An unidentified and potentially conspecific whitefish subpopulation inhabits adjacent Lake Mond (de. Mondsee). It is known to reproduce one month earlier than C. atterensis, although individuals displaying this behaviour were recorded in Lake Atter following translocation during the early 20^th century.

They do not appear to have become established, and the taxonomic status of the Lake Mond subpopulation remains in need of clarification.

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).

The Atter Whitefish is endemic to Europe, where it is restricted to Lake Atter (de. Attersee) in Austria. The species has a restricted extent of occurrence (EOO) of c. 66 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.

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

This species is endemic to Lake Atter (de. Attersee) in the Salzkammergut region, upper Danube River system, Austria.

This species' population size and trend have not been quantified, and any accurate estimation is likely to be complicated due to current management practices (see 'Use and Trade').

Lake Atter is a perialpine, oligotrophic lake formed after the last glacial period. It has a maximum depth of 169 metres, and is connected to adjacent Lake Mond (de. Mondsee) via the inflowing Seeache River.

Current knowledge suggests that the Atter Whitefish predominantly occupies shallow to midwater (c. 10-30 metres depth) habitats in the pelagic zone, where it feeds on zooplankton and occasionally descends to forage for benthic prey.

Maximum lifespan is c. 8 years and individuals become sexually mature at age 2-3+. The annual reproductive period extends from early February to mid-March, when spawning occurs in open water at depths of c. 1-40 metres.

This species is plausibly threatened by resource competition and/or introgressive hybridisation with non-native whitefish from the Baltic region, that have been routinely introduced to Lake Atter for decades (see 'Use and Trade').

These individuals derive from a commercial strain of the Maraena Whitefish (Coregonus maraena) developed in the Czech Republic during the 19^th century, which has been reared in Austrian hatcheries and used for stocking the lakes of the Salzkammergut region under the local name "Maraene" since the 1950s. This strain is possibly introgressed with the North American Lake Whitefish (Coregonus clupeaformis). Analyses carried out in nearby Lake Mond (see 'Taxonomic Notes') have demonstrated the existence of unquantified levels of introgression between native and non-native whitefishes, but also confirmed persistence of the native lineage. This has been attributed to partial reproductive isolation, since the native species mostly spawns c. one month later than the introduced taxon. Moreover, commercial fishers and fisheries managers operating in the Salzkammergut lakes draw distinguish the two taxa by the source of stocking plus aspects of morphology such as body shape.

However, no separation is made between whitefishes that are native to the Salzkammergut region but putatively endemic to individual lake systems, which in addition to those occurring in lakes Atter and Mond include the Lake Traun Whitefish (Coregonus danneri) plus lineages inhabiting lakes Fuschl (de. Fuschlsee), Wolfgang (de. Wolfgangsee) and Hallstatt (de. Hallstätter See). Some or all of these are reared by a number of local hatcheries and referred to collectively as "Coregonus sp.", "Salzkammergut Reinanke" or "Renken". The possible translocation, hybridisation and introgression of these lineages has not been fully-explored.

There is a long-established commercial fishery targeting the Atter Whitefish.

Other non-native fish species, e.g., European Eel (Anguilla anguilla), have also been stocked in Lake Atter. The potential negative effects on native fishes, such as resource competition, predation or transmission of pathogens, have not been extensively investigated, although there is preliminary evidence that stocks of small-bodied fish species have declined.

Additional threats include water level regulations, degradation of littoral habitats for urban development, and shifts in the temperature regime of Lake Atter due to climate change.

The lake became increasingly eutrophic due to human activities during the mid-20^th century. This process may have exerted an effect on whitefish population structure and selection, but is not currently considered to represent a threat (see 'Conservation').

Whitefishes comprise an important cultural element in the Salzkammergut region, where they are widely served in restaurants and marketed fresh, frozen or smoked as "Salzkammergut Reinanke".

A long-established commercial fishery targeting the Atter Whitefish operates from April to October each year, during which period it is also fished intensively by recreational anglers.

Large numbers of hatchery-reared, non-native juvenile whitefish are stocked on an annual basis, e.g., c. 1.5-2 million individuals per year from 2020-2022, in order to improve the commercial yield and make the lake more attractive to anglers (see 'Threats'). These are not harvested immediately, with fishers employing gear designed to capture age 3+ individuals with a minimum size limit of 37 centimetres. There is also a closed fishing season that runs from November 1 to December 31 each year.

This species is included (as Coregonus spp.) in Appendix III of the Bern Convention and Annex V of the European Union Habitats Directive. Lake Atter is located entirely within the boundaries of a protected area that is included in the European Union's Natura 2000 network (site AT3117000).

Investment in wastewater treatment facilities since the 1970s has driven an improvement in the water quality of Lake Atter, which has returned to its former oligotrophic state. Regular monitoring efforts are carried out by the Institute for Aquatic Ecology and Fisheries Management, located at Scharfling near Lake Mond.

This species is not well understood, and research investigating its abundance, population trend, life history and response to the ongoing threats is strongly recommended. In particular, understanding the extent of introgression with introduced whitefish lineages should be considered a priority in order to establish the need for conservation actions.

The taxonomic status of the whitefish subpopulation inhabiting Lake Mond is also in need of clarification.