ohrid Summer Trout is not specifically mentioned in regulations concerning annual quotas, minimum landing sizes or closed fishing seasons. Steps have been taken towards the harmonisation of these legal frameworks in order to supervise the fishery on a collaborative basis, which may involve reinstatement of the joint fisheries commission (see 'Use and Trade'). Points of concern include improved control of unauthorised harvesting, establishment of new annual landing quotas for both countries, a review of stocking procedures to include analysis of their effectiveness, modernisation of existing hatchery facilities, and an update of approved fishing gear types.

Recommendations for the design of a transboundary fish monitoring programme were made following a multi-species stock assessment carried out from 2013-2015, but it is unclear whether these were subsequently implemented. These surveys took place in the littoral zone and thus did not target the Ohrid Summer Trout's putative offshore habitat.

In 2018, a pilot study investigating the presence of lost, abandoned or discarded fishing gear in the North Macedonian part of the lake resulted in the removal of more than 12,000 metres of nets from the lake.

Improved wastewater treatment measures are required to prevent the ongoing eutrophication of Lake Ohrid. Since the late 1980s, around 65% of domestic wastewater produced in the Republic of Macedonia is collected by a ring system and transported to a treatment plant near the town of Struga, from where the effluent is pumped into the Black Drin River. However, the system was never fully-completed and is thus partly dysfunctional. In Albania, a wastewater treatment plant was installed at the urban centre of Pogradec in 2006, but the phosphorous-rich effluent is discharged directly into the lake. Moreover, most rural communities still lack access to proper wastewater facilities, and continue to release untreated effluent into the lake or its tributaries.

Protection and restoration measures for the littoral zone and its macrophyte community has been recommended as a potentially effective measure to protect the pelagic from increasing nutrient levels. Restoring the original course of the Satesca river has also been proposed.

A deeper understanding of the Ohrid Summer Trout's demographics (population size and trend), life history (e.g., diet and habitat preferences, locations of nursery zones and key spawning sites) and response to the ongoing threats would undoubtedly prove beneficial in the design of any structured management plan.