Holotype. NMBE-1077208, Switzerland, Lake Thun (46°40'N, 7°46'E), 194 mm SL, male.

Paratypes. NMBE-1077161–1077179, NMBE-1077203–1077207, NMBE-1077209–1077211, Switzerland, Lake Thun (46°40'N, 7°46'E), N = 27, 188–316 mm SL.

Coregonus profundus is a small whitefish species with moderate pigmentation of all fins and the body; brown-orange colouration on the flanks above the lateral line; elongate slender body; long head; large eye with a thick and triangular shaped eye socket; tip of snout is fleshy and roundish; few (15–27) and short gill rakers.

Coregonus profundus occurs only in Lake Thun and we therefore compare the characters of this species specifically with the species of Lake Thun. The differential diagnoses against C. albellus, C. alpinus, C. fatioi, and C. steinmanni are given under those species’ accounts. The lower number of gill rakers of C. profundus (total gill raker number: 15–27, mode = 21) distinguishes this species from all other 5 whitefish species, C. albellus (32–44, mode = 38), C. alpinus (25–34, mode = 30), C. fatioi (32–43, mode = 38), C. steinmanni (30–35, mode = 31), and C. acrinasus (30–40, mode = 36) (Suppl. material 1: Table S6).

Coregonus profundus–Coregonus acrinasus

Coregonus profundus can be distinguished from C. acrinasus by having shorter gill rakers (middle gill raker length: 7.6–11.7% HL, mean = 9.2 vs. 9.1–16.6% HL, mean = 13.4; longest gill raker length: 7.8–12.4% HL, mean = 10.1 vs. 11.4–16.9% HL, mean = 14.5) and a longer head (15.5–18.4% HL, mean = 16.4 vs. 13.8–16.1% HL, mean = 15.2) (Tables 8, 9).

General appearance is shown in Figure 9. Morphological and meristic characters of both sexes can be found in Table 8 and Suppl. material 1: Table S6 and first- and second-best ratios for both sexes combined can be found in Table 10. The description is valid for both sexes.

Shape: Body elongate. Slender bodied with greatest body depth anterior of the dorsal fin. 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. Snout often 60° 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 is wide (i.e., width of upper and lower jaw), rather short and mostly strongly sub-terminal and only rarely terminal. Snout is weakly pronounced, since the tip of the snout is often fleshy and roundish. Eye rather large with a large eye cavity and a thick and triangular eye-socket (i.e., sickle-shaped). Pectoral fin long and moderately tapered. Dorsal fin long with the anterior unbranched ray of the erected dorsal fin approx. 70–80° angle to body axis and only slightly bent posteriorly at the end of the ray. Caudal peduncle narrow and short with caudal fin forked and sometimes moderately to strongly asymmetrical with either the ventral or dorsal part being longer. Unbranched ray of anal fin straight and rarely bent posteriorly at the end of the ray. Anal fin is longest anteriorly and progressively shortening posteriorly with the outer margin of the anal fin slightly concave and only rarely straight.

Meristics : Very few and very short gill rakers.

Colour: Pigmentation of fins and body is overall moderate in live specimens. Pectoral fin is translucent or yellowish in colouration with moderate pigmentation at the median to distal parts of the fin. Dorsal, adipose, pelvic, anal and caudal fins are moderately pigmented. Silvery appearance along the flanks and dorsally above the lateral line the silvery appearance changes to a pale brown-orange colouration (e.g., RGB (232, 172, 52)) and very rarely the brown-orange colouration can have a hint of light greenish colour (e.g., RGB (136, 245, 205)). Sometimes the colouration above the lateral line is pale rose (e.g., RGB (247, 187, 175)) and then towards the dorsum becomes a brown-orange. This transition from one colouration to another can also be observed in C. albellus. For a comparison to the main colouration found in the other species see Suppl. material 1: Figure S8. Dorsal part of the head is moderately pigmented. Snout around the nostrils is moderately pigmented and rarely with a gap of less pigmentation posteriorly of the nostrils up to the height of the middle of the eyes. The 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)).

Distribution and notes on biology.Coregonus profundus is found in Lake Thun (46°40'N, 7°46'E). It is believed to have been endemic to this lake. Yet, based on matching genetic (microsatellite) and morphological (gill raker number, morphological characters) evidence one ripe specimen of C. profundus has been caught by a local fisherman, Stefan Dasen, in 2016 in Lake Biel (47°05'N, 7°10'E) (Suppl. material 1: Figure S9). Lake Biel has been artificially connected with Lake Thun through the river Aare since the Jura water correction from 1868–1878, where the river Aare was artificially bypassed downstream from Lake Thun into Lake Biel. For another Lake Thun species, C. albellus, it had been known since at least 2004 that it can be found in Lake Biel (see details in the note on biology for C. albellus) (Bittner 2009; this study Suppl. material 1: Figure S9).

It is important to note that native whitefish species of Lake Biel were only known to spawn in the winter months (Fatio 1885; Steinmann 1950; Rufli 1978), whereas C. profundus as well as C. albellus spawn in late summer and winter. Our study reports the first record of C. profundus in Lake Biel. It is unclear though if C. profundus has established as a self-sustaining population in Lake Biel. So far, we only know of one ripe specimen of C. profundus from Lake Biel, whereas for C. albellus reasonable numbers of ripe specimen have been caught for several years in Lake Biel during what is the normal spawning period (late summer) of this species in lakes Thun and Brienz (Bittner 2009; 2016: Suppl. material 1: Figure S9). Based on isotopic signatures C. profundus feeds on benthic prey items (Selz 2008; Hudson 2011; Ingram et al. 2012) and has a slow growth rate (Bittner et al. unpublished). Interestingly specimens of C. profundus that have been caught on the spawning grounds of C. albellus were often in past-spawning condition and occasional stomach content analysis revealed that these fish had been heavily preying on whitefish eggs (Bittner 2009). Earlier stomach content analysis of C. profundus from the months of October and February of 1971 and 1972, respectively, showed that C. profundus mainly feed on chironomid larvae and occasionally on fish eggs (Rufli 1979). Even earlier stomach content analysis by Steinmann (1950) also show that they feed on chironomid larvae, but also on pisidium and other benthic invertebrates. Habitat-stratified random sampling of Lake Thun (mid-October 2013: Vonlanthen et al. 2015) shows that C. profundus occupies mostly the moderately deep to the deepest waters in the benthic habitat (approx. 15 – 210 m; N = 16) and rarely the moderately deep pelagic waters (approx. 15 – 45 m; N = 3)(Dönz et al. 2018). The habitat-stratified random sampling did not distinguish between ripe and unripe specimens, and thus in the case of C. profundus, the distribution pattern along the depth in the benthic zone is biased by the spawning aggregation of this species since the sampling period coincides partially with the spawning season of this species. Coregonus profundus phenotypically resembles superficially C. albellus. The average size (total length) at 3 years of age for specimens used in this study is 263±16 mm (mean and standard deviation, N = 11) (Suppl. material 1: Figures S4, S6). The size of 3-year-old specimens of C. profundus is similar to that of C. albellus and C. fatioi, but smaller than that of C. acrinasus and considerably smaller than that of C. alpinus and C. steinmanni (Suppl. material 1: Figure S6). Coregonus profundus has a moderately long spawning season from August to December with one major peak from late August to late September / early October (Suppl. material 1: Figure S3; Bittner 2009; Dönz et al. 2018). Spawning depth varies with spawning season and can range from approx. 30 m to 150 m (Suppl. material 1: Figure S3; Bittner 2009; Dönz et al. 2018). The spawning season and depth of C. profundus partially overlaps with that of C. steinmanni, C. fatioi, and C. albellus (Suppl. material 1: Figure S3; Bittner 2009; Dönz et al. 2018).

Coregonus profundus is known by the common name "Kropfer" and has previously been described under the name C. alpinus (Kottelat (1997) and Kottelat and Freyhof (2007)). As we explain in detail under the species account of C. alpinus, the designated lectotype of C. alpinus is incongruent with the description of the species (with the common name "Kropfer": Kottelat (1997) and Kottelat and Freyhof (2007)). We have thus retained the name C. alpinus for the lectotype designated by Kottelat (1997) and provided a new description of this taxon. For the species otherwise described by Kottelat (1997) and Kottelat and Freyhof (2007) as C. alpinus (with the common name "Kropfer") we designated a new name, C. profundus.

The adjective profundus means deep in Latin and is used for C. profundus to describe the species unique ecology of living and breeding in great depths in Lake Thun.

Kropfer.