European regional assessment: Data Deficient (DD)
EU 27 regional assessment: Not Recorded

In the European region, this species occurs at the westernmost limit of its range and is confined to rivers draining the western slopes of the Ural Mountains in the Russian Federation. Although it has undoubtedly declined (AOO, EOO and number of mature individuals) since the mid-20^th century, its current abundance, distribution and population trend within the European portion of its range are not clearly understood. The Siberian Taimen is therefore assessed as Data Deficient for Europe, pending additional research. The species does not occur in the EU27 Member States.

This species is widely distributed in fluvial ecosystems of northern and eastern Eurasia (Kottelat and Freyhof 2007, Rand 2013).

In Europe, which comprises the westernmost limit of its range, the species is naturally confined to rivers draining the western slopes of the Ural Mountains in the Russian Federation (Holčík et al. 1988, Galland et al. 2021). Its native European range thus comprises the Kama River system, which is the principal left-bank affluent of the Volga River in the Caspian Sea basin, plus major right-bank tributaries of the Pechora River, which flows to the Barents Sea in the Arctic Basin (Holčík et al. 1988).

Until the late 1960s, it occurred in the Volga main stem from its confluence with the Kama downstream to the so-called Samara Bend, located between the cities of Tolyatti and Samara in Samara Oblast (Shakirova and Severov 2014). It is today restricted to the Kama watershed, with the only contemporary reports pertaining to the Votkinsk accumulation lake on the lower Kama main stem, plus the Belaya and Vishera tributary systems (Marić et al. 2014, Mikheev and Ogorodov 2015, Pozdeev et al. 2022). There are no recent records of previously-identified subpopulations inhabiting other affluents of the Kama such as the Vyatka, Siva, Yayva, Kosva or Chusovaya rivers, but some parts of these catchments were not sampled during field campaigns carried out since the turn of the century (e.g., Pozdeev et al. 2022).

The majority of records from the Pechora watershed refer to the river's main stem plus the Ilych, Shchugor and Usa (in the latter, left-bank tributaries including the Bolshaya Synya and Kosyu rivers) affluent systems. There are no documented reports from anywhere within the Pechora catchment since 1973, although anecdotal evidence obtained from local sources indicates that it might remain extant at one or more unspecified locations (Mikheev and Ogorodov 2015, Degteva 2019).

Outside of Europe, this species inhabits rivers draining to the Arctic Ocean east of the Pechora River to the Khroma River in the Republic of Sakha (Bogdanov and Knizhin 2022). In the Western Pacific region, it is also present in the Uda, Tugur, and Amur rivers, plus some small watersheds draining the northwestern tip of Sakhalin Island (e.g., the Bol'shaya, Langry, Vagis and Pogibi rivers) in the Sea of Okhotsk basin (Holčík et al. 1988, Rand 2013, Dyldin and Orlov 2016).

As a large-bodied obligate predator (see 'Habitat and Ecology'), this species naturally exists at relatively low densities compared with most sympatric fish species. Its abundance in the European region has declined significantly since the mid-20^th century, but its current population size and trend have not been quantified.

It has not been reported from the Volga River main stem or from the lower Kama River since the 1950s, or anywhere in the Pechora River system since the early 1970s, but exhaustive surveys do not appear to have been carried out.

It is regarded as Possibly Extinct in the Pechora River (Degteva 2019), while the extent of its presence in the Kama River beyond areas identified in recent reports (e.g., Pozdeev et al. 2022) is unclear.

Major declines have been recorded in parts of its non-European range, e.g., commercial catches in the Republic of Sakha (Yakutia), Russian Federation, declined by more than 70 times during the period 1943-2010 (Mikodina and Sivtseva 2010).

In Europe, this species is an exclusive inhabitant of larger, primarily upland (> 200 metres asl) river channels where it demonstrates a preference for lotic stretches characterised by cool, well-oxygenated water (Holčík et al. 1988, Kottelat and Freyhof 2007). Adult individuals tend to occupy deeper pools and glides located beneath rapids or waterfalls, in river bends or at confluences, but have extended and variably-sized (from < 10 to > 100 kilometres of linear river length, depending on the location and individual fish) home ranges within which they undertake marked seasonal movements (Gilroy et al. 2010, Kaus et al. 2016, Galland et al. 2021). They are somewhat territorial and select positions in dominance hierarchies based on maximising their energy intake, thus larger specimens tend to occupy the upstream portions of favourable reaches. In contrast, juveniles and subadults have no fixed territories and are often observed in riffles and runs. In addition, this species is occasionally encountered in fluvial accumulation lakes (Holčík et al. 1988, Mikheev and Ogorodov 2015).

Each year, both the Kama and Pechora rivers freeze from mid-November or early December until April, and this regime is understood to influence Siberian Taimen activity. Adult individuals tend to spend both winter and summer in deep, low-velocity depressions and pools, but forage in fast-flowing reaches after the surface ice thaws and the water temperature remains ≤ 15°C. When conditions become warmer, they may move into cool upstream stretches, areas influenced by upwellings of groundwater, or return to deeper environments. Large individuals are normally solitary, but aggregations comprising 20-30 individuals have been observed in such thermal/hydrological refuge habitats during both warm and cold periods of the year (Matveyev et al. 1998).

This species is a visual predator. Older age classes demonstrate a pronounced tendency towards piscivory, but also prey opportunistically on small mammals, waterbirds, amphibians and reptiles. Cannibalism has been observed on a regular basis (Holčík et al. 1988).

It is the world's largest salmonid species, capable of growing to at least 2 m in length with a body mass exceeding 100 kg (Holčík et al. 1988), although individuals measuring > 1.7 metres and weighing > 65 kg are very rare (Zolotukhin and Shcherbovich 2016, Karpova et al. 2023). It is long-lived, with a lifespan of at least 35 years (although some sources report 60 years, e.g., Kaus et al. 2016). Maturity is attained at an age of 5-9+, total length of 500-700 mm and weight of 2-3 kg (Matveyev et al. 1998).

The annual reproductive period extends from May to June, and typically coincides with increased river discharge due to the thawing of snow and surface ice, when water temperatures rise to 6-8°C (Gilroy et al. 2010). This species is iteroparous and potamodromous, and nuptial individuals thus migrate from deeper, larger rivers to specific upstream spawning sites comprising well-washed gravel beds in shallow reaches with moderate flow, which are often located in tributaries (Esteve et al. 2009). In some years, mature individuals undergo a reproductive resting phase whereby they fail to produce gametes and thus do not participate in spawning (Kaus et al. 2016). Nuptial individuals develop a conspicuous epigamic colour pattern comprising orange to red pigmentation on the posterior and ventral portions of the body (Karpova et al. 2023).

Unlike most salmonids, this species forms pair bonds several days or weeks prior to migration. Upon arrival at spawning sites, pairs remain together and the male aggressively defends the site to prevent other males from entering. Meanwhile, the female creates an elliptical depression (redd) in the substrate, into which the gametes are deposited (Esteve et al. 2009). Females produce 4,000–35,000 eggs, depending on the size of the individual. The presence of unclogged, well-oxygenated interstitial spaces within each redd is considered to be crucial for successful incubation and early development. Alevins typically emerge after around one month, and begin to feed in shallow marginal areas close to the spawning site once their yolk sacs are absorbed. Juveniles (parr) usually disperse downstream in riffles or runs, and often aggregate in the vicinity of cover such as undercut banks or submerged wood. They feed on aquatic invertebrates and the fry of other fish species until age 2+, at which point they become almost entirely piscivorous (Matveyev et al. 1998).

This species' European subpopulations are understood to be threatened by a series of largely ongoing factors.

In particular, its decline has been driven by river regulation and other forms of habitat degradation, which have resulted in the widespread loss of the heterogeneous, interconnected fluvial habitats required to complete its life cycle. In particular, the construction of dams, weirs and other barriers in the Kama and Volga rivers (e.g., the Zhiguli and Votkinsk hydroelectric dams during the 1950s, the Yumagusinsky dam in the late 1990s) during the 20^th century has altered natural flow and sedimentation regimes, blocked seasonal migration routes and access to spawning sites, fragmented subpopulations, and generally reduced the extent of suitable habitat for all life stages. The quality of habitat has been further diminished by bank stabilisation, channelisation and other efforts to enhance flood protection or exploit water for human development, e.g., irrigation (Mineeva et al. 2022, Pozdeev et al. 2022).

Hydroelectric dams have also created unnatural fluctuations in discharge and water temperature (hydropeaking and thermopeaking) which cause dewatering of foraging and spawning sites, the loss of stable nursery habitat for juveniles, and downstream displacement. The combined effect of hydropeaking, dam flushing operations, changes in land use, and the removal of riparian vegetation is also likely to have increased the accumulation of fine sediments at spawning sites, thus impairing the hatching and survival rates of eggs and larvae. The industrial extraction of riverine gravel and other sediments for urban development may have further reduced the extent and quality of available spawning sites.

This species is also understood to be threatened by diffuse and point-source agricultural, domestic and industrial pollution, which has resulted in eutrophication or discharge of toxic substances at some locations (Lephikin et al. 2020, Tselishcheva and Lazareva 2021, Mineeva et al. 2022). For example, the Pechora River has experienced a number of severe oil spills within the past few decades (Lukin et al. 2011), while its principal tributary the Usa River is heavily contaminated due to extensive coal mining (Walker et al. 2009). The Kama River is impacted by the discharge of domestic and industrial effluents in addition to significant thermal pollution from a power station located at the city of Perm, which discharges warm water into the river's main stem (Lephikin et al. 2020, Mineeva et al. 2022).

Some of this species' traits, such as large body size and late sexual maturity, are understood to render it susceptible to overfishing (Jensen et al. 2009). Licensed and unregulated harvesting and angling may thus have contributed to the inferred decline in the number of mature individuals inhabiting European rivers (e.g., Degteva 2019), although few precise details are available.

Rising water temperatures due to climate change represent a plausible threat, since they may interfere with food availability, lifespan, and the timing of reproductive processes (Rand 2013, Kaus et al. 2016).

This species is extensively utilised in recreational fisheries and is particularly targeted by fly fishing enthusiasts, with private tourist companies offering bespoke angling packages to a number of destinations in the eastern part of its range (Vander Zanden et al. 2007). The situation in Europe is not well-documented, but the extant subpopulations are likely to be of interest to local anglers.

During the mid-20^th century, commercial fisheries operated in both the Pechora and Kama river systems, but recent reports pertain only to a "limited fishery" in the Vishera River (Kama watershed). Elsewhere within its range, it is harvested with gill or seine nets, or as bycatch in whitefish (Coregonus spp.) fisheries. Both its flesh and eggs are viewed as delicacies, and are often eaten raw (Karpova et al. 2023).

Both licensed and unregulated commercial and recreational harvesting take place during the annual spawning migration (see 'Habitat and Ecology'), and overall offtake is thus under-represented by official fisheries data (Rand 2013).

This species is produced in aquaculture for restocking of both commercial and recreational fisheries in some central and eastern parts of its range (Balakirev et al. 2013), but it is unclear whether these practices currently take place in Europe.

In the European region (Russian Federation), this species is listed as threatened in the Komi Republic, Udmurt Republic, Republic of Bashkortostan and Perm Krai (Rand 2013). In Komi Republic, which contains the majority of the Pechora River system, it is currently assessed as Possibly Extinct (Degteva 2019).

The Syninsky Biological Reserve and Pechoro-Ilychsky State Nature Reserve were established on the Bolshaya Synya and Ilych rivers in the Pechora watershed in order to conserve local Siberian Taimen subpopulations (Degteva 2019). In the Kama River system, this species is present within the boundaries of several protected areas, most notably the Vishera Nature Reserve (Mineeva et al. 2022).

However, no specific management measures appear to be in place, and almost all relevant conservation research has been carried out in Mongolia, China and non-European parts of the Russian Federation (Vander Zanden et al. 2007, Jensen et al. 2009, Gilroy et al. 2010, Rand 2013; Kaus et al. 2016, 2019; Galland et al. 2021, Karpova et al. 2023). A significantly deeper understanding of this species' autecology, genetic diversity, abundance and current distribution in Europe is thus required in order to clearly identify priority conservation areas and recommended management practices.