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Fish

Ruffe

Gymnocephalus cernua

Ruffe

General Overview

The petrel is a small percid, abundant in the lakes and slow waters of the Danube Delta, having a characteristic appearance with a large head and bulbous eyes. It is one of the most common perches in stagnant and slow-moving waters. It has partially fused dorsal fins, unlike the *Zingel* genus. Robust body covered with rough scales. Day and night activity. Gregarious, forms banks. It has become invasive in the North American Great Lakes where it causes damage to native ecosystems.

Physical Characteristics

The ruffe has a robust body, slightly compressed laterally, with a large head and large bulging eyes — distinctive features of the genus Gymnocephalus. Unlike related species of the genus Zingel, the ruffe has its two dorsal fins partially united or very close together, without a clear separation. The first dorsal fin has strong spiny rays, while the second has soft rays. Ctenoid scales, rough to the touch, cover the entire body including the cheeks, unlike the common perch. The coloration is brownish-grey or olive with dark spots and mottling on the flanks, providing excellent camouflage on muddy bottoms. The belly is lighter in color, almost whitish. Typical dimensions are 10–15 cm, with a maximum of 20 cm and weights of 80–150 grams.

Habitat & Distribution

The ruffe has a wide distribution across Europe and northern Asia, from European lowland rivers to Siberia. In Romania it is present in the Danube, the Danube Delta, lakes, ponds, and slow-flowing rivers of the lowlands. It prefers stagnant or slow-current waters with soft bottoms (mud, sand) and moderate depths. It is a benthic, gregarious species that forms large schools in lake waters and the delta's canals. It tolerates temperature variations and moderate levels of eutrophication well, which explains its abundance in the less clean waters of the inner delta. Preferred depths range between 0.5 and 5 metres, often associated with submerged vegetation or bottom structures.

Behavior & Feeding

The ruffe is an active benthic carnivore, with both diurnal and nocturnal activity. It hunts in large schools, consuming larvae of aquatic insects, small crustaceans, molluscs, worms, and especially the eggs of other fish — a behavior that can significantly affect the recruitment of economically valuable species. Its capacity to consume large quantities of eggs makes it a disruptive factor during the spawning periods of other species. Schools of ruffe systematically patrol the water bottom, using good eyesight and the lateral line to detect prey. The species is tolerant of conspecifics, forming dense aggregations during periods of active feeding.

Life Cycle & Reproduction

Sexual maturity is reached at 2–3 years of age. Reproduction takes place between April and June, when water temperature exceeds 10–12°C. Females deposit 50,000–100,000 small eggs on aquatic vegetation or stones, with no subsequent parental care. Incubation lasts 5–12 days depending on temperature. Growth is relatively rapid in the first years, with individuals reaching maturity at dimensions of 10–12 cm. Lifespan is 7–11 years, although natural mortality through predation is high in the first years of life.

Conservation Status

The ruffe is classified as Least Concern (LC) by the IUCN, with native European populations being stable and abundant. In the Danube Delta it has no protected species status and there is no minimum legal fishing size. Its ecological role is complex: on one hand it contributes to the control of invertebrate populations, on the other hand the mass consumption of eggs can negatively affect valuable fish species. The most critical situation regarding the ruffe is its invasive status in the Great Lakes of North America, where it was accidentally introduced through ships' ballast water in the 1980s and has caused significant declines in native species through competition and egg consumption. The main threats in European habitats are water quality degradation and excessive eutrophication.

Sources

  • FishBase: Gymnocephalus cernua
  • Wikipedia: Ruffe (Gymnocephalus cernua)
  • IUCN Red List: Gymnocephalus cernua