The sterlet sturgeon, Acipenser ruthenus, is one of the smallest species in the ancient Acipenseridae family. Native to Eurasia, this fish inhabits large rivers that drain into the Black, Azov, and Caspian Seas. Its lineage traces back to the late Miocene, with modern species populating the Black Sea basin during the Pleistocene era.
Distinctive Physical Traits
The sterlet possesses a slender, elongated body, which sets it apart from its larger, more robust relatives. Its body is protected by five rows of bony plates called scutes, a characteristic feature of sturgeons. There are 11–18 dorsal scutes, 56–71 lateral scutes, and 10–20 ventral scutes, with numerous smaller bony plates scattered between these main rows.
A key identifying feature is its long, narrow, and slightly upturned snout, though its length and shape can be quite variable. Younger specimens often have more pointed snouts, while older fish may develop blunter ones. Below the snout hang four heavily fringed barbels, which are sensitive appendages used to detect food. The lower lip is distinctly split in the middle.
The back is a dark greyish-brown, fading to a yellowish-white on the belly. The scutes are often a contrasting dirty white color. While they can reach a maximum length of 125 cm and a weight of 16 kg, most sterlets are much smaller, usually under 100 cm.
Ecological Niche
The sterlet is a freshwater fish that thrives in the currents of large rivers, seldom venturing into large lakes. It occupies the benthopelagic zone, meaning it lives and feeds in the water column just above the riverbed. The fish shows a preference for deep depressions in the riverbed, although younger sterlets can be found in shallower, sandy areas.
As a bottom-dweller, its diet consists mainly of benthic organisms. Using its sensitive barbels to probe the sediment, it locates and consumes crustaceans, mollusks, worms, and various insect larvae. This feeding strategy helps control invertebrate populations on the river floor.
Unlike many other sturgeon species known for extensive migrations, the sterlet behaves as a resident fish. It undertakes shorter migrations, moving upstream within the river system for spawning purposes in the spring. This reproductive journey begins when water temperatures reach 9 to 11°C for males and 12 to 13°C for females, with spawning occurring at an optimal temperature range of 12 to 17°C.
Role in Aquaculture and Conservation
The sterlet’s characteristics make it a significant species for aquaculture. Its relatively small size, coupled with a faster rate of maturation compared to other sturgeons, makes it well-suited for farming. Males mature between 3-7 years and females between 4-12 years. Its adaptability has also made it a popular ornamental fish for large private ponds and aquariums.
Despite its prevalence in aquaculture, wild sterlet populations face considerable threats. The International Union for Conservation of Nature (IUCN) lists the species as vulnerable due to pressures like overfishing, pollution, and habitat degradation. The construction of dams has been particularly damaging, as it fragments habitats and blocks the short migratory paths necessary for spawning.
Conservation efforts are underway to protect wild populations. In some regions, such as Hungary’s Danube water system, the sterlet has been classified as a “non-catchable” species to allow stocks to recover. Another concern is hybridization with non-native sturgeon species, such as the Siberian sturgeon, which threatens the genetic integrity of native populations.