Loch Rannoch is occupied by three sympatric forms of charr, which for global Red List purposes are currently understood to represent at least two valid species (Kottelat and Freyhof 2007, Fricke et al. 2024). However, in the majority of published literature and the most recent national assessment (Nunn et al. 2023), they are considered to represent a single polymorphic subpopulation of the Arctic Charr, Salvelinus alpinus (Linnaeus 1758).

Under both of the above concepts they have typically been segregated by morphology related to dietary and habitat preferences, and thus comprise benthivorous, piscivorous and planktivorous forms. These also differ in parasite loads and some life history traits (Adams et al. 1998, Adams and Huntingford 2004, Bryce et al. 2016).

Molecular analyses have detected genetic separation indicating that the three forms are reproductively isolated. In addition, the benthivorous and planktivorous forms appear to have evolved comparatively recently within the lake itself, while the planktivorous form diverged earlier and in allopatry (Verspoor et al. 2010, Garduño-Paz et al. 2012).

For the purposes of the present global assessment, the planktivorous form is considered to represent S. struanensis (Kottelat and Freyhof 2007, Fricke et al. 2024).

At the broader scale, the taxonomic status of Salvelinus subpopulations inhabiting the British Isles has not been definitively resolved since a series of endemic species were described between the mid-19^th and early 20^th centuries (Adams and Maitland 2007).

In the United Kingdom and Ireland, all of these taxa are treated as junior synonyms of the congeneric Arctic Charr, Salvelinus alpinus (Linnaeus 1758), the name of which has been routinely applied to a widespread complex of polymorphic charr populations occurring throughout the Holarctic region (Jonsson and Jonsson 2001). However, there remains considerable uncertainty regarding the systematics of the genus across the majority of this range (Reist et al. 2013, Taylor 2016, Whiteley et al. 2019).

Members of this "Arctic Charr complex" exhibit bewildering subpopulation-scale ecological and morphological variability. 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 Struan Charr has a restricted range (extent of occurrence (EOO) c. 26 km²), which meets the threshold for the Critically Endangered category under Criterion B1 (EOO < 100 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 with a plausible future threat of introduced non-native species and/or climate change that could drive it to Critically Endangered or Extinct in a very short time period.

This species is endemic to Loch Rannoch in the Tay River system, central Scotland, United Kingdom.

A putatively conspecific subpopulation inhabits adjacent Loch Ericht, which is connected to Loch Rannoch via the short Ericht River. However, its identity has never been explicitly confirmed and it is not considered for the purposes of this assessment.

This species' current population size and trend have not been quantified, but there is no evidence of continuing decline based on recent surveys.

Loch Rannoch is an oligotrophic freshwater lake formed after the last glacial period.

It has a maximum depth of 134 metres and a stony shoreline. The lake is separated into a small, relatively shallow western basin and a deep main basin. It occupies a catchment dominated by mixed relict deciduous and coniferous woodlands with areas of rough grazing and marginal cultivation. It was converted into a hydroelectric reservoir in 1928, and a low barrage at its eastern end limits water level changes to a maximum of 2.74 metres, therefore the littoral zone is never substantially dewatered.

Loch Rannoch is among the few European lakes to be inhabited by three sympatric forms of charr, among which the Struan Charr concept applied in this assessment comprises the planktivorous form (see 'Taxonomic Notes'). It possesses a relatively fusiform body shape with a small, delicate head and large eyes, and inhabits the limnetic zone where it feeds predominantly on zooplankton. Cladocerans comprise the bulk of the diet, with chironomid larvae also contributing significantly at certain times of the year.

In contrast, the benthivorous charr possesses a deeper, more robust head and jaw structure, and is found in the littoral zone of the western basin where it feeds on zoobenthos. The piscivorous charr has a very robust head shape and a large gape, and mostly occupies the profundal zone of the main basin where it preys on smaller fishes and larger macroinvertebrates.

This species' maximum recorded lifespan is 7 years, with sexual maturity reached at age 2-3+. Some mature individuals do not spawn every year.

The annual reproductive period extends from October to November and overlaps with that of the benthivorous form, although they utilise different spawning sites.

Nuptial individuals develop an intense epigamic colour pattern, and spawning takes place on coarse stony substrata in shallow littoral or sublittoral habitats.

Loch Rannoch is considered to be of relatively pristine ecological status despite being exploited for hydropower generation.

The resident charr community is not currently threatened, although a number of plausible threats have been identified.

These include the introduction of non-native fish species, with Crucian Carp (Carassius carassius) having first been observed in the lake during 1997 and hatchery-reared Brown Trout (Salmo trutta) stocked on a regular basis. Such introductions could drive alterations in food-web structure, although Crucian Carp is not currently abundant.

Trout stocking is also understood to have driven the establishment of at least four previously unrecorded parasite taxa in Loch Rannoch, while the infection load of native parasites in charr has also increased since the early 1990s. The growth rate and gonadal development of some individuals could be inhibited as a result of these changes.

Rising water temperatures due to climate change represents a plausible future threat.

Recreational angling under permit is allowed on Loch Rannoch, and this species is sometimes targeted. It is not otherwise used or traded.

The Struan Charr is included (as Salvelinus alpinus) as a priority species of conservation concern in the U.K. Post-2010 Biodiversity Framework.

Some parts of the Loch Ericht shoreline fall within the Ben Alder and Aonach Beag, Coire Bhachdaidh and Drumochter Hills Sites of Special Scientific Interest (SSSIs).

Loch Rannoch lies entirely within the Loch Rannoch and Glen Lyon National Scenic Area and is thus protected from inappropriate development, and the Black Wood of Rannoch SSSI covers a portion of its southern shoreline.

This species is not currently recognised by the relevant authorities in Scotland or the United Kingdom, where it is treated as a subpopulation of the Arctic Charr (Salvelinus alpinus). 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.