Lectotype. MHNG-809.059, Switzerland, Lake Thun (46°40'N, 7°46'E), 154.5 mm SL, sex unknown.

Non-types. NMBE-1077133, NMBE-1077180–1077185, NMBE-1077135– 1077157, Switzerland, Lake Thun (46°40'N, 7°46'E), N = 30, 191–288 mm SL; NMBE-1077342, NMBE-1077291–1077317, NMBE-1077266, NMBE-1077267, Switzerland, Lake Brienz (46°43'N, 7°57'E), N = 30, 132–244 mm SL.

Coregonus fatioi is a medium-sized whitefish with weak pigmentation of all fins and body; light to dark green colour on the flanks above the lateral line; moderate to many pigmented small dots on the scales along the flank and the dorsum; slender, elongated and slightly torpedo-like body; long head; tip of snout is fleshy and roundish; small eye with a thin and triangular to roundish eye socket for individuals from Lake Thun and a thick and triangular shaped eye socket for individuals from Lake Brienz; many and long gill rakers.

Differential diagnoses against C. albellus and C. alpinus are given under those species’ accounts.

Coregonus fatioi-Coregonus brienzii

In Brienz C. fatioi can be differentiated from C. brienzii by being deeper bodied (22.1–26.2% SL, mean = 23.9 vs. 19.6–25.1% SL, mean = 22.6) and having a smaller eye (eye depth: 21.2–27.6% HL, mean = 24.8 vs. 23.1–28.3% SL, mean = 25.3) (Tables 5, 7, 11).

Coregonus fatioi-Coregonus steinmanni

The specimens of C. fatioi from Lake Thun differ from those of C. steinmanni by having longer gill rakers (middle gill raker length: 12.5–21.3% HL, mean = 15.8 vs. 9.1–14.3% HL, mean = 11.5; longest gill raker length: 12.8–22.6% HL, mean = 16.9 vs. 10–14.4% HL, mean = 12.1), a longer and wider underjaw (under jaw length: 28–34.1% HL, mean = 30.5 vs. 25.2–30% HL, mean = 27.3; under jaw width: 21–30.3% HL, mean = 24.7 vs. 19.3–25% HL, mean = 23). Based on ratios C. fatioi can be differentiated from C. steinmanni by having a smaller "caudal peduncle depth / upper jaw length" ratio (CD/UJ: 1.02–1.34 vs. 1.36–1.55) and "caudal peduncle depth / postdorsal length" ratio (CD/PostD: 0.14–0.17 vs. 0.17–0.20 (Tables 5, 6, 10).

Coregonus fatioi–Coregonus profundus

Coregonus fatioi from Lake Thun can be distinguished from C. profundus by having more and longer gill rakers (upper arch gill raker number: 10– 16, mode = 14 vs. 5–10, mode = 9; lower arch gill raker number: 22– 27, mode = 24 vs. 10–18, mode = 14; total gill raker number: 32–43, mode = 38 vs. 15–27, mode = 21; middle gill raker length: 12.5– 21.3% HL, mean = 15.8 vs. 7.6–11.7% HL, mean = 9.2; longest gill raker length: 12.8–22.6% HL, mean = 16.9 vs. 7.8–12.4% HL, mean = 10.1), shorter pectoral fin (pectoral fin 1 length: 13.3–18.9% SL, mean = 16.5 vs. 16.6–21% SL, mean = 18.4; pectoral fin 2 length: 13.8–20.6% SL, mean = 17.7 vs. 17.7–23.2% SL, mean = 20.2), a shorter head (13.6–16.2% SL, mean = 14.8 vs. 15.5–18.4% SL, mean = 16.4), a longer postdorsal length (41.6–50.7% SL, mean = 44.9 vs. 38.9–44.5% SL, mean = 42.5), and a longer upper jaw (28–34.1% HL, mean = 30.5 vs. 26.4–30.6% HL, mean = 28.7) (Tables 5, 8, Suppl. material 1: Table S6).

Coregonus fatioi-Coregonus acrinasus

Coregonus fatioi can be distinguished from C. acrinasus by having a longer postdorsal length (41.6–50.7% SL, mean = 44.9 vs. 40.3– 45.6% SL, mean = 43) and longer gill rakers (middle gill raker length: 12.5–21.3% HL, mean = 15.8 vs. 9.1–16.6% HL, mean = 13.4; longest gill raker length: 12.8–22.6% HL, mean = 16.9 vs. 11.4–16.9% HL, mean = 14.5) (Tables 5, 9).

General appearance is shown in Figure 6. Morphological and meristic characters of both sexes can be found in Table 5 and Suppl. material 1: Table S6 and first- and second-best ratios for both sexes combined can be found in Tables 10, 11. The description is valid for both sexes and both lakes; differences between the populations of lakes Thun and Brienz are mentioned.

Shape: Elongated. Slender bodied with greatest body depth anterior of the dorsal fin resulting in a slightly torpedo-like form. Dorsal and ventral profile similar and slightly arched. Dorsal and ventral profile from tip of snout to interorbital area mostly straight and then slightly convex to dorsal and pelvic fin origin respectively. Head long. Very rarely does the snout have an approx. 40–50° angle to the body axis anterior of the eye, such that the profile from the tip of the snout to the vertical projection where the anterior part of the eye crosses the dorsal profile is straight and afterwards slightly convex. Mouth thick (i.e., width of upper and lower jaw), long and often terminal and only rarely slightly sub-terminal. Snout mostly wider than deep, not strongly pronounced, since the tip of the snout is often fleshy and roundish. Specimens from Lake Thun have a thin, roundish and rarely triangular shaped eye-socket, whereas specimens from Lake Brienz have an eye-socket that is thick and triangular (i.e., sickle-shaped). Pectoral fin moderately tapered. Anterior unbranched ray of the erected dorsal fin ranges from almost vertically straight to an approx. 60–80° angle to body axis and only bent slightly posteriorly at the end of the ray. Caudal peduncle narrow and elongated with caudal fin forked in specimens from both lakes and sometimes moderately asymmetrical (mostly the ventral part is longer) in specimens from Lake Thun but very rarely in specimens from Lake Brienz. Unbranched ray of anal fin straight and rarely bent posteriorly at the end of the ray. Anal fin longest anteriorly and progressively shortening posteriorly with the outer margin of the anal fin ranging from being straight to slightly concave.

Meristics : Specimens of Lake Thun have many and long gill rakers, whereas specimens from Lake Brienz have a bit less and moderately long gill rakers.

Colour: Pigmentation of fins and body overall weak in live specimens. In specimens from Lake Thun the pectoral fin is translucent, sometimes yellowish with faint pigmentation at the median to distal parts of the fin. In Thun the pelvic fin ranges from completely translucent to moderately pigmented and the dorsal, adipose, anal and caudal fins are moderately pigmented. Specimens from Lake Brienz have a fully translucent pectoral fin that sometimes has a faint pigmentation on the unbranched ray. Pelvic and anal fins range from fully transparent to moderately pigmented and dorsal, adipose and caudal fins are moderately pigmented. In both lakes fish have a silvery appearance along the flanks. Specimens from both lakes sometimes have many pigmented small dots on the scales along the flank and the dorsum, which is rare in specimens from Lake Thun and common in specimens from Lake Brienz. Distribution of the dots is bound to the scale patterning such that the dots are found at the edge of the scales or at the boundary point of two scales (as can be found for the species of C. alpinus, C. brienzii and C. steinmanni). Colouration on the dorsum above the lateral line of specimens from Lake Thun ranges from a light green colouration (e.g., RGB (136, 245, 205)) to an olive-green colouration (e.g., RGB (176, 192, 125)), where the former is more common. In specimens from Lake Brienz the upper dorsum is light greenish in colouration (e.g., RGB (136, 245, 205)). For a comparison to the main colouration found in the other species see Suppl. material 1: Figure S8. The dorsal part of the head of specimens of Lake Brienz is weakly pigmented, whereas that of specimens from Lake Thun is moderately pigmented. The snout around the nostrils is weakly (Lake Brienz) to moderately (Lake Thun) pigmented with a gap of very weak (Brienz) to moderate (Thun) pigmentation posteriorly of the nostrils up to the height of the middle of the eyes. Operculum and pre-operculum are silvery with one black dot on the lower margin of the pre-operculum. Preserved specimens are pale in colouration with similar pigmentation as described for live specimens. Silvery, translucent, not coloured or unpigmented parts of the body become brown-yellowish (e.g., RGB (239, 210, 40)), whereas the pigmented parts are conserved and the coloured parts (dorsally above the lateral line) become brownish (e.g., RGB (186, 140, 100)).

Coregonus fatioi is found in the lakes Thun (46°40'N, 7°46'E) and Brienz (46°43'N, 7°57'E) that are connected through the river Aare at Interlaken. Based on isotopic signatures C. fatioi feeds predominantly on zooplankton (Selz 2008; Hudson 2011; Ingram et al. 2012). Stomach content analyses of specimens of C. fatioi from Lake Brienz suggest that C. fatioi feeds on a mix of zooplankton and benthic prey (Maurer and Guthruf 2005; Müller et al. 2007). Coregonus fatioi has a moderately fast growth rate (Both lakes: Kirchhofer 1995; Bittner et al. unpublished; Lake Brienz: Müller et al. 2007). It has to be noted that the work by Kirchhofer (1995), Maurer and Guthruf (2005) and Müller et al. (2007) did not distinguish between all species in lakes Thun or Brienz and thus lumped different species together into few groups. Maurer and Guthruf (2005) and Müller et al. (2007) differentiated between "small-type" and "large-type" whitefish based on cohort-specific threshold values for length-at-age. Based on morphology and ecology Kirchhofer (1995) differentiated in Lake Thun between "Albock" (comprising most likely of C. alpinus, C. steinmanni and C. acrinasus), "Brienzlig" (comprising most likely of C. albellus and C. fatioi) and "Kropfer" (C. profundus) and in Lake Brienz between "Felchen" (comprising most likely of C. alpinus, C. fatioi and C. brienzii) and "Brienzlig" and "Winter-Brienzlig" (comprising of summer- and winter-spawning specimens of C. albellus). The gill raker number and length of C. fatioi (many and long gill rakers) suggests, based on the functional properties of the number of gill rakers (Lundsgaard-Hansen et al. 2013; Roesch et al. 2013), that C. fatioi feeds more on zooplankton and less on benthic prey. However, this assumption needs to be verified with stomach content analyses that distinguish between the different species within a lake. Habitat stratified random sampling of Lake Thun (mid-October 2013; Vonlanthen et al. 2015) and Brienz (mid-September 2011; Vonlanthen et al. 2013) shows, that C. fatioi occupies the moderately shallow (Brienz: approx. 1–48 m, N = 9; Thun: approx. 25–140 m, N = 4) to the deepest waters of the benthic habitat in both lakes (down to 217 m and 261 m in lakes Thun and Brienz, respectively) (Dönz et al. 2018). In the pelagic habitat C. fatioi aggregates in lakes Thun and Brienz in moderate water depths (Brienz: approx. 1–100 m, N = 10; Thun: approx. 10–40 m, N = 9) (Dönz et al. 2018). Note that the habitat-stratified random sampling data for both lakes only covers a short period of time (one month in late summer) and it is thus not clear how the species are distributed spatially through the rest of the year. Furthermore, the habitat-stratified random sampling in the both lakes did not distinguish between ripe and unripe specimens, and thus in the case of C. fatioi the distribution pattern along the depth in the benthic zone is biased by the spawning aggregation of this species since the sampling period in both lakes coincides partially with the spawning season of this species. Coregonus fatioi resembles phenotypically C. albellus and to some extent C. profundus. Interestingly, Steinmann (1950) already mentioned for Lake Thun that the ecotype "Coregonus lavaretus L. nat. arurenis, oekot. nanus" (most likely C. albellus) should be grouped based on its ecology closely to the ecotype "Coregonus lavaretus L. nat. arurenis, oekot. pelagicus" (most likely C. fatioi). Steinmann mentions the German name "Schwebalbock" for the ecotype "pelagicus", which means verbally translated the "floating whitefish" and mentions that the "nanus" ecotype seems to be a small species with similar ecological properties. For the large whitefish species in Lake Thun, Steinmann (1950) defined one central ecotype, the "primigenius" ecotoype, which he places – based on the size – with two other ecotypes namely the "litoralis" ecotoype (most likely C. alpinus) and the "pelagicus" ecotype (most likely C. fatioi). Besides referring to a "primigenius" ecotype, Steinmann (1950) also refers to a "primigenius"-group, which most likely comprises of the "pelagicus" and "litoralis" ecotypes. A further indication of this is that he also mentions that yet another ecotype, namely the "profundus" ecotype, can be directly deduced from the "primigenius" ecotype. Steinmann (1950) further mentions that specimens, which he places in the "primigenius"-group, used to migrate before the construction of water gates (see below) upstream from Lake Thun into the river Aare, which connects Lake Thun with Lake Brienz. Steinmann (1950) mentions that these fish belong to the "primigenius"-group, but did not specify if the migrating population constituted of individuals of the "litoralis" or the "pelagicus" ecotype or both. This migrating population was referred to as "Wanderalbock" (i.e. migrating whitefish) in German and historically migrated from Lake Thun into Lake Brienz during the spawning season, before migration became impossible due to the construction of water gates in 1856 (Fatio 1890; Dönz et al. 2018). Fatio (1890) mentioned that a large part of the population of C. fatioi "disappeared" at the beginning of the spawning season in late August and was caught by fishermen in the river Aare downstream (near the city of Thun or Bern) or upstream (near the city of Interlaken) of Lake Thun before and after the construction of the water gate. We compared six whitefish specimens from the museum collections of the MHNG and NMBE, which had no species designation but where it was mentioned that they were caught in the river Aare near the city of Bern (in the years 1881 and 1895), Thun (in the year 1950) and Interlaken (in the year 1945), to the contemporary specimens of Lake Thun including the type specimens of C. albellus, C. fatioi and C. alpinus. All the specimens were caught after the construction of the water gate, when free movement between the lakes was already constrained. All six specimens from the river Aare group in morphospace within the range or adjacent to the range of the contemporary specimens of C. fatioi including the type specimen (Suppl. material 1: Figure S11a–c), suggesting that the historically migrating population of whitefish from Lake Thun most likely belonged to the species C. fatioi. Bittner (2009) sampled and genotyped individuals of a population of whitefish spawning in the river Aare near Interlaken. Dönz et al. (2018) re-analysed those individuals and was able to assign 4 individuals with high assignment probability (>70%) to several different contemporary species of Lake Thun, namely C. alpinus (individual assignment probability of THL15N18 = 86%), C. acrinasus (ind. assign. prob. of THL15N07 and THL15N23 = 77% and 80%, respectively) and C. fatioi (ind. assign. prob. of THL15NfS1124 = 92%). This suggest either that historically more species than just C. fatioi migrated to the river Aare for spawning and were missed both by Fatio (1890) and Steinmann (1950) and are thus not represented in our PCA morphospace of Aare river whitefish (Suppl. material 1: Figure S11 a–c). Or the historical migratory population consisted – as has been suggested by Fatio (1890) and Steinmann (1950) – of individuals of C. fatioi. The average size (total length) at 3 years of age for specimens in this study is 266±15 mm (mean and standard deviation, N = 14) and 244±14 mm (N = 16) for lakes Thun and Brienz respectively (Suppl. material 1: Figures S4–S6). In Lake Brienz the size of 3-year-old specimens of C. fatioi is considerably larger than that of C. albellus and similar to that of C. alpinus and C. brienzii, whereas in Lake Thun it is similar to that of C. profundus and C. albellus and smaller than that of C. alpinus, C. steinmanni, and C. acrinasus (Suppl. material 1: Figure S6). Coregonus fatioi has a long spawning season with two peaks. One spawning peak is in late summer/early autumn from August to October, which seems more common in Lake Thun than Lake Brienz, and the second peak is in early to late winter from December to March (Suppl. material 1: Figure S3; Bittner 2009; Dönz et al. 2018). Spawning depth varies with spawning season and can range from approx. 40 m down to the max. depth of 210 m and 261 m in lakes Thun and Brienz, respectively (Suppl. material 1: Figure S3; Bittner 2009; Dönz et al. 2018). Occasionally C. fatioi can be found spawning shallower (up to 10 m), but generally it spawns in deeper waters. The spawning season and depth of C. fatioi partially overlaps with that of C. steinmanni, C. albellus, C. acrinasus, and C. profundus in Lake Thun and with that of C. albellus and C. brienzii in Lake Brienz.

The name given to this species by Fatio (1890) was preoccupied by another species described by Fatio (1885). Kottelat (1997) proposed C. fatioi as a replacement name. The specific epithet fatioi is the genitive of Fatio. It was named by Kottelat (1997) after the late researcher Viktor Fatio, a zoologist from Switzerland who wrote a standard reference work on the Swiss vertebrates entitled "Faune des Vertébrés de la Suisse Partie 1–3"and in which he also described part of the whitefish species diversity of Switzerland.

Tiefenalbock in Lake Thun and Felchen in Lake Brienz.