Lake Þingvallavatn is occupied by four sympatric forms of charr, which for Red List purposes are understood to represent at least two species (Kottelat & Freyhof 2007; Fricke et al. 2024). However, in most published literature they are considered to represent a single polymorphic subpopulation or subspecies of the Arctic Charr (Salvelinus alpinus).

Under both of the above concepts, they have typically been segregated by morphology related to dietary and habitat preferences, and thus comprise "large benthic", "small benthic", "piscivorous" and "planktivorous" forms. These also differ in parasite loads and some life history traits, e.g., age and length at maturity, maximum size, mortality rate and spawning period (Sandlund 1992; Ferguson et al. 2019).

Molecular analyses have detected genetic separation indicating that the four forms are reproductively isolated, but suggest that the piscivorous form is less distinct than the other three. Specifically, most piscivorous individuals are closely-related to the planktivorous form, but others display similarities with the large benthic form (Guðbrandsson et al. 2019).

For the purposes of this assessment, the piscivorous and planktivorous forms are considered together to represent S. murta (Kottelat & Freyhof 2007; Fricke et al. 2024).

At the broader scale, there remains considerable uncertainty regarding the evolutionary relationships and taxonomy of the genus Salvelinus (Reist et al. 2013; Taylor 2016; Whiteley et al. 2019).

Members of this genus exhibit bewildering subpopulation-scale ecological and morphological variability throughout their global range. When such divergence occurs within a single lake system, the different sympatric forms are often referred to as “morphs”, “morphotypes”, "ecomorphs" or "ecotypes" (Snorrason et al. 1994; Adams et al. 1998; Knudsen et al. 2006; Klemetsen 2010; Muir et al. 2016).

Some of these subpopulations and sympatric forms have over time been described as nominal species, including at least 15 from North America, around 30 from Europe and 12 from Siberia and the Far East. However, these taxa encompass only a small fraction of charr distribution and diversity, and there exist significant differences in opinion regarding which of them should be considered valid (Savvaitova 1995; Adams & Maitland 2007; Kottelat & Freyhof 2007; Klemetsen 2010; Whiteley et al. 2019).

With the above in mind, there is an emerging consensus that the striking genetic and phenotypic diversity exhibited by members of this genus cannot be adequately represented by a single accepted taxonomic system (Whiteley et al. 2019).

The Red List currently follows the nomenclature provided by Fricke et al. (2024), albeit a species-oriented conservation management approach is unlikely to prove appropriate for members of this genus (Barthelemy et al. 2023; also see 'Conservation').

Global and European regional assessment: Vulnerable (VU)
EU 27 regional assessment: Not Recorded

The Murta Charr has a restricted range (extent of occurrence (EOO) c. 116 km², area of occupancy (AOO) c. 84 km²), which meets the thresholds for the Endangered category under Criterion B1 (EOO < 5,000 km²) and Criterion B2 (AOO < 500 km²). It is restricted to one location, but there is no indication of continuing decline or extreme fluctuations, hence it does not qualify for a threatened category under Criterion B.

There is no evidence of any population size reduction that would approach the threshold for Vulnerable under Criterion A (≥ 30% over the past ten years or three generations). The population size does not approach the thresholds for Vulnerable under Criterion C (< 10,000 mature individuals) or D1, and there exists no quantitative analysis of extinction probability which would permit application of Criterion E.

Therefore, this species is assessed as Vulnerable under Criterion D2, based on its presence at one location where the threats of pollution, climate change, and/or the introduction of non-native species could drive it to Critically Endangered or Extinct in a very short time period.

This species is endemic to Lake Þingvallavatn (sometimes anglicised as Thingvallavatn) in southwestern Iceland.

This species' population size is unknown, but it is understood to exceed the minimum threshold for Red List criteria (< 10,000 mature individuals).

Charr dominate the fish community of Lake Þingvallavatn, among which the planktivorous form of Murta Charr is most abundant and comprises c. 80% of the overall stock, with the piscivorous form comparatively rare (see 'Habitat and Ecology'). The current population trend has not been quantified, but there is no evidence of continuing decline.

Dimictic, oligotrophic Lake Þingvallavatn was formed after the last glacial period. It is the largest natural lake in Iceland, is primarily fed by groundwater and has a maximum depth of 114 metres.

The lake is inhabited by four sympatric forms of charr which are differentiated by ecological traits, among which the Murta Charr concept applied in this assessment includes piscivorous and planktivorous forms (see 'Taxonomic Notes'). Both of these occupy the pelagic zone, with the former also inhabiting the epibenthic zone.

The piscivorous form feeds mainly on smaller fishes, predominantly Icelandic Stickleback (Gasterosteus islandicus) and juvenile charr. It grows significantly larger than the planktivorous form, which preys on zooplankton and emerging insects such as chironomid pupae. The planktivorous form also comprises the primary prey of the resident Brown Trout (Salmo trutta) subpopulation.

The maximum recoreded lifespan is c. 18 years, and sexual maturity is reached at age 3-6+.

The annual reproductive period of the planktivorous form extends from mid-September until the end of October, whereas the piscivorous form spawns from September to November.

Reproductively-active individuals develop an intense epigamic colour pattern at these times, and spawning takes place on coarse stony substrata in shallow littoral or sublittoral habitats.

The incubation period lasts for c. 5 months and the eggs hatch in early spring. Juveniles occupy littoral areas of the lake once the winter surface ice has melted.

Lake Þingvallavatn is considered to be of relatively pristine ecological status, although a number of plausible threats have been identified.

For example, a dam and power plant constructed on the outflowing Sogið (aka Sog) River in 1959 led to irregular annual water level fluctuations of c. 0.5 metres and a collapse in the formerly abundant Brown Trout population, since the river comprised its most important spawning site in the catchment. The trout decline has been linked to fluctuating Murta Charr commercial landings after the dam became operational (see 'Use and Trade').

In the 1980s, a second power plant designed to exploit the Nesjavellir geothermal field in the southwestern part of the Þingvallavatn was constructed. Warm effluents from the plant are discharged into a nearby stream and later into nearby sink wells and drill holes. The effluents then mix with groundwater which flows through the porous lava bedrock before entering the lake at its southwestern shore. Both chemical and thermal pollution have been observed at the outlet sites, but chemical analysis of biological samples, including fish, have failed to detect any widespread negative impacts.

Climate change perhaps represents the most significant ongoing and future threat. For example, c. 30% of groundwater flowing into Lake Þingvallavatn originates from glacial meltwater, and the nearby Langjökull glacier has been rapidly decreasing in mass since the mid-1990s.

In addition, mean summer water temperatures have increased by c. 1.5°C since the early 1960s, while the period of winter ice cover has become dramatically shorter. Increasing water temperatures may drive longer periods of stratification and depleted oxygen levels in the profundal zone during summer and/or interfere with zooplankton community structure, potentially reducing the extent of suitable habitat and food availability. Moreover, the shallow littoral zone is likely to experience the most severe warming, which may negatively impact the quality of spawning and nursery habitats.

Iceland is the world's largest producer of farmed Artic Charr (Salvelinus alpinus), and introduction of non-native individuals to Lake Þingvallavatn represents a plausible future threat.

The subsistence fishery for Murta Charr in Lake Þingvallavatn dates back to at least the early 18^th century, while commercial fishing began around the turn of the 20^th century.

The fish were initially salted and sold to local communities, but in 1937 a canning factory opened close to Reykjavik, exports commenced and landings increased significantly.

The commercial fishery was timed to coincide with the fish moving to their spawning sites during a short period in September and October. The majority of landings comprised the planktivorous form, but the piscivorous form was also harvested.

Annual landings fluctuated significantly between the 1960s-1990s, e.g., 72 tonnes in 1964 dropping to 5.2 tonnes in 1967, mean of c. 60 tonnes from 1977-1986 dropping to zero in 1987. In 1989 the fishery was abandoned entirely due to the decline in productivity.

Subsequent research demonstrated that there was no reduction in fish abundance, and that the crash in landings was caused by a decrease in the size of mature adults, rendering commercial nets designed to catch fish of a certain minimum length ineffective. No signals of recruitment loss or overfishing were detected, but the fluctuations may have been related to a decrease in predation pressure following the decline of Brown Trout in the lake (see 'Threats').

Recreational catch-and-release fishing and associated tourism at Lake Þingvallavatn is popular, and charr are targeted alongside Brown Trout. From 1993-2004 a large number of Brown Trout parr reared using broodstock from the inflowing Öxará River were stocked into the lake to supplement the declining subpopulation, and this has resulted in recovery of the adult stock.

Around 25% of the Lake Þingvallavatn catchment is located within the Þingvellir National Park, part of which is also a UNESCO World Heritage Site.

In 2005, a series of measures aiming to protect charr habitat and spawning sites in the lake were introduced. In particular, any pollution of both ground- and surface-water was forbidden and limits were placed on the permitted extent of water level fluctuations caused by the outlet dam.

The northern portion of the catchment has been protected from grazing since 1975, and aquaculture on the lake is generally prohibited.

The biology and ecology of the resident charr species have been extensively-studied, although their systematic status remains questionable. In particular, the Murta Charr is not recognised as a valid species by the relevant authorities in Iceland, where it is treated as a subpopulation of the Arctic Charr. The taxonomy of Eurasian charrs is in need of review (see 'Taxonomic Notes'), and it has been widely recommended that their conservation management must be considered independent of their systematic classification. 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 should ideally be coordinated at local, national or regional scales.