Sterlet sturgeon

Acipenser ruthenus

Family: Acipenseridae | Genus: Acipenser

The sterlet sturgeon (Acipenser ruthenus) is the smallest member of the sturgeon family, found in the major Eurasian rivers flowing into the Black, Caspian and White Seas. It is easily recognised by its elongated body, rows of bony scutes, and a long, pointed snout with sensory barbels. Moreover, the sterlet sturgeon lives in shoals near the riverbed and, like other sturgeons, loses its teeth as it matures. This long-lived species can survive for more than two decades and reach up to 1 metre in length, though larger individuals have occasionally been recorded.

The sterlet is a threatened species, currently classified as Endangered (EN) by the IUCN, due to overfishing, river habitat degradation, and the interruption of its migratory routes.

Geographical distribution

The sterlet is widely distributed across Eurasia, inhabiting large rivers that drain into the Caspian, Black, Azov and White Seas, as well as Siberian River basins extending to the Yenisei River.

Unlike other anadromous sturgeons (species that live in salt water but migrate to fresh water to spawn), the sterlet is potamodromous, meaning it completes its entire life cycle within freshwater environments. The truth is that many populations have suffered severe declines and fragmentation due to dams, pollution  and  hydrological changes. Conservation, restocking and captive breeding programmes are currently underway, particularly in the middle and upper Danube River.

Migratory (anadromous) populations have now disappeared, and the species’ range has been greatly reduced by human impacts and habitat fragmentation.

Reintroduction and breeding efforts are helping to sustain surviving populations in parts of Central and Eastern Europe.

Habitat

This species is strictly freshwater and prefers:

Large, deep rivers with strong currents and sandy or gravelly bottoms.
Well-oxygenated, cool waters (10–18 °C).
Flooded areas during river rises, which provide additional feeding grounds.

Although the sterlet can adapt to pond farming, its optimal habitat remains natural river sections with moderate to strong flow and clean substrates.

Lifestyle and behaviour

The sterlet shows clear adaptations to a benthic lifestyle, reflected in its social structure, diurnal activity patterns, reproductive strategies, and interaction with the river ecosystem.

Social behaviour:

Lives in shoals near the riverbed, forming  gregarious, non-territorial groups that often coexist with other bottom-dwelling fish species.
Most commonly found in groups during the juvenile stage and outside the breeding season.

Daily activity:

Primarily diurnal: active during the day while foraging on the river bottom using its sensitive barbels.
At night, or when inactive, it remains close to or resting on the riverbed.

Reproduction:

Oviparous species: during spring spawning season, adults migrate upstream to gravel or stony riverbeds with strong currents.
Females lay 10,000–150,000 eggs on average (depending on size), which adhere to the substrate and hatch after about four days.
Males mature at 3–5 years, and females at 5–8 years.
Spawning frequency varies in natural conditions; reproduction occurs every 1–2 years.

Interaction with its environment:

Plays an important role as a benthic predator, feeding on invertebrates and small fish.
Its bony scutes provide protection from predators, while its barbels detect prey in turbid waters.
The absence of teeth in adults determines its suction-feeding behaviour.
Extremely sensitive to pollution and river alteration, it serves as a bioindicator of environmental quality.

Diet

In the wild

Molluscs, crustaceans and small fish.
Insect larvae and other benthic invertebrates.

Under human care

Protein-rich diets formulated for sturgeons and bottom-feeding fish.
Feeds containing fish meal, larvae, and animal protein supplements.

Conservation status

The sterlet sturgeon (Acipenser ruthenus) is listed as Endangered (EN) on the IUCN Red List.

Main threats:

Overfishing, especially for its meat and caviar.
Habitat fragmentation and pollution, caused by dams, canals and other barriers that block spawning migrations.
Reduced natural reproduction due to disrupted river systems.

Captive breeding, reintroduction and habitat restoration programmes are being implemented in several European countries.
However, wild populations continue to decline, and urgent, coordinated conservation actions are essential for the species’ survival.