Gymnocephalus baloni Holcik & Hensel, 1974
Conservation status: in Bulgaria: Vulnerable VU [B1bc]; International: IUCN [DD], BeC-III.
General distribution. The Danube River and some of its bigger tributaries in Germany, Austria, the Czech Republic, Slovakia, Hungary, Croatia, Serbia, Bulgaria, Romania and Moldova.
Distribution and abundance in Bulgaria. The species was found along the entire stretch of the Danube River and in the mouths of its tributaries Ogosta, Iskar, Vit, Osam, Yantra and Rusenski Lom [1, 2, 3]. In 2005-2006, single specimens were recorded in the Danube River near Vrav, Novo selo, Vidin, Stanevo, Marten and Sandrovo [our data]. In recent years, the occurrence of the species in the Danube tributaries has not been confirmed.
Habitats. Permanent large rivers.
Biology. It prefers running waters. It reaches sexual maturity in the second year. Spawning occurs at water temperature in the range from 8 to 20°C, in the period April-June. The fecundity of females varies between 13 000 and 53 200 eggs. The species is active during the night. It feeds on benthic invertebrate animals, mainly larvae of insects. It reaches a maximum body length of 132 mm and a weight of 84 g. The life span is 5-6 years. 
Similar species. The ruffe (Gymnocephalus cernuus)and the Schraetzer (G. schraetser). The Danube ruffe is distinguished by the higher depth of the body, the presence of 2 spines on the operculum and the arch-shaped first ray of the anal fin.
Negative factors. River regulation by dams and weirs; water pollution.
Conservation measures taken. None.
Conservation measures needed. Restriction on the river regulation activities and protection of the Danube River against pollution.
References. 1. Karapetkova, 1994; 2. Karapetkova, Zivkov, 1995; 3. Sivkov, 1985; 4. Holcik, 1998.
Authors: Teodora Trichkova, Tihomir Stefanov