This taxon is currently regarded as valid for global Red List purposes, but was treated as a subpopulation of the Arctic charr, Salvelinus alpinus (Linnaeus 1758), for the most recent national assessment (Nunn et al. 2023).

The taxonomic status of Salvelinus subpopulations inhabiting the British Isles has not been definitively resolved since a series of endemic species were described between the mid-19^th and early 20^th centuries (Adams and Maitland 2007).

In the United Kingdom and Ireland, all of these taxa are treated as junior synonyms of the congeneric Arctic Charr, Salvelinus alpinus (Linnaeus 1758), the name of which has been routinely applied to a widespread complex of polymorphic charr populations occurring throughout the Holarctic region (Jonsson and Jonsson 2001). However, there remains considerable uncertainty regarding the systematics of the genus across the majority of this range (Reist et al. 2013, Taylor 2016, Whiteley et al. 2019).

Members of this "Arctic Charr complex" exhibit bewildering subpopulation-scale ecological and morphological variability. 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 and Maitland 2007, Kottelat and 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: Critically Endangered (CR)
EU 27 regional assessment: Not Recorded

Willoughby's Charr has an extremely restricted range (extent of occurrence (EOO) c. 35 km²), which meets the threshold for the Critically Endangered category under Criterion B1 (EOO < 100 km²). It is present at a single location where the quality of habitat is estimated to be declining and the number of mature individuals is inferred to be declining. Therefore, this species is assessed as Critically Endangered under Criterion B (B1ab(iii,v)).

This species is endemic to Lake Windermere in the Leven River system, northwestern England, United Kingdom.

Purportedly conspecific subpopulations have been reported from Lake Ennerdale Water plus a number of other lakes in northwestern England and Scotland. However, their identities have never been explicitly confirmed and they are not considered for the purposes of this assessment.

This species' population size and trend have not been explicitly quantified, but monitoring data have demonstrated a marked decline in abundance throughout Lake Windermere since the 1970s which has accelerated since the turn of the century.

The initial stages of this decline followed a population increase driven by greater productivity of the lake during the early stages of eutrophication in the 1950s and 1960s, coupled with a decrease in predation pressure due to the impact of fisheries on Northern Pike (Esox lucius) abundance (see 'Threats').

Catch-per-unit-effort data obtained from fishers suggests that abundance has today returned to a level comparable with the 1940s, when the lake was still of relatively pristine ecological status but the Willoughby's Charr population was significantly depressed due to overfishing (see 'Use and Trade').

The decline has been most evident in the lake's south basin, where the negative impact of the identified threats has been considerably more pronounced (see 'Threats').

There is evidence to suggest that individuals spawning in spring (see 'Habitat and Ecology') naturally comprise only c. 15% of the total population, and that they may be declining more rapidly than those spawning in autumn.

Windermere is the largest natural freshwater lake in England and was formed after the last glacial period. It has a maximum depth of 64 metres and two main basins comprising a deeper north basin and shallower south basin, which are separated by a shallow sill with an average depth of c. 10 metres.

The lake is naturally classified as polymictic and oligotrophic, but it is presently monomictic with the north basin mesotrophic and the south basin eutrophic (see 'Threats'). It becomes stratified from April onwards, with remixing tending to take place in late October or early November in the south basin and November or December in the north basin.

It is exploited as a municipal water supply for Northwest England, and is host to a thriving leisure industry with c. 16 million tourists visiting the surrounding national park each year. There is extensive sheep grazing on uplands located within the catchment. The lake level is partially controlled by a weir at the outlet which disrupts natural drainage processes to a limited extent.

For the majority of the year, Willoughby's Charr occupies the pelagic zone at most available depths, tending to avoid the upper few metres of the water column. Maximum lifespan appears to be 9-10 years and sexual maturity is reached at age 5-6+.

It exhibits two annual reproductive periods, with part of the population spawning in late autumn (mainly November) and the remainder in spring (mainly February to March). Both the north and south basins are inhabited by autumn- and spring- spawning individuals, while a further subset of autumn-spawners migrates a short distance into the inflowing Brathay River. Nuptial individuals develop an intense epigamic colour pattern.

Spawning takes place at specific littoral and sublittoral sites comprising patches of coarse, well-washed stony substrata with well-oxygenated interstitial spaces. Individuals spawning in autumn utilise sites located at shallower depths than those in spring, but recent studies suggest that only a handful of suitable deeper sites remain (see 'Threats').

The eggs of autumn-spawning individuals hatch at the beginning of March, and those of spring-spawners hatch around the first week of May.

Genetic differentiation between autumn- and spring-spawning stocks has been detected in both basins, but is more pronounced in the south basin. There is in addition a small but significant difference between the two basins themselves, which is further supported by contrasting growth rates, parasite loads and historical evidence that little inter-basin movement occurred prior to eutrophication (see 'Threats').

This species feeds predominantly on zooplankton, but switches to benthic invertebrates from December to April. It appears to undertake diel vertical migrations related to planktivorous foraging during warmer months of the year.

Studies carried out in the mid- to late-20^th century demonstrated evidence of trophic polymorphism in that the spring-spawning stock possessed a greater number of more slender gill rakers than the autumn-spawning stock. These traits are suggestive of a specialisation on zooplankton, and led to the conclusion that spring-spawners were more likely to consume pelagic prey and autumn-spawners benthic prey.

Recent analyses have shown that some individuals possess a comparatively slender head and body shape indicative of pelagic foraging, whereas others have a more robust morphology consistent with benthic feeding. These differences were more evident in the south basin, with all fish in the north basin tending towards pelagic morphology. However, there was no correlation between spawning time and phenotype, with autumn-spawning females and spring-spawning males both possessing a pelagic body shape. Moreover, there is currently no significant difference in gill raker morphology between the different stocks.

These latter morphological changes appear to have evolved within just a few decades and are potentially reflective of deteriorating environmental conditions (see 'Threats'), which may be driving a shift towards benthivory and/or a reversal of fomerly differentiating factors between the two spawning stocks in the south basin.

This species' decline has been linked to a combination of ongoing threats, the impacts of which are more pronounced in Lake Windermere's south basin.

The trophic status of the lake increased gradually from the mid-19^th century due to an expanding human population, increased discharge of domestic wastewater and changes in agricultural practices, with a marked acceleration between the 1940s and 1980s.

Eutrophication of the south basin has led to profundal anoxia during late summer and autumn, and a significant reduction in the extent of suitable charr habitat for several months per year. Extensive deoxygenation also occurs in the north basin, although fully anoxic conditions have not been reported. These processes are ongoing despite the introduction of measures to mitigate nutrient enrichment (see 'Conservation'), and the lack of recovery may be related to the delayed release of phosphorous accumulated in sediments, and/or high levels of nitrogen in treated wastewater being discharged to the lake. In addition, raw sewage continues to be discharged into the lake from storm spillage overflows during periods of heavy rainfall.

Sedimentation is known to be a key driver of reproductive failure in charr, and recent surveys have revealed that a number of known spawning sites in the lake are now completely covered by layers of fine sediment. These are likely to be of algal origin and associated with the eutrophication process, and accumulation appears to have been greatest at deeper sites favoured by spring-spawning individuals.

Warming of the lake due to climate change has been identified as a major ongoing and future threat. Average water temperatures between 2017-2021 were almost a degree greater than during the period 1981-2010, and this appears to be driving increasingly long periods of summer stratification. Warmer temperatures may also hamper the development of charr eggs and other early life stages while favouring the abundance of potentially competitive fish species (see below).

A rise in abundance of the piscivorous Northern Pike after the 1970s may have contributed to the initial decline of Willoughby's Charr. However, predation pressure has decreased since the mid-1990s due to increasing abundance of the non-native Eurasian Perch (Perca fluviatilis) and Common Roach (Rutilus rutilus), which is in itself a cause for concern. The Common Roach has been present since the early 1900s, when it was probably introduced by anglers live-baiting for Northern Pike. It remained relatively scarce until the 1990s, when a significant increase linked to rising water temperatures occurred. This species now comprises an important component of the littoral and epilimnetic fish communities, and it is believed to compete with Willoughby's Charr for zooplankton resources.

The abundance of non-native Common Bream (Abramis brama) is also increasing, while presence of the benthic Eurasian Ruffe (Gymnocephalus cernua), which is known to prey extensively on the eggs of other fish species, was confirmed in 2019.

This species was harvested from at least the early 13^th century, and a commercial fishery utilising seine nets was established around 1660. However, by the mid-19^th century landings had become unsustainable and the stock crashed at a time when it was considered a high-value delicacy and typically sold potted with spices and clarified butter. This decline led to a change in regulations, and the last commercial netting took place in 1921.

From the 1840s, an alternative plumb-line technique utilising a pair of purpose-built rods attached to a rowing boat was introduced. This unique approach is today considered an important aspect of local culture and is still employed by a small number of fishers, with a negligible-impact regulated fishery running from March 15 to September 30 each year. The minimum size limit is 20 centimetres, and a few of the landed individuals are sold to local restaurants.

Willoughby's Charr is included (as Salvelinus alpinus) as a priority species of conservation concern in the U.K. Post-2010 Biodiversity Framework.

Lake Windermere is located within the Lake District National Park protected area, and in 2017 the presence of charr in several lakes contributed to the area becoming a UNESCO World Heritage site. A stretch of the lake's northwestern shore has been designated a Site of Special Scientific Interest.

The use of any freshwater fish species as live or dead bait for angling is now prohibited in order to prevent further introductions of non-native taxa.

Windermere is perhaps the most comprehensively-studied natural lake in the U.K., with collection of biological and physiochemical data ongoing since the 1940s. The population dynamics of Willoughby's Charr have been analysed by gill netting since 1939, collection of fishery catch-per-unit-effort data since 1966 and hydroacoustic surveys since 1989, while the use of environmental DNA has recently been explored.

An attempt to establish a "refuge" subpopulation took place from 1989-1991, when hatchery-reared fingerlings obtained by stripping eggs from wild autumn-spawning individuals were introduced to the artificial Grimwith Reservoir in the Trent River system, Yorkshire. Subsequent monitoring efforts indicated that some of these individuals remained present until at least 1998, but recent surveys have shown that the site is no longer occupied.

Actions taken to address eutrophication in Lake Windermere include the stripping of phosphates from wastewater discharges since 1991-1992, but this has only been partially successful (see 'Threats'). In early 2023 the company responsible for water management in North West England announced a £19 million investment to reduce storm-driven sewage spills from wastewater facilities discharging into the lake, but warned that they would not be eliminated entirely.

Willoughby's Charr is viewed as an important symbol of local culture, tourism and ecosystem health. For example, from 2010-2014 it featured prominently within the U.K. Heritage Lottery-funded project 'Windermere Reflections', which sought to conserve and enhance the Windermere catchment through a series of short-term actions, and by promoting longer-term attitudinal and behavioural changes in residents, businesses and visitors, encouraging engagement and a sense of responsibility through interpretation, education and training. The species is highlighted in exhibits at local museums, a sculpture is on permanent display in the tourist centre of Ambleside, and it has been included in numerous books, media articles and promotional materials focussed on the Lake District National Park. In 2017, a 12-minute film centred on Windermere's charr fishery was released.

This species is not currently recognised by the relevant authorities in England or the United Kingdom, where it is treated as a subpopulation of the Arctic Charr (Salvelinus alpinus). 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.