Danube bleak branched rays while the anal fin has fifteen to nineteen. This fish is morphologically quite variable which may be due to local adaptations to various habitats but is more likely to be due to several species being involved. The Danube bleak occurs in slow flowing stretches of rivers flowing into the Black Sea and the Caspian Sea and in the Aral Sea basin, in Alpine rivers among others. It is a migratory fish, spawning in the headwaters of the rivers and then moving down to the lower parts, estuaries and brackish areas of sea. These migratory patterns have been disrupted
Common species because common species shape ecosystems, contribute disproportionately to ecosystem functioning, and can show rapid population declines, conservation should look more closely at how the trade-off between species extinctions and the depletion of populations. Common species Common species and uncommon species are designations used in ecology to describe the population status of a species. Commonness is closely related to abundance. Abundance refers to the frequency with which a species is found in controlled samples; in contrast, species are defined as common or uncommon based on their overall presence in the environment. A species may be locally abundant without being common. However,