Acipenser ruthenus Linnaeus, 1758
Conservation status: in Bulgaria: Endangered EN [A1a,b], BDA-II, IV; International: IUCN [VU A1c+2d], BeC-III, CITES-II, HD-V.
General distribution. The rivers flowing into the Caspian Sea, the Black Sea, the Sea of Azov, the Baltic Sea, the White Sea, the Barents Sea and the Kara Sea. In the Danube River and some of its bigger tributaries – as far upstream as Austria, and in the past, upstream to Germany.
Distribution and abundance in Bulgaria. The species is found only in the Danube River [1, 2, 3, 4]. In the past, it was also recorded in the lower reaches of some of its tributaries: the Iskar River, upstream to the region of Lukovit , and the Yantra River upstream to the village of Dolna Studena . In the period 1960-1974, the sterlet comprised about 3.7% of the total catch of all fish species, and 58.7% of the catch of sturgeons in the Danube River . In 1995-2002, its share in the catch of sturgeons considerably decreased: only 3.7% .
Habitats. Permanent large rivers with swift current and gravel bottom. The species is found rarely in estuaries.
Biology. A bottom, mainly fresh water species. It is found very rarely in brackish water, always near the mouths of rivers . In winter, it aggregates in bottom holes, and in spring (April-June) it migrates upstream for spawning. The males reach sexual maturity at the age of 3-6 years, 1-2 years earlier than the females. Eggs are laid in swift current over gravel bottom at water temperature of 12-17°C. The fecundity of females ranges between 4 900 and 140 600 eggs. It feeds on crustaceans, insect larvae, worms, small mollusks and eggs of other fish. The life span is 22-24 years, and females live longer than males .
Similar species. In the Bulgarian section of the Danube River and the Black Sea, another 5 species of sturgeons have occurred: the beluga (Huso huso), the Russian sturgeon (Acipenser gueldenstaedtii), the ship sturgeon (A. nudiventris), the starry sturgeon (A. stellatus) and the common sturgeon (A. sturio). The sterlet is distinguished from the beluga, the Russian sturgeon, the starry sturgeon and the common sturgeon by the higher number of lateral scutes (over 50), and from the ship sturgeon by the interrupted at center lower lip.
Negative factors. The main threats to the species are water pollution, overfishing and poaching. To a lesser degree, river regulation by dams and weirs.
Conservation measures taken. The species was included in Annexes II and IV of the Biological Diversity Act (2002). An Action Plan for the Conservation of Sturgeons in Bulgaria has been adopted (Ministry of Environment and Water, 2004). A temporary ban on catch of sturgeons in the Black Sea was imposed (Ministry of Agriculture and Forestry, 2006).
Conservation measures needed. Imposition of a total moratorium on sturgeon fishery in the Danube River and the Black Sea. Reintroduction of the species into the Danube River. Protection of the Danube River against pollution.
References. 1. Kovatcheff, 1923; 2. Drensky, 1951; 3. Marinov, 1978; 4.; Karapetkova, Zivkov, 1995; 5. Chichkoff, 1939; 6. Karapetkova, 1972; 7. Vassilev, Pehlivanov, 2003; 8. Svetovidov, 1964.
Authors: Teodora Trichkova, Tihomir Stefanov