Stenodus leucichthys

Stenodus leucichthys is a species of freshwater whitefish in the family Salmonidae. In the strict sense its natural distribution is restricted to the Caspian Sea basin. It is now considered extinct in the wild, but survives in cultured stocks. The nelma (Stenodus nelma), a more widespread species of Eurasian and North America, is sometimes considered its subspecies.

Systematics
Alternatively, the name Stenodus leucichtys has been used in a broader sense, referring to a widespread species composed of two subspecies. 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. Nelma, also known as the sheefish or inconnu, is currently often considered as a distinct species Stenodus nelma.

At a higher level, the genus Stenodus is not phylogenetically distinct from the broader lake whitefish genus Coregonus, although it is phenotypically characterized by a specialized predator morphology.

Description and status
The fish has a large mouth with a protruding lower jaw and a high and pointed dorsal fin. It is generally silver in color with a green, blue or brown back. The meat is white, flaky and somewhat oily. An adult fish weighs from 14 to 25 kg. The fish eat plankton for their first year of life and then become predators of smaller fish.

Aqbalyq used to inhabit particularly the Volga, Ural and Terek rivers, and migrate up to 3000 km upstream from the Caspian to their spawning grounds in the spring. Following the construction of dams and hydropower reservoirs, the migration and natural reproduction has been impeded, and the taxon is now considered as extinct in the wild by the IUCN. The stock however survives in hatcheries and some populations are maintained by stocking.