Dominant fish species in the Danube

Protection and help for rare fish species: The LIFE+ project's measures are targeted at the fish population of the Danube. Through these measures there should be an improvement of the living conditions for the Danube's fish fauna. From the many fish species in the Danube, certain species are highly endangered and have been listed on an EU-wide register. These species are called FFH species and should particularly benefit from the planned measures.

The common nase (Chondrostoma nasus)

Danube fish: the common nase
Photo: Gerhard Pock

The nase (scientific name "Chondrostoma nasus") is a gregarious fish from the large class of cyprinids. Prominent characteristics are the protruding snout and the downward facing, broad mouth with a horny lower lip and sharp edges. The body exhibits a grey-blue to grey-green colour and a light-coloured belly. The fins are reddish, and during the spawning season nase fish have white spots, the so-called spawning tubercles. These disappear again afterwards.

The nase can grow to more than 50 cm in length and weigh more than 2 kg. Nase fish live in flowing waters which for the most part belong to the barbel region (= epipotamal zone). The diet of these bottom-dwelling shoaling fish consists mainly of algae which they graze from rocks, and of the small organisms that live in this algae growth. In the spring, at spawning time, the nase undertakes extended upstream migration over long distances to reach the spawning grounds. The spawning grounds are gravel banks, on the pebbles of which large amounts of the sticky eggs are laid. During the spawning run, hundreds of nase fish collect in groups, and spawning is an impressive natural spectacle during which the fish often slap their tail fins in the shallow water. After spawning, the nase fish return to their original habitats downstream again.

The nase used to be found in large quantities, but now this fish species is greatly endangered. The regulation of rivers and the concurrent loss of gravel banks and deep pools, as well as the interruption of the migratory paths with transverse structures such as weir systems or ground sills, are the major reasons for the sharp decline over the last decades. Many river-related renaturation projects or measures for fish connectivity have been carried out in sections where the nase is present. By re-establishing connectivity in the rivers and creating suitable habitats, it is hoped that the stocks will recover and spread out in their original habitats again.

The sterlet (Acipenser ruthenus)

Danube fish: sterlet
Photo: Clemens Ratschan

The sterlet belongs to the class of Danube sturgeons and when fully grown is up to about one metre in length. It is not dependent on migration to the Black Sea and lives entirely in the fresh water of the rivers. It is true that the sterlet is not a dominant Danube fish species in the sense of the Water Framework Directive improvement measures, but it is an FFH species and therefore an object of protection in the framework of the LIFE+ project.

Relatively little is known about the sterlet’s mode of life, particularly in respect to the preferred habitats in the river. Gravel banks and rocky structures as well as deep pools are important for spawning and as a habitat. Sterlets still occur in a small population in the Upper Austrian part of the Danube, while there are larger stocks in Slovakia. Over the last years, various initiatives have been founded for the protection of the Danube sturgeons, such as the "Danube Sturgeon Task Force", and in various international EU-sponsored projects, measures for the protection of sturgeons and therefore also the sterlets are being carried out.

The huchen (Hucho hucho)

Two huchens side by side
Photo: Gerhard Pock

The huchen belongs to the family of salmonids, as do for example the trout species. With a weight of up to about 60 kg and a maximum length of about 150 cm, it is the largest member of this family. Nowadays these sizes are no longer achieved by huchens. This can be attributed to the lack of habitats on the one hand, and on the other to the considerably smaller food supply. Huchens prefer quickly flowing, cold, clear, oxygen-rich bodies of water with a varied structure to the riverbed. The huchen is a predatory fish and feeds primarily on other fish.

The huchen is also found in the Danube and is dependent on opportunities to migrate into tributaries. The LIFE+ "Network Danube" project is intended to contribute to the survival and strengthening of the huchen population by constructing fish bypasses and creating extensive connecting waterways. The huchen is a so-called "protected good" in the Natura 2000 areas. Measures to improve their habitats can be supported by EU LIFE funds.

In Austria, the huchen was present in larger rivers in the Danube’s drainage basin. Here it inhabits the grayling and barbel zone. Today, the largest concentration is in the Mur River. Other known huchen rivers are the Danube, Pielach, Melk and Mank, as well as the Drava and Gail. Huchens are also occasionally to be found in the Ybbs, the Salzach, the Inn, the Enns and their tributaries, and usually stem from stocking. The huchen can now only be found in a fraction of the historic bodies of water and is considered highly endangered.

Adult huchens primarily live in deep potholes or trenches. Juvenile fish mainly live in more shallow river areas near gravel banks. Huchens become sexually mature at between 3 and 5 years of age and lay their eggs on gravel banks. This process is called spawning. To reach suitable gravel banks, huchens migrate upstream, often into tributaries as well. This can be observed even today, for example in the Pielach. During oviposition a male (milter) and a female (spawner) position themselves close together over a hollow in the riverbed which the female has made with its tailfin (so-called "spawning pit"). Oviposition and fertilisation take place simultaneously. The water temperature during spawning is about 10 °C.

The incubation period, meaning the time from oviposition and fertilisation until hatching, likewise depends on the water temperature and at a water temperature of 10 degrees is about 26 days.

The striped ruffe (Gymnocephalus schraetser)

The striped ruffe, a dominant Danube fish species, on the gravelly bottom
Photo: EZB TB Zauner

The striped ruffe, also called schraetzer, is a so-called groundfish and therefore mainly to be found at the river bottom. It prefers deeper, sandy-gravelly, slow-flowing parts of the barbel and bream zone. Its diet consists of soil-dwelling organisms, and spawning is from April to May. For this, the spawning individuals migrate upstream to seek out deep, overflowed gravel banks. The sticky eggs adhere to stones and submersed branches in broad jelly-like strings (so-called egg-strings). The striped raffe usually grows to 15-25 cm. It belongs to the Percidae family.