Species » Acipenser ruthenus
Sterlet (Acipenser ruthenus). Photo credit: Vasile Oţel – Danube Delta National Institute, Tulcea, Romania
1- General information
The common name of this species is “Sterlet”. The scientific name “ruthenus” derived from the medieval Latin name for Russia which is “Ruthenia”.
The Sterlet is a Eurasian freshwater species inhabiting rivers flowing into the Caspian, Black, Azov, Baltic, White, Barents and Kara Seas. While it is nowadays still the most widely distributed sturgeon species in the Danube River Basin, it formerly ranged from the Delta as far as Ulm in Germany in much higher numbers and also in numerous tributaries like Isar, Inn and its tributary the Salzach, Sio, Raba, Drava, Mura, Sava and its tributaries the Lonja and Kupa, the Morava, Vah and its tributaries the Nitra and Zitava, the Hron, Ipel, Tisza and its tributaries the Bega and Mures, the Zagyva and Bodrog with the tributaries Latorica, Laborec, Uh, Somes and Tamis.
A. ruthenus is the smallest and most short-lived sturgeon species of the Danube River. A maximum length and age of 1.25 m and 24 years, respectively, was reported for the Danube. The Sterlet inhabits the lowland and foothill zones of rivers and only occasionally moves into the brackish water of estuaries for feeding. Tagging experiments have revealed a maximum migration distance in the Danube River of more than 300 km.
The Sterlet usually stays in the current in deep depressions of the riverbed, over stony, gravelly or sandy bottoms. When the water level rises in spring, the fish move onto the flooded lowlands to feed. As the water temperature decreases, the fish form large shoals in the deepest sections of the river or in depressions in the river bed to overwinter without feeding. The main food items in all rivers consist of small bottom-dwelling organisms. Males and females need up to 5 and 7 years, respectively, to mature. The female spawners can carry 7,000 to 108,000 eggs, depending on their size.
4- Spawning migration
During spring floods and suitable water temperature, adult A. ruthenus swim upstream for spawning. In the Middle Danube, timing ranges from April to May when water temperatures are between 8 and 19 °C at water depths of up to 10 m. The optimal temperature for reproduction ranges from 12 to 17 °C. Spawning on the riverbed occurs at depths of 7 to 15 m. The eggs are laid on pebbles and rarely on gravelly-sand bottoms in river stretches with a high current velocity.
After spawning, the spent fishes (= adult fishes after spawning) move slowly downstream to bays, sandy shallows or channels with muddy bottoms, where they feed intensively. Larval and juvenile fish remain at the spawning sites among rocks and stones during their early development, but later the juveniles disperse to feeding grounds.
5- Status and conservation
According to the IUCN Red List (2010) Acipenser ruthenus is vulnerable. After a decline of stocks in the Upper and Middle Danube River in previous centuries, the species recovered from the 1980s. Although it has almost vanished from the Upper Danube (continued occurrence in Germany and Austria depends strongly on stocking, with the exception of one supposed autochthonous relict population on the Austrian–German border), stocks in the Middle Danube (Slovakia, Hungary and Serbia) seemingly recuperated. Thus, in 1980, A. ruthenus reappeared in the Morava River as far as the Suchohrad region, in the lower reaches of the Vah River, and now occurs regularly as far upstream as Bratislava. Main reasons for this reappearance were improved water quality and efforts of artificial propagation and restocking. After 2005, however, an increasing and alarming number of signs showed that the population of the Middle Danube has become unstable and is again rapidly decreasing. A research programme on the reasons for this worrisome development and to determine effective countermeasures is urgently needed.