Species » Huso huso
Beluga sturgeon (Huso huso). Photo credit: Vasile Oţel – Danube Delta National Institute, Tulcea, Romania
1- General information
The common name for this sturgeon species is “Beluga Sturgeon” or “Great Sturgeon”. The scientific name “Huso” is derived from the Greek word “hus” which means “swine”, most probably hinting at the sturdy and robust outer appearance of this fish.
Huso huso inhabits the Black, Azov, Caspian, and Adriatic Seas and occurs naturally only as a sea-migrating form. In the Black Sea Basin, the species formerly congregated in considerable numbers for spawning in the large rivers of the region, including the Danube, Dniester, Southern Bug, and Dnieper, as well as rivers along the eastern shore of the Basin.
The winter race regularly migrated upstream the Danube as far as Slovakia (river km 1,860 – 1,870) and occasionally reached the Austrian and even the German stretch (river km 2,320). The species also entered the lower Morava River, the Vah River, the Zitava River, the Drava River, the Tisza River and its tributaries the Zagyva, Körös and Maros Rivers, the Sava River and its tributary the Kupa River, and the lower courses of the Velika Morava and Olt Rivers.
The main spawning grounds in the Middle Danube River were formerly situated in the so-called Zitny Ostrov reach below Bratislava (river kms 1,766 – 1,866), as well as between Budapest and Szentendre. The main fishery was in the Little Danube (the northern branch of the Danube River) near the mouth of the Vah River at the village of Kolarovo, and in the Danube River, between Komarno and Sap.
The Beluga Sturgeon is the largest of all Danube River sturgeon species and also the largest freshwater fish species in the world. Scientists often doubt the claim that this sturgeon may grow up to 8 meters in length, but maximum lengths of up to 6 meters have been documented in the past. Nowadays, most Beluga Sturgeons do not reach this size anymore. The same holds true for the documented maximum age of over hundred years.
Beluga males and females need 13 and at least 15 years, respectively, to reach maturity. Female spawners can carry 230,000 to 1,000,000 eggs, depending on their size. This sturgeon is also the source of the famed Beluga caviar.
H. huso is the only true predator among the Danube River sturgeons as it hunts and feeds on a large variety of both freshwater and marine fish, depending on the location during this species’ migrations. Larger individuals may even take aquatic birds and baby seals. While bottom-living and drifting tiny animals form the initial main food supply for juveniles, other fishes are taken as early as the young Beluga sturgeon has reached a length of just 2 to 3 cm. Other fishes constitute the main diet when the young Beluga has reached a length of 9 cm.
When living in marine waters, the species inhabits mainly the shallow water of the coastal zone and is probably confined to regions with muddy bottom substrates. For the first year of life, juveniles remain in shallow, relatively warm habitats on the continental shelf. During both the seaward and the spawning migrations, the fish travel in the deepest parts of the riverbed.
4- Spawning migration
Like with other sturgeon species, there are two sorts of spawning migrations that have been documented. Spawners that enter the river in spring to spawn the same year (spring or summer race) and spawners that enter the river in fall to overwinter and spawn the following year (fall or winter race). According to some experts these are not two different races of one species but rather two different spawning migration strategies of one and the same species.
In the Danube River, spawning migrations can be observed almost all year round. Nevertheless, two peak periods have been noted, one for the winter race and one for the spring race. The spring run is observed from January to April, beginning soon after ice-melt at temperatures of 4 to 5 °C. The autumn run begins in August and reaches its peak in October and November.
Regardless of the season of entry into a given river H. huso breeds during the period of high discharge in spring. In the Danube River, this usually occurs in April and May. This sturgeon spawns at a lower temperature and within a narrower temperature range than other migratory sturgeon species, and spawning usually coincides with a flood peak. The optimal temperature for spawning is reported to lie within the range 9 to 17 °C. It has also been reported that autumn spawning occurs in the Danube River during October and November at water temperatures close to those prevailing in spring, but this information requires confirmation. The hatchlings start to disperse downstream back to the sea soon after hatching. Depending on the location of the respective spawning site, this downstream dispersal may have lasted more than a year in the past. Nowadays, with the spawning sites upstream of the Iron Gate dams being lost for the sturgeons it can be assumed that the young Belugas reach the Danube estuary and the Black Sea in the year of their spawning.
The location of spawning sites does not depend on distance from the river mouth but rather on the presence of conditions conducive to reproduction, such as the right type of bottom substrate and a suitable current velocity. In general, H. huso spawns further upstream than any other sea-migrating sturgeon species and therefore the regulation of the water flow and construction of dams have had the greatest impact on the natural reproduction of this species. Spawning H. huso seek out a section of river with a high current velocity and a hard stony, gravelly or rocky bottom, only very rarely using sand or clay. Spawning usually takes place at a depth of 4 to 15 m, but may occur as deep as 40 m.
The main spawning sites are in the river bed, but temporary sites in floodplains may also be used. Limited spawning may also occur in the lower courses of the river. For instance, there are reports from the Kilia Arm of the Danube River Delta, in a stretch of river where an especially strong current exposes a stony bottom. Recent field studies in Bulgaria and Romania revealed some spawning sites in the Lower Danube below the Iron Gate dams, where this species still spawns. Other details about the spawning habits of H. huso are still unknown.
5- Status and conservation
H. huso was once among the most widely distributed and abundant of the migratory sturgeons in the Danube Basin. During the middle ages a major fishery in the middle stretches of the Danube River in Slovakia and Hungary was based on the large winter race. A significant decline in catches was already noted at the beginning of the 16th century, but exploitation continued through the 17th and 18th century, so that by the 19th century only a few individuals were still being caught in the Middle Danube. The last specimen recorded in the Slovakian-Hungarian stretch of the river was a female (3.1 m / 150 kg) taken near the town of Sturovo in 1925.
Due to the extensive river modifications carried out in recent decades (e.g. irrigation and hydropower dams, dyke construction and channelisation), the species has suffered further population decline and range restriction, with its migratory movements now confined by dams. H. huso nowadays only reaches the Iron Gates II dam, 863 km from the sea. According to the IUCN Red List (2010) Huso huso is critically endangered.
Regular annual observations of Beluga Sturgeon juveniles by researchers in Romania document still successful spawning in river stretches downstream of the Iron Gate dams. Nevertheless, with increasing human pressure on the Danube River such as navigation, river regulation and economic development, this species is still utterly endangered and in need of continuous protection. To further strengthen the Danube and Black Sea population it is absolutely necessary to restore this species also in the Middle Danube by making the Iron Gate passable again for the upstream spawning migration of the adult sturgeons and the downstream migration of spent adults and dispersal of the juvenile.