Beluga, Beluga sturgeon, Great sturgeon
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 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.
The common name for the sturgeon, as for the unrelated beluga whale, is derived from the Russian word белый (belyj), meaning "white," probably referring to the extensive pale colour on the flanks and belly in beluga compared to that of other sturgeons.
Huso huso shows typical characteristics of other sturgeon, such as an elongated body, heterocercal tail, partially cartilaginous skeleton, naked skin and longitudinal series of scutes.
The dorsal fin has 48 to 81 soft rays, and the anal fin, much shorter, has 22 to 41 soft rays. There are five in a series of longitudinal scutes: dorsal (one series, 9–17 scutes), lateral (two series, one per side, 28–60 scutes each) and ventral series (two series, one per side, 7–14 scutes each). The surface of the skin is covered by fine denticles. The rostrum is conical and contains numerous sensory pits on both ventral and dorsal surfaces. The mouth is large, crescent-shaped and protractile, with the upper lip continuous while the lower lip is interrupted by a large gap. The barbels are laterally compressed with foliate appendages, arranged in two pairs, originating midway or closer to the mouth than to the tip of the snout.
However, during growth, the beluga sturgeons show evident morphologic changes:
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 1,571 kg (3,463 lb) and 7.2 m (23 ft 7 in). Another specimen reportedly weighed 1,220 kg (2,690 lb) and measured 6.1 m (20 ft 0 in) in length. Claims about greater length (10 m (32 ft 10 in), or even 12 m (39 ft 4 in)); and weight (2,000 kg (4,400 lb), 2,500 kg (5,500 lb), or even 3,000 kg (6,600 lb)) are disputed and unconfirmed; but they are not impossible. Several other records of aged sturgeon exceed 5 m (16 ft 5 in). Among sturgeons, only the closely related Kaluga (Huso dauricus) can attain similar size, with a maximum weight of 1,000 kg (2,200 lb).
Beluga of such great sizes are very old (continuing to grow throughout life) and have become increasingly rare in recent decades because of heavy fishing of the species. Today, mature belugas that are caught are generally 142–328 cm (4 ft 8 in – 10 ft 9 in) long and weigh 19–264 kg (42–582 lb). The female beluga is typically 20% larger than the male. An exceptionally large beluga recently caught weighed 960 kg (2,120 lb) and measured 3.4 m (11 ft 2 in).
Beluga sturgeon are considered euryhaline, capable of moving freely between freshwater and estuaries, and thus can live in waters of varying saline content. Sturgeons are quite a durable species and can survive some of the most altered and polluted rivers in the world.
Historically, beluga sturgeon were found in the Caspian Sea, Black Sea, Adriatic Sea, Sea of Azov, and all rivers interconnecting these waterways. Unfortunately, this range has been greatly reduced in modern times to the Caspian Sea, the Black Sea, and a few rivers such as the Danube, with attempts to reintroduce Belugas into various historic locations.
Due to rampant overfishing, the average lifespan of beluga sturgeon is unknown, with no specimens living past their 56th year. However the species is reportedly quite long-lived, being capable of surviving over 100 years in the wild.
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. The prey is sucked into the mouth opening extremely quickly.
Juveniles feed on benthic invertebrates in rivers and shallow coastal waters, where they grow quickly. At the length of 8–10 cm, they become largely piscivorous.Different diets have been observed throughout the distribution range of beluga sturgeon, as well as according to spawners' migration stage. Adults mainly eat a great diversity of large fish (73% of the diet). Additional food items may include molluscs and crustaceans, aquatic birds and young seals (Caspian seals, Pusa caspica).
The piscivorous diet of beluga sturgeon tends to change with age: in the Caspian Sea, it mainly consists of Clupeonella sp. for juveniles smaller than 40 cm, different species of Gobiidae for fish ranging between 40 and 280 cm and then mullets, Alosa sp. and other sturgeons for the largest.
In brackish environments of the Ponto-Caspian basin, the genera Alosa, Aspius and Engraulis are the preferred prey. In estuaries and rivers of the same area, migrating spawners eat various cyprinids, mainly Cyprinus carpio and Rutilus rutilus, Sander lucioperca and, among sturgeons, Acipenser ruthenus is the main 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.
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 by this magnificent species.
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. The sturgeon remains a protected species listed in Appendix III of the Bern Convention, and its trade is restricted under CITES Appendix II. The Mediterranean population is strongly protected under Appendix II of the Bern Convention, prohibiting any intentional killing of these fish.
The United States Fish and Wildlife Service has banned imports of beluga caviar and other beluga products from the Caspian Sea since 6 October 2005, after listing beluga sturgeon under the U.S. Endangered Species Act.
