Physical Characteristics
The bleak has an elongated, fusiform body, strongly compressed laterally, elegant and perfectly adapted for fast swimming in compact shoals at the water surface. The head is small, elongated, and strongly compressed medio-laterally, with large and prominent eyes situated in the anterior half. The snout is short, and the mouth is small, obliquely upward, slightly superior (lower jaw pushed ahead of the upper) - a perfect adaptation for surface feeding on insects that fall on the water. The mouth lacks barbels. The body is covered with very small, thin, extremely deciduous cycloid scales (they detach very easily at touch), creating the characteristic shiny-silver appearance with intense metallic reflections.
The dorsal fin is placed at the middle of the back with a short base and straight or slightly concave margin. The anal fin is very long and characteristic (18-23 rays). The caudal fin is deeply forked with nearly equal lobes. The lateral line is complete and curved. Coloration is spectacular: greenish or bluish back with green-blue metallic reflections, flanks and abdomen bright white-silver with intense metallic sheen, dorsal and caudal fins grey. The iris is silver or yellowish with dark grey spots. Average dimensions are 10-15 cm and 20-40 grams, but exceptional specimens can reach 20-30 cm and 100-350 grams.
Habitat & Distribution
The bleak is a strictly pelagic surface fish that occupies the upper water layers (0-2 meters depth), almost completely avoiding the bottom and very deep zones. In the Danube Delta, the bleak is ubiquitous and extremely abundant in almost all types of aquatic habitats: large and small lakes, permanent and temporary marshes, wide and narrow channels, the main Danube arms with gentle current, and even the moderate brackish zones of the Razelm-Sinoe complex.
It prefers well-oxygenated waters with weak to moderate currents and reasonable clarity. The ideal habitat includes open areas without excessive surface vegetation, where shoals can freely patrol and detect insects falling on the water. The bleak is extremely gregarious and forms spectacular shoals, sometimes numbering thousands or even tens of thousands of similarly sized specimens. It tolerates temperatures between 0°C and over 25°C, with the optimum being 15-22°C. It is sensitive to pollution and low oxygen, functioning as an excellent bio-indicator of water quality.
Behavior & Feeding
The bleak is an opportunistic omnivore specialized in surface feeding, with an extremely varied diet that adapts to the seasonal availability of resources. The main food consists of terrestrial insects that fall on the water (flies, hymenoptera, butterflies, beetles), aquatic insect larvae rising to the surface, zooplankton (daphnia, copepods), phytoplankton (microscopic algae), and occasionally fish eggs and fry when available.
The feeding behavior is characteristic and spectacular: the bleak constantly patrols the water surface in compact shoals, detecting vibrations produced by insects, then attacks rapidly, often jumping completely out of the water to catch prey - this behavior creates characteristic "circles" at the surface that betray the presence of shoals. When a predator attacks, the shoal explodes in all directions in a spectacular silver "explosion", then quickly reforms. Feeding activity is intense in the early morning and evening when insects are most active.
Life Cycle & Reproduction
The bleak reaches sexual maturity very early, at 1-2 years when it is 8-12 cm long, allowing populations to recover rapidly from pressure. Reproduction takes place in late spring and early summer, between May and July, when water temperature reaches and exceeds 15-20°C. The bleak migrates from deeper lakes and channels to very shallow areas (20-80 cm) with flooded vegetation for egg laying.
Fecundity varies with size: a female lays between 1,500-4,000 eggs per year. The eggs are very small (1-1.5 mm diameter), yellowish-translucent, sticky, and adhere to submerged vegetation. Hatching occurs after 3-6 days depending on temperature, with larvae being pelagic and initially feeding on fine zooplankton. Growth is rapid in the first year: the bleak reaches 5-8 cm at 6 months and becomes sexually mature at 1 year. Longevity is relatively low (5-8 years) due to intense predation pressure and rapid metabolism.
Conservation Status
The bleak is classified as "Least Concern" (LC) by IUCN, being one of the most abundant and widely distributed cyprinid species in Europe and Romania. In the Danube Delta, populations are extremely healthy, numerous, and stable. The species benefits from minimal protection in Romania: a minimum legal retention size of 12 cm, a reproduction prohibition period (May-June), but no strict catch limits due to abundance.
The bleak plays an absolutely fundamental and irreplaceable ecological role: it is the central link in the aquatic food chain, transferring energy from plankton and insects to large predators; it constitutes the primary food source for pike, pikeperch, catfish, perch in juvenile stages and for fishing birds; and functions as an excellent bio-indicator of water quality - the presence of numerous bleak shoals signals well-oxygenated, clean, and productive waters. Threats are limited: water pollution, excessive eutrophication, and overfishing with nets for use as live bait.
Sources
