In some published literature, this species is referred to as a subpopulation of the Arctic Charr (Salvelinus alpinus), and 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: Endangered (EN)
EU 27 regional assessment: Endangered (EN)

The Constance Deepwater Charr has a restricted range (area of occupancy (AOO) c. 536 km²), which meets the threshold for the Endangered category under Criterion B1 (EOO < 5,000 km²). It is present at a single location where the extent and quality of habitat are estimated to be declining. Therefore, this species is assessed as Endangered under Criterion B (B1ab(iii)), both globally and for the EU 27 member states.

This species is endemic to Lake Constance (fr. Lac de Constance; de. Bodensee) in the Rhine River system, located at the boundary between Switzerland, Germany and Austria.

This species was presumed extinct between the 1970s and 2010s, but a single specimen was reported in 2011 and more than 100 individuals were subsequently captured during field surveys carried out between 2014 and 2017.

Analyses of samples collected from Lake Constance during the 1980s and 1990s have revealed that it was in fact present throughout this period, but declined markedly after 1985.

The current population size and trend are unknown.

Lake Constance is a monomictic, perialpine, naturally oligotrophic lake formed after the last glacial period. It has a maximum depth of 254 metres, and two main basins comprising the deep Upper Lake referred to locally as the "Obersee" plus the much smaller and shallower Lower Lake or "Untersee".

This species is restricted to the Upper Lake, where it inhabits the profundal zone at depths greater than 80 metres. It feeds largely on turbellarians, supplemented by other benthic invertebrates and some zooplankton. The stomach contents of samples collected between 1980-1998 contained a comparatively higher proportion of zooplankton and fewer turbellarians, and this has been attributed to an enforced dietary switch during the eutrophic phase of Lake Constance (see 'Threats').

It exhibits a number of specialised morphological traits linked to profundal ecology, including small body size and a subinferior mouth with a strongly overlapping upper jaw.

Its life history has not been studied in detail, but spawning is believed to take place during the summer at depths of 70-150 metres.

Lake Constance is also inhabited by a pelagic subpopulation of the Arctic Charr (Salvelinus alpinus), which predominantly inhabits shallower waters of the lake and spawns during winter. This subpopulation might today be composed entirely of hybrids due to intensive stocking with juvenile charr of various non-native provenances (e.g., Switzerland, Bavaria, Sweden) from 1977 until at least 1992. In total, an estimated 9.2 million hatchery-reared charr individuals were introduced to the lake between 1977-2019, of which at least 2.2 million were of non-native origin.

According to historical records, a second form of small-bodied charr which occupied profundal pelagic habitats and fed on a now extinct zooplankton species occurred in Lake Constance prior to eutrophication. Genetic and morphological analysis of recently-collected individuals suggest that trophic polymorphism might be ongoing in the profundal zone of the lake, but it is unclear whether this is the result of speciation processes or recent introgression with Arctic Charr (see 'Threats').

Lake Constance has experienced pronounced alterations in trophic status since the mid-20^th century. A rising human population in the catchment after 1900 drove the development of industry and agriculture, which led to increasing pollution from domestic wastewater and runoff.

The lake subsequently entered a phase of eutrophication which peaked in 1979, and the associated development of anoxic conditions in the profundal zone probably drove this species' decline since its eggs could no longer develop in the oxygen-depleted substrata. The extent of suitable foraging and spawning habitat is also likely to have been significantly reduced during this period. It is possible that sediments remain starved of oxygen in some parts of the lake, and this may impact recruitment at some current spawning sites.

A secondary outcome of such changes in the available range of spawning depths is the potential for overlap and increased gene flow between different species that were formerly segregated along depth gradients. This raises the possibility of speciation reversal and extinction events due to hybridisation and introgression. However, the reproductive periods of the Constance Deepwater Charr and the pelagic Arctic Charr (see 'Habitat and Ecology') do not seem to overlap, and they are believed to spawn at different sites within the lake. The persistence of these putative temporal and spatial barriers has possibly contributed to the maintenance of reproductive isolation, although molecular data are consistent with gene flow between the two taxa.

The invasive Quagga Mussel (Dreissena bugensis) has spread rapidly throughout Lake Constance since it was first recorded in 2016. It is able to survive at depths greater than 100 metres, and appears to be driving significant food web alterations. It is here considered a plausible threat due to the potential for it to colonise, and thus reduce the extent of, the Constance Deepwater Charr's spawning sites.

Warming of the lake due to climate change constitutes a plausible ongoing and future threat, and is already driving longer periods of stratification and less frequent mixing. This may lead to long-term oxygen depletion in the hypolimnion and hamper recruitment as previously occurred during the eutrophic phase of the lake.

This species was formerly harvested on a commercial basis, but is not currently used or traded.

This species was assessed as Critically Endangered in the latest (2022) iteration of the Swiss National Red List.

Lake Constance is surrounded by various protected areas, several of which are included in the European Union's Natura 2000 network.

Investment in wastewater treatment facilities and other policy-led measures developed since the 1980s has driven an improvement in water quality, and the lake had returned to its former oligotrophic state by the turn of the 21^st century. The extent of suitable habitat is believed to have increased due to reoxygenation of the hypolimnion, but not all spawning sites may have recovered and the situation now appears to be worsening due to climate change (see 'Threats').

No specific conservation actions are in place, and a deeper understanding of this species' demographics (population size and trend) plus the locations of viable spawning sites would likely prove beneficial in the development of any future management plan.

In addition, 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.