Beluga (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. Etymology. 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. Description. "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 sturgeon show evident morphologic changes: Size. Among all extant bony fishes , the beluga sturgeon rivals the ocean sunfish ("Mola" sp.) as the most massive fish and is the second-longest 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. Several other records of aged sturgeon exceed . Among sturgeons, only the closely related Kaluga ("Huso dauricus ) can attain similar size, with a maximum weight of . 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 long and weigh . The female beluga is typically 20% larger than the male. An exceptionally large beluga recently caught weighed and measured . Biology. 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 	Beluga (sturgeon)
27108407	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 1,700 km upstream, while others spawn just 100 km. To make the long journey to very distant spawning grounds, the sturgeon adapted a two-stage migration strategy, entering the river in fall and staying over winter in reaches of the river offering adequate substrate and water-flow 	Beluga (sturgeon)
27108408	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 	Beluga (sturgeon)
27108409	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 . Thereafter, the larvae usually subsist on benthic invertebrates, but when reaching at least in length, they will switch to a fish-based diet. While swimming back to the ocean, the young sturgeon may cover up to a day. Diet. 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 Adriatic population. It has been reported that in marine and brackish environments, adult  H. huso forage 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 feed mainly on local Cyprinidae. Uses. 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 endangered status has made its caviar even  more expensive throughout the world. The beluga's air bladder is said to make the best isinglass. Status. IUCN classifies the beluga as critically endangered. It is 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. In July 2016, Sturgeon Aquafarms in Bascom, Florida became the first and only facility in the world to obtain a permit exemption for the sale of beluga sturgeon and its caviar in the U.S. Since 2017, the company has assisted in beluga sturgeon repopulation efforts across the world by providing over 160,000 fertilized eggs to the Caspian Sea region.