Sterlet – Acipenser ruthenus (Hegediš, 2007)
DINOSAUR FISH: Sturgeon fish species of the Danube River
Mirjana Lenhardt, Marija Smederevac-Lalić
Key words: sturgeon, biology, importance
Sturgeons are very old group of fish species dating back more than 250 million years ago. Their ancestors lived together with dinosaurs, but the sturgeons managed to survive the period when dinosaurs became extinct. That’s why they look like living fossils.
Figure 1. Neighbors from the Jurassic period (Jarić, 2009)
Do you know how many sturgeon fish species live in the Danube?
- Beluga (Huso huso)
- Russian sturgeon (Acipenser gueldenstaedtii)
- Ship sturgeon (Acipenser nudiventris)
- Stellate sturgeon (Acipenser stellatus)
- Sterlet (Acipenser ruthenus)
- European sturgeon (Acipenser sturio)
Figure 2. Representatives of sturgeon family characteristic for Danube River Basin (Vassilev, 2006)
Nowadays family comprises 27 sturgeon species inhabiting the northern hemisphere. Once in the Danube lived six sturgeon species, but one of them disappeared (Atlantic sturgeon – Acipenser sturio) and 1950s it was last seen in the Danube. While Ship sturgeon (Acipenser nudiventris) is almost extinct, last was seen in 2009. The other four species: beluga (Huso huso), Russian sturgeon (Acipenser gueldenstadtii), stellate sturgeon (Acipenser stellatus) and sterlet (Acipenser ruthenus) still live in the waters of the Danube River basin.
Beluga, Russian sturgeon and stellate sturgeon are anadromous species that grows in the river, then younger migrates downstream and enters into the Black Sea, where it lives until the fish become fully mature and go to spawn in the rivers. The largest among them is a beluga that can live over a hundred years and reach a weight more than 1000 kilograms. Males become fully mature at age 11, but females later, when they reach 13 years old. Russian sturgeon is smaller and reaches a length of 236 cm, and maximum of 33 years. Males become fully mature at 11 years old and females at 12 years. The stellate sturgeon is the smallest of anadromous sturgeons and reaches a maximum length of 218 cm and 35-years, while males become fully mature at 3 and females at 7 years. The ship sturgeon can be as long as 221 cm and 36 years old, and males become fully mature at age 6, females at 12 years old. Sterlet is a potamodromous species, migrating only within the river, and it is the smallest sturgeon species with a maximum length of 125 cm and can reach the age of 24 years. Sterlet males achieve fully maturity with 3 years and females at 4 years.
Sturgeon species is easily distinguished from the other types of fish by five longitudinal rows of bone shields, scutes, one set along the middle line back, two on the sides of the body and two rows along the ventral side of the body. Scutes are modified ganoid scales. In addition to shields, sturgeons are recognizable also for their elongated bodies, characteristic head with long flattened rostrum, barbels, and elongated upper tail lobes.
Figure 3. Two juvenile sturgeon species (Suciu, 2015)
The mouth is always on the lower side of the head, and in front of them, that is, between the mouth and the top of the throat, there are four barbels. These barbels serve as sensors to find food. They have mouths that can be dislodged and suck up food like a vacuum cleaner. Sturgeons do not have teeth. Beluga fed primarily to fish, and crustaceans and molluscs, while Russian sturgeon, stellate and ship sturgeon feeding on benthic organisms invertebrates and occasionally small fish. Sterlet feeds mainly on benthic invertebrates, but also on fish eggs and larvae.
Figure 4. Sturgeon mouth (https://dnr.wi.gov/topic/Fishing/images/sturgeon/sturgeon_mouth.jpg)
Sturgeons have pectoral and abdominal paired fins, and odd dorsal, anal and caudal fin. Caudal fin is heterocercal, just like sharks. Cross section of the pectoral fin bones is used to determine their age. On the cross-section, annual rings are seen as at the intersection of the tree.
Figure 5. This is cross section of the starlet pectoral fin bone. Can you guess the age of this starlet?(Jarić, 2010)
Sturgeons are characterized by very long life cycles of up to over 150 years, late maturation and many species grow very large (up to 6 – 7 meters). From the drawing below one can see how large they can be compared to human kind.
Figure 6. The size of the person is presented in relation to the maximum size that can reach each of the sturgeon species: beluga, European (Atlantic) sturgeon, Russian sturgeon, stellate sturgeon, ship and starlet (Jarić, 2010).
Sturgeon species through history
An archaeological site from the Mesolithic and early neoliths (about 6000 years BC), the settlement Lepenski Vir, located on the Danube bank near the big vortices, known as a significant fishing place (Figure 1). There are the remains of sturgeon species, and it is assumed that the sturgeon species represented idols for the residents of this settlement. From this period there are sculptures, whose mouth resembles the mouth of the sturgeon and which have bone shields on the head (Figure 2).
Figure 7. Archaeological findings of preserved buildings in Lepenski Vir (Živaljević I., 2012)
Figure 8. Sculptures that represent fishlike human being (Živaljević I., 2012)
Figure 9. The shape of the mouth in two types of sturgeon (Živaljević I., 2012)
Special stone poles were found to be used for hunting sturgeons, especially beluga (Figure 10).
Figure 10. Fishing tools used for sturgeon species (above) and the way they were hunted (bottom picture) (Živaljević I., 2012)
Fences on the Danube
In XIX century wooden fences were constructed on the Danube River, which were used for catching sturgeon species, while there were still river vortices and until the Danube was still unregulated. Lease was paid for the setting up these structures at places where the river vortices were located and where the catchment sites were suitable for sturgeons.
Figure 11. Wooden structures on the vortices of the Danube River (Петровић, 1998).
Figure 12. Fences (wooden structures) on the Danube River (Friedrich, 2012)
After the regulation of the Danube at the end of the nineteenth century there was no longer possible to find a suitable place for this kind of sturgeon fishing. Later, in the XX century, the longline strands were used to catch sturgeon, which are made of rope on which were listed the big hook, but without bait. These strands were placed at the bottom and each hook had its float that allowed her to hover in the water. The hooks are regularly sharpen and when the beluga and other sturgeon passed by the hook, the hook would be stucked to her skin (scutes) and thus would be caught.
Figure 12. Scheme of longline with hooks and caught beluga sturgeon (Ristić, 1963)
Figure 13. Cork floats, multilayers fishing (Ristić, 1963)
Figure 14. The fishermen on the boat prepare to throw longline (Lenhardt, 2004)
The catch of sturgeon species was banned on the Danube River in 2006, but there is still an illegal catch.
Figure 15. This specimen of beluga was preserved and kept in the Museum of Natural History in Vienna. This sturgeon swam the Danube from the Black Sea almost to Vienna (Lenhardt,2008)
After building the Iron Gate dams at the end of XIX century migrations of sturgeon were blocked at the 863 river km of the Danube.
Due to human impact sturgeon species and endangered. These, important ancient species, should be protected and saved from extinction because they have ecological value, economical and cultural value for the society. We should respect and guard them much more, because they are on the planet much longer then us and have enabled our ancestors to survive in the past. Therefore it is important do all necessary steps to protect them. More about the topic of protection sturgeon you can find in the article Is eating caviar a treat to the survival of sturgeons?
- Friedrich, T. (2012). Master Thesis: Historical distribution, current situation and future potential of sturgeons in Austrian rivers. Vienna.
- Jarić, I. (2009). Master’s thesis: Population viability analysis of the Danube sturgeon. Uppsala University.
- Jarić, I. (2010). PhD thesis: Histopathological changes and heavy metal accumulation in sterlet (Acipenser ruthenus L.) populations from the Danube River. Belgrade University.
- Lenhardt, M., Hegediš, A. & Jarić, I. (2005). Action plan for sturgeon species management in fishery waters of Republic Serbia. – Institute for Biological Research “Siniša Stanković”, Belgrade; Developed for Ministry of Science and Environmental Protection of Republic Serbia, 1-21.
- Петровић, М. (1998). Сабрана дела Књига 14 Рибарство, Завод за уџбенике и наставна средства Београд.
- Ristić, M.D. (1963). Ribarstvo biološka studija Acipenseridae Jugoslovenskog dela donjeg Dunava. Ribarstvo Jugoslavije, godina XVII sveska 2, 3, 4. –Zagreb.
- Suciu, R. (2015). Final Report: Fish behaviour preparatory study at Iron Gate Hydropower dams and reservoirs. European Investment bank.
- Vassilev, M. (2006). Lower Danube – the latest refuge for surviving of sturgeon fishes in the Black Sea Region. In Hubert, P. (ed.) Water Observation and Information System for Decision Support. Conference Proceedings, Balwois, Ohrid, Macedonia.
- Živaljević, I. (2012). Big Fish Hunting: Interpretation of stone clubs from Lepenski Vir, in N. Vasić (ed.) Harmony of Nature and Spirituality in Stone (Proceedings of the 2nd International Conference in Kragujevac, Serbia, March 15-16, 2012), Stone Studio Association: Belgrade: 195-206. PDF
Top of Form
anadromous – fish born in freshwater, then migrate to the ocean as juveniles where they grow into adults before migrating back into freshwater to spawn.
fully mature – fully grown, become adult, able to reproduce
potamodromous – fish born in upstream freshwater habitats, then migrate downstream (still in freshwater) as juveniles to grow into adults before migrating back upstream to spawn.
bone shields, scutes – sturgeon cover, hard plates instead of scales in ordinary fish
ganoid scales – specific scales in sturgeons are greatly enlarged into armour plates along the sides and back
rostrum – beak, structures in different groups of animals, evolved from their upper jawbones
heterocercal fin – a tail fin with unequal lobes in which the vertebral column turns upward into the larger lobe as in sharks