Taxonomic Notes
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 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 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.
Justification
Although this species' population trend may be decreasing due to habitat degradation, there is no evidence that the rate of decline approaches the minimum threshold for Vulnerable under Criterion A (≥ 30% over the longer of 10 years or 3 generations). It does not approach the range thresholds for Vulnerable under Criterion B (extent of occurrence (EOO) < 20,000 km2, area of occupancy (AOO) < 2,000 km2) or D2. The population size far exceeds 10,000 mature individuals, hence it does not approach the thresholds for Criteria C or D. There exists no quantitative analysis which would permit application of Criterion E.
Therefore, the Inconnu does not currently meet the thresholds for any Red List criteria, and it is assessed as Least Concern.
Geographic Range Information
This species has a northern Holarctic distribution, and is present along coastlines of the northern Russian Federation, northwestern United States (Alaska) and northwestern Canada.
In the Russian Federation, its range extends eastwards from the northern White Sea basin to Kolyuchin Bay on the Chukchi Peninsula.
In North America, it occurs from the Kuskokwin River in Alaska (United States) to the Anderson River in the Northwest Territories (Canada). However, there exists only a single record from the area between Kotzebue Sound and the Mackenzie Delta region, which spans about 1,200 km of coastline. It is also found in the upper Mackenzie and Yukon river systems upstream to northern British Columbia, where it is apparently abundant.
During the mid-1960s, it was introduced to the Volga River system (Caspian Sea basin) in order to supplement commercial fisheries, but a lack of recent records suggest that it did not become established.
Population Information
This species' population size is unknown, but it significantly exceeds the minimum threshold for Red List criteria (< 10,000 mature individuals). The current population trend has not been quantified, and the number of subpopulations is unclear.
There exist multiple recent reports of stock declines in subpopulations exploited by commercial fisheries (see 'Threats').
Habitat and Ecology Information
This species has evolved anadromous, semi-anadromous and potamodromous life histories, depending on subpopulation.
In most cases, adult individuals forage and overwinter in the lower reaches of large lowland rivers, deltas, estuaries and at sea, sometimes rather far from the coast. They migrate into rivers annually for spawning, moving upstream for long distances sometimes in excess of 1,000 kilometres.
However, some subpopulations are landlocked in freshwater lakes from where they migrate into tributaries to spawn. Others remain resident in rivers and migrate upstream to spawn.
In some North American rivers, both anadromous and potamodromous subpopulations co-occur.
Individuals belonging to anadromous subpopulations are believed to become sexually mature at a size of 600-900 mm standard length and an age of 8-16+, with females maturing later than males. Those within freshwater subpopulations appear to mature earlier, at age 4-5+, and have a shorter life cycle. Most individuals spawn every 2-3 years, and only 1-2 times during their lifetimes.
The annual spawning migration begins in early spring once the winter ice cover subsides. Spawning itself occurs from September to October in fast-flowing, shallow water with a depth of 2-3 metres and substrata of gravel or cobbles. Each female is capable of releasing 80-420,000 non-adhesive eggs. Adults begin to feed immediately post-spawning, and spend the winter and following spring in the river, returning to the lower reaches or sea by the summer.
Embryonic development lasts for 250-260 days, with hatched larvae measuring 12-14 mm standard length appearing from May to June of the following year. Once actively feeding, the fry may migrate downriver to the sea, or remain in the river for 2-3 years. Juveniles exhibit rapid early growth and prey on mysids, insect larvae and fish fry, becoming exclusively piscivorous at a size of c. 300 mm standard length.
Adults are predatory and feed almost exclusively on smaller fishes.
This species occasionally forms hybrids with members of the genus Coregonus, and in the Volga River system may have hybridised with the native Caspian Inconnu (Stenodus leucichthys).
Threats Information
This species exhibits rapid early growth and reaches sexual maturity late, which renders it vulnerable to fishing gear prior to becoming mature.
Some subpopulations in North America have been extirpated or declined significantly due to overharvesting. Similar stock declines have also been noted in parts of the Russian Federation, e.g., the Yenisei River.
The construction of dams and commercial mining activities have reduced the extent of available spawning habitat in some of its native river systems, while pollution from domestic and industrial sources has affected others.
Use and Trade Information
This species is of major importance in comercial, subsistence and recreational fisheries.
Large numbers of individuals are produced by hatcheries in some parts of the Russian Federation.
Conservation Actions Information
Management actions such as seasonal and year-round fishing bans have been implemented for some threatened subpopulations in North America, but their effectiveness has apparently varied.
No conservation management schemes are known to be in place elsewhere within the species' range.
A deeper understanding of this species' demographics (population size and trend), current distribution and life history, particularly the identification of key spawning sites, would likely prove useful in the development of future management efforts. Given the extent of its range, such efforts should be coordinated at local or regional scales.