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.

Among these, C. pennantii is most closely related to C. clupeoides Lacépède 1803 from central Scotland and C. stigmaticus Regan 1908 from northwestern England. A number of recent studies have demonstrated that there is little morphological or genetic support for considering these as distinct species when they are compared with one another (Etheridge et al. 2012, Crotti et al. 2020).

In the United Kingdom, all three of these taxa are treated as junior synonyms of the congener Coregonus lavaretus (Linnaeus 1758), the name of which has since the mid-20^th century been applied to a widespread complex of polymorphic whitefish populations occurring across northwestern Eurasia (Crotti et al. 2021). However, the systematics of this putative assemblage and the native range of C. lavaretus remain questionable (see further discussion below).

C. pennantii is currently regarded as valid for global Red List purposes following Fricke et al. (2023), and the present assessment should therefore be considered equivalent to the most recent Welsh national assessment for C. lavaretus (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 Gwyniad is endemic to Llyn Tegid and Llyn Arenig Fawr (introduced for conservation purposes and included within this assessment), northwestern Wales, United Kingdom. It has an extremely restricted extent of occurrence (EOO) of c.27 km², which meets the thresholds for the Critically Endangered category under Criterion B1 (EOO < 100 km²). The area of occupancy (AOO) of c.8 km² is considered likely to be an under-estimate, and in the absence or a reliable figure for AOO, the EOO only is used here. The species is present at two locations, at one of which the quality of habitat is observed to be declining.

Therefore, this species is assessed as Endangered (EN) under Criterion B (B1ab(iii)). The species does not occur in the EU27 Member States.

This species is endemic to Llyn Tegid (en. Bala Lake) in the upper Dee River system, Gwynedd county, northwestern Wales, United Kingdom.

An additional subpopulation is established in nearby Llyn Arenig Fawr as a result of benign introduction. This subpopulation is included in the present assessment since the known intent of the introduction was to reduce the extinction risk of the taxon, the occupied site is geographically close to the natural range, viable offspring have been produced, and at least five years have passed since the translocation took place. However, Llyn Arenig Fawr could not be included on the range map accompanying this assessment due to its small size.

This species' population size is unknown, but it is understood to exceed the minimum threshold for Red List criteria (< 10,000 mature individuals). The current population trend has not been quantified.

The results of annual hydroacoustic surveys carried out at Llyn Tegid in most years from 2003-2014 indicate that although abundance was relatively high compared with other whitefish locations in the U.K., the demographic structure of the population was indicative of weak recruitment due to the relatively low proportion of age 0+ and 1+ individuals. Overall, the population was regarded as being in an "unfavourable" condition in all but one of the nine years in which the surveys took place.

However, samples obtained by gillnetting in 2003 indicated that the population comprises a relatively high number of year classes in apparently good condition. This discrepancy is possibly related to young individuals occupying the near-surface zone which could not be detected by vertical hydroacoustic techniques.

At the time of the last survey in 2014, the overall population inhabiting Llyn Tegid was estimated to comprise c. 135,000 individuals.

Naturally oligotrophic Llyn Tegid is the largest natural lake in Wales and has a maximum depth of 42 metres. Llyn Arenig Fawr is a small oligotrophic lake located c. 250 metres higher in altitude than Llyn Tegid and has a maximum depth of 35.5 metres. Both lakes were formed after the last glacial period.

The Gwyniad occupies benthopelagic habitats in deeper parts of the lakes but undertakes diel vertical migrations related to foraging during warmer months of the year. It feeds primarily on zoobenthos such as chironomid larvae and sphaeriid clams, with zooplankton comprising a significant proportion of the diet in summer and autumn. Foraging is generally less intensive from January to April. This species' generation length is unclear. The annual reproductive period extends from January to February when spawning takes place on shallow shoreline gravel banks.

Llyn Tegid is a highly managed system that has been controlled by sluice gates at its outflow since the onset of the 19^th century. Water levels in the lake are managed within the multi-purpose River Dee Regulation Scheme, which regulates flow in the river for flood alleviation, storage for downstream water supply and the provision of recreational facilities on the lake. During the 1950s, the sluice gates were replaced and the overall water level was reduced by c. 2 metres, but it is raised above the seasonal norm each summer in order to optimise conditions for water-based recreation activities. The sluice system also allows water from the adjacent Tryweryn River to be diverted into Llyn Tegid for flood protection or to boost storage.

These modifications have resulted in considerable alteration of the natural nutrient and sedimentation regimes, particularly since the 1980s. Agriculture in the Llyn Tegid basin has also intensified, with major rises in the density of livestock and the application of fertilisers and manure. The combined effects of nutrient enrichment through diffuse agricultural pollution, discharge of effluents from wastewater treatment works and increased sedimentation have resulted in encroaching eutrophication.

The Gwyniad is thus plausibly threatened by a reduction in habitat quality, with signals of profundal hypoxia detected in the lake during some summers. Additional threats include clogging and exposure of spawning sites due to sedimentation, and fluctuating water levels leading to the dewatering of spawning and incubation sites.

In addition, a series of blue-green algae (Cyanobacteria) blooms linked to increased nutrient concentrations have occurred since 1995.

A number of non-native fish species have been introduced to the lake, among which the Eurasian Ruffe (Gymnocephalus cernua) is of particular concern. First recorded during the early 1980s and abundant since the 1990s, it is an opportunistic, largely benthivorous predator that is believed to feed heavily on Gwyniad 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, although in Llyn Tegid there is currently no evidence of a long-term negative impact.

The invasive Australian Swamp Stonecrop (Crassula helmsii) has recently become established in Llyn Tegid. There is a significant risk that this non-native plant may threaten the resident Gwyniad subpopulation by smothering its spawning sites.

A small dam has been constructed on the outlet of Llyn Arenig Fawr in order to control the water level, but no immediate threats have been identified.

Warming of both Llyn Tegid and Llyn Arenig Fawr due to climate change represents a plausible ongoing and future threat. Warmer conditions may interfere with the development of early life stages, increase the duration of summer stratification events (thus aggravating the risk of deoxygenation both alone and in combination with the effects of eutrophication) and increase habitat suitability for non-native species.

This species is not 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 lavaretus) in Schedule 5 of the Wildlife and Countryside Act 1981, Schedule 3 of the Conservation (Natural Habitats, &c.) Regulations 1994, and is listed as a priority species of conservation concern in the U.K. Post-2010 Biodiversity Framework.

Llyn Tegid is located within the boundaries of Eryri (sometimes referred to as Snowdonia) National Park, while the lake is itself a Ramsar site under the Convention on Wetlands of International Importance (site no. 535).

Efforts to establish a refuge population resulted in 81,000 fertilised eggs being translocated to Llyn Arenig Fawr from 2005-2007, following a trial in 2003. Signs of reproduction were evident during subsequent surveys conducted in 2012.

Catchment Sensitive Farming schemes have reduced pollution in two of Llyn Tegid's small tributaries, but there is currently no long-term strategy in place to deal with the majority of diffuse pollution or sediment transport impacting the lake.

Population assessments using hydroacoustic techniques were carried out in most years from 2004-2014, and were accompanied by detailed reporting.

Gwyniad research has been largely abandoned since 2014, and the reinstatement of annual monitoring campaigns is strongly recommended in order to inform appropriate future management decisions.

The taxonomic status of British Coregonus species is in need of review, but there exist a number of complications (see 'Taxonomic Notes').