The sterlet, Acipenser ruthenus, is an ancient, cartilaginous fish belonging to the sturgeon family, which has existed for over 200 million years. This species is the smallest among the Eurasian sturgeons. The sterlet is an exclusively freshwater inhabitant, distinguishing it from many of its larger, ocean-migrating relatives. Its existence is deeply intertwined with the history of the great river systems of Eurasia.
Unique Physical Traits and Identification
The sterlet is the smallest sturgeon species in the Eurasian region, typically reaching a maximum length of 90 to 125 centimeters and a weight of up to 16 kilograms. A highly distinctive feature is its long, narrow, and pointed snout, which can be quite variable in shape.
Underneath the snout, four long, thread-like barbels hang down, which reach back toward the mouth. These sensory organs detect food along the riverbed, complementing the fish’s toothless mouth, which has a clearly split lower lip. Instead of scales, the sterlet is protected by five rows of bony plates, called scutes, running the length of its body.
These scutes are arranged in specific numbers: 11 to 18 along the back (dorsal), 56 to 71 along the sides (lateral), and 10 to 20 along the belly (ventral). The coloration is generally dark brown or grey on the back, fading to a yellowish-white on the belly. The lateral scutes and the edges of its fins are often lighter, creating a series of pale lines down its sides that aid in identification.
Geographic Distribution and Behavior
The sterlet’s native range spans the river systems of Eurasia, primarily those that drain into the Black, Caspian, and Azov Seas. Its distribution extends eastward through Russia and into the Siberian river basins, such as the Ob and Yenisei rivers. This species is potamodromous, conducting its migrations entirely within river systems, unlike many sturgeons that move between saltwater and freshwater.
Sterlets are demersal fish that favor the deep depressions and channels of large rivers with a steady current. They forage on the riverbed, using their sensitive barbels to locate food. Their diet consists mainly of benthic organisms, including various crustaceans, aquatic insect larvae, and worms.
The life cycle includes an annual spawning migration that occurs in the spring, from mid-April to early June. They move upstream to specific spawning grounds characterized by strong currents and firm substrates of gravel or pebble-stone. Males reach sexual maturity around three to seven years, while females mature between five and twelve years, laying clutches of eggs that can number between 15,000 and 44,000.
Commercial Importance and Conservation Efforts
The sterlet is prized for its flavorful meat and, historically, its roe for caviar. The golden-colored caviar was once considered a delicacy reserved for royalty. The species’ smaller size and faster growth rate have made it a suitable candidate for commercial aquaculture and farming.
Wild sterlet populations are highly threatened across their native distribution. Major factors contributing to this precarious status include overfishing, both legal and illegal, driven by demand for its products. Habitat degradation is a significant threat, particularly the construction of dams and other river barriers that block the fish’s upstream spawning migrations.
Pollution from industrial and agricultural runoff further degrades the water quality and spawning grounds required for reproduction. Conservation efforts focus on curbing these declines through protective measures like temporary fishing bans in heavily affected regions, such as parts of the Danube River. Artificial propagation and restocking programs are being implemented to supplement wild numbers and restore populations.