Acipenser stellatus

Stellate sturgeon (A. stellatus) Photo credit: Vasile Oţel – Danube Delta National Institute, Tulcea, Romania.

 

1- General information

The common name for this species is “Stellate Sturgeon”. The scientific name “stellatus” is a Latin word meaning “covered with stars”. This refers to the star-like shape of the bony plates covering this sturgeon’s body.

 

2- Occurrence

Acipenser stellatus inhabits the Caspian, Azov, Black, and Aegean Seas, from where it enters tributaries including the Danube. It was always rare in the Middle and Upper Danube, only occasionally migrating upstream as far as Komarno, Bratislava, Austria or even Germany. During its spawning migrations A. stellatus also entered tributaries of the Lower Danube River, such as the Prut, Siret, Olt and Jiul Rivers. In the Middle Danube it migrated into the Tisza River (up to Tokaj) and the lower courses of its tributaries, the Maros and Körös Rivers, as well as the mouth of the Zagyva River, the lower courses of the Drava and Sava Rivers and the mouth of the Morava River.

As a result of river regulation and damming, the range of A. stellatus in the Caspian, Azov, and Black Sea watersheds has decreased significantly. The extent of spawning grounds has been reduced considerably and migration routes and timing have changed. Nowadays, most individuals in the Danube River migrate only as far as the Iron Gates dams at river km 863.

 

3- Biology

The Stellate Sturgeon can grow up to more than 2 meters and may reach a maximum age of 35 years. It takes males and females up to 6 and 10 years, respectively, to mature. The female spawners can carry 70,000 to 430,000 eggs, depending on their size.

A. stellatus typically is found in the shallow water of the sea coast and the lowland sections of rivers. Small bottom-living animals are the main food source for adults and plankton plays an important role in the nutrition of the early larval stages.

This species spawns on river banks inundated by spring floods and above the stony bottom of the river bed at fast currents. The eggs are laid on beds of scattered stones, pebbles, and gravel mixed with shell fragments and coarse sand. The optimal spawning conditions include a high flow velocity and a clean gravel bottom. A decrease in current velocity after spawning and during the development of the eggs can lead to increased losses of embryos. In the Danube River spawning occurs from May to June at temperatures between 17 and 23 °C. Not much is known about the spawning habits of this species.

The larvae of A. stellatus inhabit not only the lower and middle water layers in the rivers after hatching but also occur at the surface. They drift downstream, and, during subsequent development, their capability of active movement increases. The distribution of juveniles on the bed of the Danube is influenced by food supply, current and turbidity. Juveniles migrate downstream at depths of 4 to 6 m. The life span in the river lasts from May to October and active feeding begins when the larvae reach 18-20 mm.

 

4- Spawning migration

Like other sturgeons, A. stellatus enters the Danube River to spawn throughout almost the entire year, but two peak periods are evident. The run begins in March at a water temperature of 8 to 11° C, reaches its peak intensity in April, and continues through May. A second, more intense migration begins in August and lasts until October. This species prefers warmer habitats than other Danube sturgeons and its spawning runs occur at water temperatures higher than those prevailing during the migrations of the other species, thus taking place immediately after those of Huso huso and A. gueldenstaedti.

 

5- Status and conservation

A. stellatus was always rare in the Middle and Upper Danube and has now been extirpated from

the Upper Danube and the (Hungarian-Slovakian) stretch of the Middle Danube River as only a few individuals succeed in passing through the ship-locks at the Iron Gate dams. The last known specimen from the Slovakian section was taken at Komarno on 20 February 1926, and the last from the Hungarian stretch was reported at Mohacs in 1965.

 

According to the IUCN Red List (2010) Acipenser stellatus is critically endangered. However, according to present observations in the Danube, it is close to extinction. The current status of the population, which has suffered severely from overfishing in the past, and the exact location of the spawning sites are not known. More research is needed to effectively conduct conservation measures for this species.