British whitefishes
The taxonomic status of Coregonus subpopulations inhabiting the British Isles has not been definitively resolved since a series of endemic species were described between the early 19^th and early 20^th centuries.

In the United Kingdom, C. vandesius is treated as a junior synonym of the congener Coregonus albula (Linnaeus 1758), the name of which has since the mid-20^th century been applied to a widespread complex of polymorphic cisco populations occurring across northwestern Eurasia (Mehner et al. 2021). However, the systematics of this putative assemblage and the native range of C. albula remain questionable (see further discussion below).

C. vandesius is currently regarded as valid for global Red List purposes following Fricke et al. (2024), and the present global assessment should be considered equivalent to the most recent English and Scottish national assessments for C. albula (Nunn et al. 2023).

European ciscoes and whitefishes
At the regional scale, the systematics of European ciscoes and whitefishes (Coregonus spp.) has also 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 evolved in some perialpine lakes, and these radiations might be even more diverse in a few 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.

With the above in mind, it appears unlikely that the striking phenotypic diversity exhibited by members of this genus will ever be adequately represented by a single accepted taxonomic system. The Red List currently follows the nomenclature provided by Fricke et al. (2024).

Global and European regional assessment: Endangered (EN)
EU 27 regional assessment: Not Recorded

The Vendace has a restricted range (extent of occurrence (EOO) c. 2,965 km², area of occupancy (AOO) c. 28 km²), which meets the thresholds for the Endangered category under Criterion B1 (EOO < 5,000 km²) and Criterion B2 (AOO < 500 km²). It is present at five severely-fragmented locations, and the quality of habitat is observed to be declining.

Therefore, this species is assessed as Endangered under Criterion B (B1ab(iii)+2ab(iii)).

This species is native to the United Kingdom, where it inhabits Bassenthwaite Lake and Derwent Water in the Derwent River system, northwestern England, and was formerly present in Castle Loch and Mill Loch in the Annan River system, southern Scotland.

Elsewhere in Scotland, introduced subpopulations are established at Loch Earn in the Earn River system, Loch Skeen in the upper reaches of the Annan River system and Daer Reservoir in the upper Clyde River system. These subpopulations are included in the present assessment, since the known intent of the introductions was to reduce the extinction risk of the taxon, the occupied sites are geographically close to the natural range, viable offspring have been produced, and at least five years have passed since the translocations took place.

With the exception of Loch Earn, the small size of the Scottish sites precludes their accurate depiction on the range map accompanying this assessment.

This species' current population size and trend have not been quantified.

Overall abundance is understood to have declined significantly during the 20^th century, and monitoring data compiled since the 1990s suggests that Derwent Water contains the largest extant stock.

The Castle Loch subpopulation was extirpated during the 1920s, and the Mill Loch subpopulation at some point between 1966 and 1975.

The Bassenthwaite Lake subpopulation declined significantly during the late 20^th century, and no individuals were observed between 2000-2012. However, in 2013 a single young fish was captured during a routine survey and a small number of additional individuals have since been recorded. It remains unclear whether these comprise a surviving remnant of the original subpopulation or at some point recolonised the lake from Derwent Water via the Derwent River. Ongoing reproduction in Bassenthwaite Lake has not been confirmed.

The translocated introduced subpopulations inhabiting Loch Earn, Loch Skeen and Daer Reservoir are currently understood to be self-sustaining.

Bassenthwaite Lake and Derwent Water are naturally mesotrophic, monomictic lakes formed after the last glacial period, and earlier comprised a single larger basin. Both are relatively shallow (mean depth < 6 metres) but contain some deeper areas, and they remain connected by a stretch of the Derwent River.

Bassenthwaite Lake has a maximum depth of 21 metres, and is largely surrounded by upland moor and improved pasture with smaller areas of rough grazing, bare rock and plantation forest. It has a very short average retention time of < 30 days and the water level can fluctuate by up to two metres during winter flood events. Thermal stratification during summer is weak due to the lake's shallow depth and exposed location, with episodic mixing events increasing oxygen concentrations at depth. The town of Keswick is located four kilometres upstream, and wastewater from its sewage treatment plant is discharged into the Derwent River prior to it draining into the lake.

Derwent Water has a maximum depth of 22 metres, and is also characterised by weak thermal stratification and susceptibility to wind-driven mixing. It is located upstream of the sewage treatment plant at Keswick and mostly surrounded by steep slopes and woodland with relatively little agriculture.

The former locations Castle Loch (maximum depth 3.4 metres) and Mill Loch (maximum depth 18 metres) are separated by just a few hundred metres and comprise remnants of a historically larger lake system. They remain in a eutrophic state, have been stocked with non-native fish and aquatic plant species, and support recreational fisheries.

The Vendace is gregarious and forms schools in the pelagic zone, where it predominantly feeds on zooplankton supplemented by smaller amounts of terrestrial insects and aquatic macroinvertebrates. It inhabits comparatively cool, deep (c. 20 metres) habitats, but undertakes diel vertical migrations related to foraging during warmer months of the year. Maximum reported lifespan is 5-10, normally 5-6, years, and individuals become sexually mature at age 2-3+. The annual reproductive period extends from late November to late December, when spawning takes place on well-washed gravel or sand banks in shallow littoral or sublittoral habitats. The eggs hatch in spring and appear to require consistently cold water (< 10°C) in order to complete their development.

Bassenthwaite Lake became increasingly eutrophic during the 20^th century, with particularly severe increases in phosphorous load during the late 1970s and early 1980s. The primary sources of nutrient enrichment were discharge of wastewater from the town of Keswick, plus three smaller sewage treatment plants and agricultural activities elsewhere in the catchment. Mitigation measures implemented since the mid-1990s (see 'Conservation') have improved the lake's trophic status, and it is currently classified as mesotrophic. However, diffuse pollution and wind-driven recycling of phosphorous stored in the sediment remain problematic.

These conditions have driven a reduction in the extent and quality of available habitat through periodic episodes of anoxia in the hypolimnion, leading to a loss of cool deepwater refugia, and excessive algal growth at spawning sites. It is plausible that alterations in zooplankton community structure have also occurred.

Moreover, increased siltation of spawning sites during episodic periods of high flow in the lake's tributaries has been driven by the erosion of surrounding uplands due to changes in land use and farming practices, channel erosion of the Derwent River, and resuspension during mixing events.

The warming of Bassenthwaite Lake due to climate change may already be exacerbating the effects of the above issues, and is expected to exert an increasingly significant influence in the future.

A number of non-native fish species have been introduced to the lake since the late 1980s, among which the Eurasian Ruffe (Gymnocephalus cernua) is of particular concern. It is an opportunistic, largely benthivorous predator that is believed to feed heavily on Vendace eggs during winter and competes for zooplankton resources in summer. This species has been strongly linked to the purported decline of the Powan (Coregonus clupeoides) in Loch Lomond, Scotland, and has become increasingly dominant in Bassenthwaite Lake.

The non-native Common Roach (Rutilus rutilus) and Common Dace (Leuciscus leuciscus) are also present in the lake, with an abundance of the former in particular being favoured by eutrophic conditions and plausibly resulting in increased competition for zooplankton resources.

The Australian Swamp Stonecrop (Crassula helmsii) is also established at Bassenthwaite Lake. This invasive, non-native plant is believed to reduce the quality of Vendace spawning habitat through smothering, since it tends to grow densely in sublittoral zones and does not die back in winter.

Derwent Water has received less nutrient enrichment than Bassenthwaite Lake and has therefore retained its mesotrophic status (see 'Habitat and Ecology'). However, Common Roach, Eurasian Ruffe and extensive beds of Australian Swamp Stonecrop are established in the lake, where they are considered to represent the most significant threat to the resident Vendace subpopulation.

Climate change-induced warming of Derwent Water constitutes a plausible ongoing and future threat since it may lead to periods within which no suitable deepwater refuge habitat remains.

In Scotland, the Castle Loch subpopulation is understood to have been extirpated due to eutrophication after 1911, when a newly-constructed sewage treatment plant began discharging wastewater into the lake.

Extirpation of the Mill Loch subpopulation is believed to have been driven by diffuse pollution from surrounding agriculture, although it is possible that the presence of non-native fish species also played a role.

This species was harvested in Scotland until around the turn of the 20^th century, but is no longer used or traded.

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

At the national scale, it is included (as Coregonus albula) in Schedule 5 of the Wildlife and Countryside Act 1981, which prohibits unlicenced capture, killing or captive maintenance. It is also listed in Schedule 3 of the Conservation (Natural Habitats, &c.) Regulations 1994, and is included as a priority species of conservation concern in both the U.K. Post-2010 Biodiversity Framework and the Scottish Biodiversity List.

The entire extant native range is located within the Lake District National Park protected area, which is also a UNESCO World Heritage Site. Bassenthwaite Lake has additionally been designated a National Nature Reserve, while Derwent Water is included in the 'River Derwent and Tributaries' Site of Special Scientific Interest and Special Area of Conservation.

In response to concerns over the long-term viability of the Bassenthwaite Lake subpopulation (see 'Threats'), a conservation and restoration programme was initiated in 1986. The first action was to establish a "refuge" location for this subpopulation and reestablish the species in Scotland. In 1988, fertilised eggs were transported from Bassenthwaite Lake to purpose-built hatcheries, and in 1989 a total of 8,379 unfed fry were introduced to Loch Earn. The Vendace was thereafter not recorded at the site until 2005, when a single individual was caught by a private angler. A second individual was reported in 2012, and a third was obtained during surveys carried out in 2016, with the ages of the fish suggesting that a small subpopulation is established and has survived for the equivalent of nine generations.

Since 1996, the conservation programme has been administered by NatureScot (formally Scottish Natural Heritage), Natural England and the English Environment Agency via the U.K. Biodiversity Action Plan Steering Group (Vendace), and has included efforts to establish three additional refuge sites in Scotland plus one in England.

Among the Scottish sites, 17,550 fry and 47,500 eyed eggs were introduced to Loch Skeen from Bassenthwaite Lake between 1997-1999, and a self-sustaining subpopulation subsequently became established there.

A total of 25 adult individuals, 12,800 unfed fry and 32,300 eggs were transferred from Derwent Water to the Daer Reservoir in 1998, 2005 and 2008. A single, recently-spawned female individual was captured during surveys in 2017, indicating successful reproduction has taken place for at least three generations.

In 2011, 70,000 eyed eggs were collected from Derwent Water and introduced to Loch Valley in the Cree River system. No individuals were recorded during surveys in 2017, but additional monitoring has been proposed. A failed attempt to translocate Vendace eggs from Mill Loch to this site took place in 1968, but water chemistry at the time is believed to have been excessively acidic.

In England, c. 134,000 eggs were in late 2005 transferred to Sprinkling Tarn in the upper Derwent River system, but no individuals were recorded during subsequent surveys.

Reintroduction of the Vendace to Castle Loch or Mill Loch is not considered viable due to their ongoing eutrophic status and abundant stocks of non-native fish species.

The Bassenthwaite Lake and Derwent Water subpopulations have been regularly monitored by a combination of gill netting and hydroacoustics since 1995 and 1998, respectively.

Improvements to the sewage treatment facilities in Keswick since 1994-1995 have led to a modest decline of phosphorus concentrations in Bassenthwaite Lake, and recovery of the system's former mesotrophic status is predicted to be slow (see 'Threats'). Efforts to reduce sedimentation through the reduction of grazing pressure and reforestation of steep surrounding slopes are also in place.

The use of in situ artificial spawning substrata was trialed at Bassenthwaite Lake during the late 1990s, but subsequently abandoned due to excessive levels of sedimentation during winter.

The use of both live- and dead-bait for angling has been prohibited at Bassenthwaite Lake and Derwent Water since 2002, and no further introductions of non-native fish species are known to have occurred at either location.