View of the Danube at Altenwörth. The focus is on the fish migration aid located next to the Danube.

Spotlight on ecological

The common nase 

Chondrostoma nasus

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 nase is a gregarious fish from the large group of cyprinids.

The striped ruffe

Gymnocephalus schraetser

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.

The scraper is a so-called demersal fish and is therefore mainly found at the bottom of the river.

The sterlet

Acipenser ruthenus

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 sterlet belongs to the group of Danube sturgeons and grows to around one meter in size when fully grown.