In some published literature, this species is treated as a subpopulation of the Bourget Whitefish (Coregonus lavaretus) or a member of the "Coregonus lavaretus species complex", and referred to by the English vernacular names "Common Whitefish" or "European Whitefish" (see below).

At the regional scale, the systematics of European ciscoes and whitefishes (Coregonus spp.) has been the subject of considerable debate since the turn of the 21^st century, with little indication of a definitive outcome.

It is widely accepted that non-anadromous members of this group have repeatedly undergone adaptive radiations in boreal, subarctic and perialpine lakes (Douglas et al. 1999, Østbye et al. 2005, Kahilainen and Østbye 2006, Harrod et al. 2010, Hudson et al. 2011).

Subpopulations inhabiting these systems largely diversified in the wake of the most recent glacial period 10,000-15,000 years ago. They are typified by parallel patterns of divergence in traits associated with foraging (i.e., gill raker counts, benthic vs. pelagic feeding ecology), physiology (i.e., growth rate, habitat depth partitioning) and reproductive ecology (i.e., reproductive timing and spawning habitat).

Up to six different sympatric forms sometimes referred to as “morphs”, "ecomorphs" or "ecotypes" have been recorded in perialpine lakes, and these radiations might be even more diverse in some large systems of northeastern Europe (Præbel et al. 2013, Doenz et al. 2018, Bitz‐Thorsen et al. 2020, Öhlund et al. 2020).

However, there exist significant differences in opinion regarding the taxonomic status of these forms.

In Northern Europe, including the United Kingdom, all except a handful of Irish subpopulations are usually regarded as belonging to two widely distributed species; Coregonus albula (ciscoes) and C. lavaretus (whitefishes), with the latter also frequently referred to as the "Coregonus lavaretus species complex” (Etheridge et al. 2012, Wanke et al. 2017, Häkli et al. 2018, Crotti et al. 2020). Within this comparatively uniform taxonomic concept, the catch-all English vernacular name “Vendace” is typically used for C. albula while "Common Whitefish" or "European Whitefish" are applied to C. lavaretus.

Conversely, researchers and fisheries authorities based in Central Europe have tended to treat the different forms as distinct taxa, leading to the recognition of more than 60 species across the European region (Kottelat and Freyhof 2007, Selz et al. 2020, De-Kayne et al. 2022, Selz and Seehausen 2023). Under this diverse taxonomic concept, the name “Vendace” is restricted to its original usage for the United Kingdom endemic Coregonus vandesius and the native range of C. lavaretus is limited to its type locality of Lake Bourget in France.

In the absence of a region-wide consensus, the Red List continues to follow the taxonomy provided by Fricke et al. (2024).

Global and European regional assessment: Endangered (EN)
EU 27 regional assessment: Endangered (EN)

The Sandfelchen has a restricted range, with an extent of occurrence (EOO) c.1,137 km², which meets the threshold for the Endangered category under Criterion B1 (EOO < 5,000 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)), both globally and for the EU 27 member states.

This species is endemic to Lake Constance (fr. Lac de Constance; de. Bodensee) in the upper Rhine River system, which is bordered by Switzerland, Germany and Austria.

It has reportedly been introduced to perialpine lakes of the Po River drainage in Italy, although there are no recent observations.

This species' current population size and trend have not been quantified, but it is naturally considered to be of low abundance.

Declines in two other whitefish species inhabiting Lake Constance have been observed since 2005, with a particularly sharp contraction since 2013 (see 'Habitat and Ecology' and 'Threats').

Lake Constance is a monomictic, perialpine, naturally oligotrophic lake formed after the last glacial period. It has a total surface area of 536 km², divided between the large (472 km²), deep (>250 metres) Upper Lake (de. Obersee) and the smaller (63 km²), shallower Lower Lake (de. Untersee).

The Sandfelchen is currently understood to be restricted to the Lower Lake. It is relatively large-bodied, primarily demersal and feeds on zoobenthos.

This species' life history is not well-understood, but the annual reproductive period extends from late November to December when spawning takes place in the littoral zone.

Four whitefish species which differ in traits related to feeding and reproductive ecology along depth gradients were originally native to the lake. In addition to the Sandfelchen, these comprise the pelagic-spawning Blaufelchen (Coregonus wartmanni), inshore-spawning Gangfisch (C. macrophthalmus) and profundal Bodensee Kilch (C. gutturosus). However, the latter became extinct during the 1970s due to anthropogenic oxygen depletion in the hypolimnion (see 'Threats').

Two additional whitefish populations have been recognised in the Lake Constance system. One mostly inhabits the Lower Lake and is sometimes referred to as Coregonus sp. "Weissfelchen", while the other occurs in the Upper Lake but migrates into the Alpine Rhine River for spawning and is therefore known as C. sp. "Alpenrheinfelchen". These might represent still-undescribed species, although genetic analyses indicate that C. sp. "Alpenrheinfelchen" is possibly conspecific with the Sandfelchen.

A rising human population in the Lake Constance 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 led to periodic stratification and anoxia in the hypolimnion. The natural reproduction of native whitefishes was reportedly inhibited during this period, since their eggs could no longer develop in the oxygen-depleted substrata.

The development of anoxic conditions in the profundal zone probably led to the extinction of the sympatric Bodensee Kilch. A hypothesised secondary outcome of this stratification is the potential for overlap and increased gene flow between different whitefish species that were formerly segregated along depth gradients, which raises the possibility of speciation reversal and extinction events due to introgressive hybridisation. However, increased gene flow between whitefish taxa might also be driven by intensive stocking and artificial breeding in hatcheries (see below). Unregulated stocking of Lake Constance with non-native whitefishes from other central Alpine lakes, Eastern Europe and North America also occurred around the end of the 19^th century, but there is no evidence of any long-term negative impact on the native taxa.

An increase in zooplankton abundance during the eutrophication process triggered significant interannual variation in whitefish biomass. These discrepancies were characterised by rapid growth rates, leading to an increased proportion of sexually immature individuals being captured by commercial fisheries and a loss of older age classes in the standing stock.

The supplementary stocking of hatchery-reared native whitefishes, whereby gametes are stripped from wild individuals and larvae reared under hatchery conditions before being released as fingerlings, may represent a further threat. These procedures are carried out in many perialpine lakes, although there is little evidence that they improve yield when ecological conditions are suitable for natural whitefish reproduction. Furthermore, studies focusing on other fish species have demonstrated that the influx of juvenile individuals can impair natural recruitment, impose artificial sexual selection on target species, and increase hybridisation rates due to the inadvertent crossing of similar-looking sympatric taxa.

However, DNA analyses published to date suggest that the above factors have caused no significant loss of genetic diversity in the extant Lake Constance whitefish community.

The observed decline in whitefish commercial landings since 2005 has been inconclusively attributed to the re-oligotrophication of the lake (see 'Conservation'), which has driven a reduction in food availability due to declining zooplankton abundance. Growth rates have also reduced considerably, although both of these outcomes are likely to represent a partial return to natural conditions.

The non-native Three-Spined Stickleback (Gasterosteus aculeatus) is currently viewed as a major threat due to its rapid expansion in the pelagic zone of the lake since around 2012-2013. This species not only competes with native whitefishes for zooplankton resources, but preys on their eggs and fry. Further study is required in order to establish the precise extent of its impact.

The invasive Quagga Mussel (Dreissena bugensis) has also spread throughout Lake Constance since it was first recorded in 2016. This filter-feeding mollusc appears to be driving significant food web alterations, including energy sources and pathways for native fishes, due to its propensity to alter zooplankton abundance, community structure and composition. On the other hand, it has been hypothesised that the mussel might provide an additional food source for the zoobenthivorous Sandfelchen and therefore benefit the species.

The piscivorous Great Cormorant (Phalacrocorax carbo) is estimated to consume c. 450 tonnes of mixed fish species per year and may be contributing to native whitefish declines.

Warming of the lake due to climate change constitutes a plausible ongoing and future threat, since it may result in habitat shifting.

Lake Constance supports one of the oldest and most valuable commercial whitefish fisheries in Central Europe, but declining stocks enforced a recent closure (see 'Population' and 'Conservation'). The number of professional fishers has fallen significantly since the 1930s, with a maximum of 80 permitted as of 2018. The Blaufelchen is the predominant target, and comprises an important feature of the local cuisine, economy and heritage. In Lower Lake Constance, the Sandfelchen is also harvested albeit largely as bycatch. Whitefishes are locally marketed both fresh and smoked, but demand currently outstrips supply and more than 50% of those sold in the area are brought in from elsewhere. Initial aquaculture experiments have been carried out, with the aim of reducing the reliance on imports. In addition, the annual yield has varied throughout the last century due to the influence of nutrient-driven ecosystem changes (see 'Threats').

When the fishery is open, the Sandfelchen is among the species targeted on a catch-and-keep basis by the c. 13,000 private anglers fishing on the lake each year.

Supplementary stocking of Lake Constance with millions of hatchery-reared whitefish fry increased from the 1950s to the early 2000s, but has recently slowed due to the declining adult stock. Hatchery-reared juvenile Sandfelchen have not been stocked since the 1990s, in order to minimise the risk of introgression with the Gangfisch.

This species is included (as Coregonus spp.) in Appendix III of the Bern Convention and Annex V of the European Union Habitats Directive. 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 has driven an improvement in water quality since the 1980s. The Upper Lake had returned to its former oligotrophic state by the turn of the 21^st century, while the Lower Lake is currently mesotrophic.

Translocation of non-native whitefishes has been prohibited in Switzerland since 1991, and no longer takes place anywhere in Lake Constance.

An international monitoring scheme is jointly coordinated by the Fisheries Research Station Langenargen (de. Fischereiforschungsstelle Langenargen) and Fish Hatchery Nonnenhorn (de. Fischbrutanlage Nonnenhorn) in Germany, and the Department of Nature, Hunting and Fishing St. Gallen (de. Amt für Natur, Jagd und Fischerei St.Gallen) in Switzerland.

As a result of declining abundance in the overall whitefish spawning stock (see 'Population'), no artificial stripping of gametes from wild reproductive adults took place in 2018 or 2022.

Furthermore, in 2023 the International Lake Constance Fishery Commission (de. Internationale Bevollmächtigtenkonferenz für die Bodenseefischerei; IBKF) agreed a new package of management measures in response to the declining whitefish stock.

These include a three-year ban on commercial and recreational fishing of all resident whitefish species starting from January 1^st 2024, stocking with larger individuals to mitigate predation pressure on fry, and the introduction of pilot schemes to investigate methods of reducing the Three-Spined Stickleback population.

Prior to the end of 2023, there was an annual closed fishing season which extended from November 1 to December 31.

The need for international efforts to manage the Great Cormorant subpopulation has also been highlighted, and monitoring campaigns designed to investigate changes in fish community composition over time are due to be established.