The beluga (), also known as the beluga sturgeon or great sturgeon (Huso huso), is a species of anadromous fish in the sturgeon family (Acipenseridae) of the order Acipenseriformes. It is found primarily in the Caspian and Black Sea basins, and formerly in the Adriatic Sea. Based on maximum size, it is the third-most-massive living species of bony fish. Heavily fished for the female's valuable roe, known as beluga caviar, wild populations have been greatly reduced by overfishing and poaching, leading IUCN to classify the species as critically endangered.

However, during growth, the beluga sturgeons show evident morphologic changes:

  • Juveniles are slender, and the head is quite narrow with a mouth ventrally placed but projecting upward. The snout is thin and pointed (almost half of the head), scutes are evident, back and flancs are dark grey or black and the belly is white.
  • Adults are heavy-set, spindle-shaped, large and humpbacked. The head is massive with a very large protractile mouth that gradually moves in an almost frontal position during growth. The snout is quite short (one-third to one-quarter of the head), and scutes gradually undergo absorption and decrease in number with age. Colouring is blue-grey or dark brown, with silver or grey flanks and white belly. The dark dorsum contrasts strongly against the rest of the body;
  • Very old specimens are stocky, with a large head and an enormous mouth.

Size

thumb|left|250px|A 1000-kg, 4.17-m-long beluga fish from the [[Volga River (National Museum of Tatarstan, Kazan, Russia)]]

Among all extant bony fishes, the beluga sturgeon rivals the ocean sunfish (Mola sp.) as the most massive fish and is the second-longest bony fish after the giant oarfish (Regalecus glesne). It is the largest freshwater fish in the world. The beluga also rivals the great white shark (Carcharodon carcharias) and the greenland shark (Somniosus microcephalus) for the title of largest actively predatory fish.

The largest accepted record is of a female taken in 1827 in the Volga estuary at and . Another specimen reportedly weighed and measured in length. Claims about greater length (, or even ); and weight (, , or even ) are disputed and unconfirmed; but they are not impossible. Several other records of aged sturgeon exceed . An exceptionally large beluga recently caught weighed and measured .

Spawning

Like most sturgeons, the beluga is anadromous, migrating upstream in rivers to spawn on clean, hard substrate, which offers both support and cover to their sticky and adhesive eggs. Spawning biology and development of larval stages of the sturgeon, the most ancient fish of the Danube, co-evolved with the formation of the Danube valley, resulting in very different survival strategies in its early life stages. This appears to explain why different individuals of the same long-migratory species spawn as far upstream as upstream, while others spawn just . To make the long journey to very distant spawning grounds, the sturgeon adapted a two-stage migration strategy, beginning in autumn when they enter the Danube River. After overwintering in the river, spawning takes place in the spring in reaches of the river offering adequate substrate and water-flow resting conditions. Very few locations of existing wintering and spawning grounds for sturgeon are presently known in the lower Danube, and none are known to exist in the river's upper reaches. The same situation concerns nursery sites upon which young sturgeon depend during their journey to the Black Sea.

Males attain sexual maturity at 12–16 years of age, whereas females do so at 16–22 years. They will spawn every four to seven years. At one time, beluga sturgeons could migrate up to upriver to spawn, but dams in almost every major tributary that they utilize have impeded historic spawning routes. The female lays her eggs on gravel from underwater. Upon hatching, the embryo are long, and 10–14 days later when they absorb their yolk sack, the length is . While swimming back to the ocean, the young sturgeon may cover up to a day.

Diet

thumb|left|Fingerling of Huso huso eating a small fish in an aquarium

Huso huso is a pelagic predator whose local distribution is not influenced by the nature of the substrates, unlike with most of the sturgeons that show demersal attitude. Additional food items may include molluscs and crustaceans, aquatic birds and young seals (Caspian seals, Pusa caspica).

thumb|An adult H. huso eating a small sturgeon

In brackish environments of the Ponto-Caspian basin, the genera Alosa, Aspius and Engraulis are the preferred prey.

Little is known about the diet of the extinct Adriatic population. It has been reported that in marine and brackish environments, adult Adriatic H. huso foraged primarily on molluscs (Cephalopoda, of which common cuttlefish, Sepia officinalis, and European squid, Loligo vulgaris, are particularly common in the Adriatic Sea) and fish belonging to the families Gadidae, Pleuronectidae, Gobiidae, Clupeidae, Scombridae and Mugilidae, but also on big crustaceans; in the rivers, they fed mainly on local Cyprinidae.

Uses

thumb|The caviar of a beluga sturgeon, the main reason for the massive overharvesting of this species

Beluga caviar is considered a delicacy worldwide. The flesh of the beluga is not particularly renowned, but it is a hearty white meat similar to that of swordfish. Beluga caviar has long been scarce and expensive and the fish's critically endangered status has made its caviar even more expensive throughout the world.

The beluga's air bladder is used to make the best isinglass.

Status

IUCN classifies the beluga as critically endangered. Due to aforementioned poaching of the sturgeon, the Danube is the only river remaining with naturally reproducing sturgeon populations within the European Union.

Repopulation efforts

Since 2015, an official captive breeding scheme has been established in Italy, with beluga from the Azov Sea. Then, after the building of a fish ladder on Isola Serafini dam, on 2019 hundreds of young microchipped beluga and 60 tagged subadults were released into the Po river, following EU Projects (Life Ticino Biosurce). Since then, many H. huso were released in the Po river, attempting to resurrect the extinct Adriatic population.

Management of sturgeon fisheries within the Caspian Sea began in the 1950s and while the initial regulations had honorable intentions, they achieved dwindling effects due to the ever present demand for the fish's caviar.

Following a World Wildlife Fund crowdfunding appeal, over 7,000 three month-old beluga sturgeons were released into the Danube River in Bulgaria. Despite repopulation efforts, the beluga sturgeon continues to face poaching threats. In 2021, two Romanian men in Grindu, Ialomita, were caught trying to smuggle a 140 kilogram, 2.5-metre beluga sturgeon in a wagon; the fish was later safely returned to the river.

Threats

The beluga sturgeon is confronted by several critical threats that imperil its existence. Illegal harvesting, habitat disruption through dam construction, and pollution are among the most pressing challenges faced.

Illegal harvesting and poaching

The beluga sturgeon faces a significant threat from illegal catches for its meat and caviar. The excessive harvesting and a sharp increase in poaching have removed the largest and most mature specimens from the population, almost eliminating natural reproduction. This exploitation has pushed the species to the brink of extinction.