Beluga (sturgeon)

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.[10][11]

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).[10][12] 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.[13] Several other records of aged sturgeon exceed 5 m (16 ft 5 in).[10] 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.[14] An exceptionally large beluga recently caught weighed 960 kg (2,120 lb) and measured 3.4 m (11 ft 2 in).[7]

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.[16] 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 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, beginning in autumn when they enter the Danube River overwinter in the river and the second stage is their spawning which takes place in the spring the river in fall and staying over winter 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.[17] 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 1,000 km (620 mi) 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 4 to 40 m (13 to 131 ft) underwater. Upon hatching, the embryo are 11 to 14 mm (0.43 to 0.55 in) long, and 10–14 days later when they absorb their yolk sack, the length is 18 to 20 mm (0.71 to 0.79 in). Thereafter, the larvae usually subsist on benthic invertebrates, but when reaching at least 10 cm (3.9 in) in length, they will switch to a fish-based diet. While swimming back to the ocean, the young sturgeon may cover up to 60 km (37 mi) a day.[18][19]

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.[7] 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.[18] 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).[20] Additional food items may include molluscs and crustaceans, aquatic birds and young seals (Caspian seals, Pusa caspica).[18]

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.[21]

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.[7] 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.[22]

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.[9]

Since 2015, an official captive breeding scheme has been established in Italy, with beluga from the Azov Sea.[28] 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).[29] Since then, many H. huso were released in the Po river, attempting to resurrect the extinct Adriatic population.[30]

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.[23] 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.[31]

Following a World Wildlife Fund crowdfunding appeal, over 7,000 three month-old beluga sturgeons were released into the Danube River.[26] 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.[32]