Common Bleak
Alburnus alburnus
The common bleak is a slender, laterally compressed freshwater fish with a superior mouth and brilliant silvery scales. It prefers the upper water layers of standing or slow-flowing waters and is widely distributed across Europe. The species is known for forming large, dense schools and for its agility in catching insects directly from the water surface.
Details
Identification
Superior mouth with a steeply upward-pointing cleft, brilliant silvery sides, long anal fin with 17-20 rays.
Social behavior
Highly gregarious schooling fish that lives in large groups in open water near the surface.
Diet
Feeds primarily on zooplankton and insects that fall onto or hover just above the water surface.
Hunting strategy
Active searching for plankton in open water and rapid striking at insects on the surface.
Spawning substrate
Gravel, stones, roots, or aquatic plants in shallow, well-oxygenated water.
Overwintering
Retreats to deeper, low-flow water zones during winter months and reduces metabolic activity.
Ecology
Ecological role
Important primary consumer of zooplankton and a central prey species for predatory fish and waterfowl.
Natural predators
Pike, pike-perch, European perch, asp, cormorant, kingfisher, and terns.
Competitor species
Other planktivorous fish such as roach, sunbleak, or juvenile bream.
Ecosystem service
Regulation of zooplankton populations and transfer of energy to higher trophic levels.
Threats
Loss of shallow water habitats due to shore engineering, heavy eutrophication, and chemical water pollution.
Scientific profile
Morphology & ID
Coloration
Back greenish-grey to bluish-green; sides and belly intensely silvery with a metallic sheen.
Distinguishing features
Superior, steeply upturned mouth; long anal fin (17-23 branched rays); scales are very loose; scaleless keel between pelvic fins and anus.
Confusion species
Sunbleak (Leucaspius delineatus - incomplete lateral line), Dace (Leuciscus leuciscus), juvenile Asp (Leuciscus aspius).
Sexual dimorphism
Males develop fine nuptial tubercles on the head and back during spawning season; females are usually deeper-bodied.
Habitat
Fish region
Bream region (metapotamal), also in large lakes and brackish waters of the Baltic Sea.
Preferred zone
Pelagic, prefers the water layers near the surface (epilimnion).
Flow preference
Limnophilic to eurytopic; prefers standing or slow-flowing waters.
Substrate preference
Gravel, sand, or submerged vegetation in shallow littoral zones.
Oxygen requirement
Moderate to high; sensitive to oxygen depletion in the surface layer.
Migration
Migration behaviour
Short-distance migrations within the water system to suitable spawning grounds near shores or in tributaries.
Reproduction
Spawning substrate
Phytolithophilic; eggs are deposited on stones, roots, or aquatic plants in shallow water.
Larval phase
After hatching, larvae briefly attach to substrate; after yolk sac absorption, they transition to a pelagic lifestyle.
Parental care
None.
Diet
Feeding type
Planktivorous and insectivorous (surface insects).
Diet juvenile
Small zooplankton such as rotifers and nauplii.
Diet adult
Zooplankton (cladocerans, copepods), insect larvae, surface insects, occasionally algae.
Feeding strategy
Active searching in the open water column and targeted snapping at insects on the water surface.
Schooling
Strongly pronounced schooling behavior; often forms massive schools in the open water zone.
Ecological role
Role in food web
Central link in the food chain; converts zooplankton into fish biomass for predators like pike-perch and pike.
Natural predators
Pike, pike-perch, asp, perch, cormorant, goosander, terns.
Competitor species
Other planktivorous species such as roach (Rutilus rutilus) or sunbleak (Leucaspius delineatus).
Parasites
Ligula intestinalis (tapeworm), Dactylogyrus species (gill flukes).
Bioindicator function
Indicator for moderately polluted waters (quality class II); indicator for high pelagic productivity.
Fisheries & legal
Fishing regulation
In most German federal states, there is no statutory closed season or minimum size.
Closed season
None
Economic use
Historically used for 'pearl essence' (guanine) for artificial pearls; today primarily used as bait fish for angling.
Protection & threats
Main threats
Riverbank stabilization and loss of shallow water habitats; heavy predation in anthropogenically altered waters.
Population trend
Stable; one of the most common fish species in Central Europe.
Conservation measures
Restoration of shore zones; maintaining the connectivity of watercourses.