Lake Þingvallavatn is occupied by four sympatric forms of charr, which for Red List purposes are understood to represent at least two species (Kottelat and 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 small benthic form is considered to represent S. thingvallensis (Kottelat and 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 and Maitland 2007, Kottelat and 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 Þingvallavatn 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 is uncertain, precluding the use of Criterion C or Criterion 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 pollution, climate change, and/or introduced 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' current population size and trend have not been quantified, but there are no reports indicative of decline.

Its abundance is naturally low compared with pelagic charr in Lake Þingvallavatn.

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.

This species inhabits epibenthic habitats in the shallow littoral zone at water depths of 0-10 metres, where it feeds on gastropods and benthic insects. It is predominantly nocturnal and takes refuge among lava stones during the day.

The maximum recorded lifespan is c. 17 years, and sexual maturity is reached at age 2-4+.

The annual reproductive period extends from August to November, with early-spawning individuals possibly confined to areas with upwellings of cold ground water.

Nuptial individuals develop a subtle epigamic colour pattern, 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.

This species is not used or traded.

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 taxonomic status remains questionable. In particular, the Þingvallavatn 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.