This species has been referred to as Coregonus sp. "Sempacherballen" or a subpopulation of the Bourget Whitefish (C. lavaretus) in some published literature.

It was formerly believed to occur in lakes Lucerne (fr. Lac des Quatre-Cantons; de. Vierwaldstättersee), Zug (fr. Lac de Zug; de. Zugersee), Alpnach (fr. Lac d’Alpnach; de. Alpnachersee), Baldegg (fr. Lac de Baldegg; de. Baldeggersee) and Hallwil (fr. Lac de Hallwil; de. Hallwilersee). These subpopulations have now been assigned to congeneric taxa with the exception of those formerly inhabiting lakes Baldegg and Hallwil, which remain unidentified in the wake of their extirpation (Selz and Seehausen 2023).

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: Vulnerable (VU)
EU 27 regional assessment: Not Recorded

The Sempach Whitefish has a restricted range (extent of occurrence (EOO) c. 15 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 climate change that could drive it to Critically Endangered or Extinct in a very short time period.

This species is endemic to Lake Sempach (fr. Lac de Sempach; de. Sempachersee) in the upper Rhine River system, central Switzerland.

Historical records suggest that this species was declining by the late 19^th century and became scarce at some point during the mid-to-late 20^th century. Its ongoing population size and trend have not been quantified, and the current population is probably composed entirely of hybrid individuals (see 'Threats').

Lake Sempach is a perialpine, naturally mesotrophic lake formed after the last glacial period. It has a maximum depth of 87 metres.

Little is known of this species' life history or ecology. The annual reproductive period is believed to occur during November, when spawning takes place in relatively shallow parts of the lake.

The 19^th century decline of native whitefishes in Lake Sempach has been attributed to artificial lowering of the water level.

There is evidence to suggest that the lake was also occupied by a small-bodied whitefish taxon until this period, which is referred to as Coregonus sp. "Bündeli" in some published literature. This may have comprised a second endemic species and was apparently displaced by the larger, originally less-abundant Sempach Whitefish following its purported extinction.

Records demonstrate that repeated introductions of whitefishes from Swiss, German and North American lakes to Lake Sempach took place in the late 19^th and early 20^th centuries, presumably leading to extensive hybridisation and introgression with the native species. Although naturally related to whitefish inhabiting Lake Hallwil (de. Hallwilersee), the extant Sempach population today exhibits marked genetic analogies with congeners from lakes Lucerne (fr. Lac des Quatre-Cantons; de. Vierwaldstättersee) and Zug (fr. Lac de Zug; de. Zugersee).

Discharge of urban waste water and agricultural runoff subsequently drove the lake to become mesotrophic around the mid-1960s and highly eutrophic by 1984. These changes conceivably gave rise to speciation reversal and extinction events in line with those observed in some other Swiss lakes.

As a result of these factors, it is unclear to what extent the genetic structure of the current whitefish population resembles that of the native Sempach Whitefish.

Warming of the lake due to climate change constitutes a plausible future threat.

Lake Sempach maintained a large and productive native whitefish fishery from the medieval period until the original population declined during the late 19^th century.

Supplementary stocking with whitefish larvae from various origins began in 1883, but yields remained low from the 1890s to the 1920s.

Landings have since recovered due to continued stocking efforts, but it is unclear which whitefish species are involved (see 'Threats').

This species is included (as Coregonus spp.) in Appendix III of the Bern Convention.

The hypolimnion of Lake Sempach has been artificially aerated since 1984, in order to mitigate anoxia during summer thermal stratification and support mixing throughout the water column in winter.

Investment in wastewater treatment facilities and the banning of phosphates in textile detergents have driven an improvement in water quality, but the lake currently remains in a eutrophic state.

Additional research to determine the extent to which the whitefish population currently inhabiting Lake Sempach can be considered to represent the original native species is strongly recommended, and is likely to require a comparison with DNA obtained from museum specimens or ancient sediment cores.