Stenodus leucichthys, also known as the inconnu, sheefish, ak balyk, or beloribitsa, is a species of large, anadromous, freshwater whitefish in the family Salmonidae. In the strict sense its natural distribution is restricted to the Caspian Sea basin. It was formerly considered extinct in the wild, but survives in cultured stocks. In addition to the landlocked subspecies Stenodus leucichthys leucichthys, it comprises the nelma, Stenodus leucichthys nelma (Pallas, 1773) which lives in Eurasian and North American rivers of the Arctic basin. Despite this designation, nelma, similarly known as the sheefish or inconnu, is currently often considered to be a distinct species Stenodus nelma, which makes the legitimacy of this use case and its inclusion of both species as one debatable.
At a higher level, the genus Stenodus is not considered phylogenetically distinct from the broader lake whitefish genus Coregonus, although it is phenotypically distinct due to its large size and specialized predator morphology. This is due to conflicting evidence regarding the monophyly of the Coregonus genus and its relationship to Stenodus, either being sister to or nested within Coregonus, with several studies backing both arguments. Possible reasons for conflicting results include the relatively new age of the clade that contains both genera, which diverged approximately 10 million years ago, and the lack of sufficient data on the effect of hybridization between and within each genera. While the most recent mitogenomic analysis does support the monophyly of Coregonus, evidence of the Atlantic whitefish species, C. huntsmani, being more basal to the whitefish genus than the Stenodus genus suggests it being paraphyletic.
Description
The inconnu has a large, terminal mouth with a protruding lower jaw, a high and pointed dorsal fin, a forked and homocercal tail, and possesses an adipose fin. It is generally silvery white on the body sides and underbelly with a green, blue, or pale brown back. The meat is white, flaky, sweet, and somewhat oily. In terms of development, inconnu larvae reach the fingerling stage at approximately 30 to 40 days and have a maturation period lasting 3 to 6 years, with males typically reaching maturity about one year earlier.
The fish eat plankton and aquatic insect larvae as juveniles and then become predators of smaller fish almost exclusively in adulthood. However, 30-day old fingerlings stomach contents have been observed to contain the larvae and juveniles of smaller fish species, indicating that this predatory transition occurs during very early life stages. As an adult, its primary food source, comprising over 90% of its diet, is small pelagic fish which mainly consisted of native kilka and silverside, with less reliance on Caspian roach and gobiids, prior to its extirpation.
Historically, ak balyk used to inhabit particularly the Volga, Ural and Terek rivers, and migrate up to upstream from the Caspian to their main spawning grounds Kama River until dam construction and river contamination limited migration past the city of Volgograd. Historically, their food supply was sustained by native small fish until the introduction of invasive comb jelly Mnemiopsis into the Caspian and the decline of prey fish stocks, especially kilka, degraded their food supplies, which contributed to its decline as well. The stock however survives in hatcheries, particularly those established in the Volga River, and some populations are maintained by stocking.