There exists considerable confusion surrounding the taxonomic identities and geographic distribution of the two species currently comprising the genus Stenodus. These circumstances have led to widespread misapplication of their scientific and vernacular names throughout the published literature.

For the purposes of this Red List assessment, the following conventions are therefore applied:

Stenodus leucichthys was described from the Volga and Ural river systems and is endemic to the Caspian Sea basin. Its most appropriate English vernacular name is the 'Caspian Inconnu'.

Stenodus nelma was described from Siberia and has a northern Holarctic distribution. It is referred to by a series of vernacular names including Inconnu, Nelma and Sheefish.

The invalid taxa, Salmo mackenzii Richardson, 1823 and Coregonus lucius Nilsson, 1855, have typically been referred to as junior synonyms of S. leucichthys. However, based on their respective type localities of the Mackenzie River system (Canada) and Arkhangelsk (Russia), they are here viewed as junior synonyms of S. nelma.

The Caspian Inconnu does not approach the range thresholds for Vulnerable under criterion B1 (extent of occurrence (EOO) <20,000 km²) or D2, and criterion B2 is precluded by its uncertain area of occupancy (AOO). The population size and trend are unknown, hence criteria A and D1 cannot be used. There exists no quantitative analysis which would permit application of criterion E.

Whilst there is uncertainty surrounding the population size of this species, it is assumed to be below 10,000, and all individuals occur within one subpopulation inhabiting the lower Volga River. This species is therefore assessed as Near Threatened, as it is close to qualifying as Vulnerable C2a(ii).

This species is endemic to the Caspian Sea drainages, where the majority of records pertain to the northern portion of the basin.

During its annual spawning migrations (see 'Habitats and Ecology'), it formerly entered the Terek, Ural and Volga river systems, penetrating further than 3,000 kilometres upstream to tributaries of the Kama River in the latter.

At the present time it remains extant only in the lower Volga River.

This species' current population size and trend have not been quantified, but it experienced a near-total population collapse after the mid-20^th century (see 'Threats'). Whilst the population size is uncertain, it is assumed to be below 10,000 mature individuals. All extant individuals are understood to belong to the same subpopulation, which inhabits the lower Volga River.

The subpopulations which entered the Terek and Ural river systems to spawn are believed to have been relatively small. Both were extirpated before the turn of the century, possibly by the end of the 1970s.

In the Volga River, the last record of individuals upstream of the Volgograd Dam dates from the mid-1990s. Some individuals reportedly persist below the dam, and this subpopulation is supplemented by stocking (see 'Conservation').

This species is anadromous and euryhaline, with mature adult individuals undertaking extensive spawning migrations upstream from the saline Caspian Sea to flowing stretches of affluent rivers. They are pelagic in the sea itself outside of this period.

Adult male individuals become sexually mature at age 5–6+ and females at age 6–8+. Spawning occurs from September to November over fine-grained substrata of gravel or sand in shallow (1–3 metres deep), flowing river stretches. The eggs sink to the bottom and lodge in the substrate. Embryonic development is extended and lasts for c. six months, with free-swimming fry appearing in spring and immediately migrating downstream. They initially feed on aquatic invertebrates, but are able to consume early life stages of other fish species c. 30 days after hatching. Post-spawning adults return to the Caspian Sea, and some are believed to spend the non-reproductive period in the southwestern part of the basin off Guilán Province (Iran).

This species is an obligate piscivore and particularly targets small schooling fishes such as sprats (Clupeonella spp.).

This species' decline was driven primarily by construction of large hydroelectric dams on the Volga River during the latter half of the 20^th century, a chain of which has completely blocked access to its former spawning sites. In the Terek and Ural rivers, extensive modification of river channels, water abstraction and pollution from agricultural, domestic and industrial sources are believed to have significantly reduced the extent of spawning and nursery habitats. Overharvesting may also have played a role in accelerating the decline, since the Caspian Inconnu exhibits rapid early growth and late maturity, which renders it vulnerable to fishing gear prior to becoming sexually mature.

In 1965, several hundred thousand juvenile individuals of the congeneric, non-native Inconnu (Stenodus nelma) were released into the Volga River by a fish hatchery. There is a suggestion that this may have led to hybridisation between the two species, although this has never been unequivocally clarified.

This species was formerly fished commercially in the lower Volga River and northern Caspian Sea. Annual catches ranged from 280–410 metric tonnes during the early 20^th century, but fell to less than one tonne after the Volgograd Dam was completed in the late 1950s, at which point the fishery was officially closed.

Recent reports indicate that small numbers continue to be harvested on an unauthorised basis.

Commercial harvesting of Caspian Inconnu in the Volga River has been prohibited since the late 1950s.

From 1961, a mechanical fish lift on the Volgograd Dam permitted the transport of anadromous fishes to the reservoir above, and at least 5,100 Caspian Inconnu individuals were artificially translocated above the dam using this method prior to 1968. The chain of dams upstream of Volgograd rendered these actions ineffective in terms of preserving migratory fish species, however, and it is unclear whether the lift remains operational.

During the 1970s, artificial spawning grounds were created downstream of the Volgograd Dam. While there is evidence to suggest that they are utilised, their effectiveness is reported to be poor due to fluctuating water levels, excessive levels of predation on eggs and fry and the occasional presence of environmental toxins from agricultural, domestic and industrial sources.

The Caspian Inconnu is propagated artificially through the use of hormones at three separate hatcheries, with captive-bred juveniles released into the lower Volga River on an annual basis in order to bolster the resident subpopulation. This project appeared to be ongoing as of the mid-2010s, but the current status is uncertain. Moreover, a number of published studies have referred to difficulties encountered in terms of the number of viable fry produced. In addition, it is unclear whether captive adult individuals were at some point hybridised with the non-native Inconnu (Stenodus nelma).

It is recommended that future research efforts initially focus on determining the current population size, distribution and the extent to which natural reproduction is ongoing in the lower Volga River.