The Danube Delta gives shelter to more than 3.400 species of vertebrate and nonvertebrate animal, distinguishing between the other biosystems through its various european and world-wide unique features.
The Danube Delta's ecosystem is well represented by a large number of fishes such as: carp (Cyprinus carpio), pike perch (Lucioperca lucioperca), sheatfish (Silurus glanis), rapacious carp (Aspius rapax), hake (Vimbo vimbo carinata), as well as the fresh water sturgeons - sterlet (Acipenser ruthenus), Pontic shad (Alosa pontica), sea sturgeons - beluga (Huso huso), Black Sea sturgeon (Acipenser guldenstaetdii), stor sturgeon (Acipenser stellatus), tench (Tinca tinca), bream (Abramis brama), red eye (Scardinius erytrophalmus), caracas fish (Carassius auratus gibelio), perch (Perca fluviatilis), pike (Esox lucius). Marsh lands liable to flooding offer the fish propitious shelters to lay down their spawn and come to maturity, shelters where many species of birds nest, the habitat of some mammals, batrachians, reptiles etc. a special place in the ecosystem being occupied by the floating reed islet, a formation which combines reed roots, herbs, organic remains and soil, which usually floats on the water.
Among the birds which cohabit in this ecosystem we can mention: tufted duck (Aythya fuligula), ferruginous duck (Aythya nyroca), red-crested pochard (Netta rufina), greylag goose (Anser anser), pygmy cormorant (Phalacrocorax pygmaeus), red heron (Ardeola pururea), the great egret (Egretta alba), the egret (Egretta garzetta), the squacco heron (Ardeola ralloides), the spoon bill (Platalea leucorodia), the gray heron (Ardeola cinerea), the night heron (Nycticorax nycticorax), the common pelican (Pelecanus onocratalus), the Dalmatian pelican (Pelecanus crispus), the mute swan (Cygnus olor), the ibis (Plegadis falcinellus).
Most of these birds are protected on strictly delimited surfaces. In the Rosca-Buhaiova reservation you can find approximately 2,000 pairs of common pelicans and 300 pairs of Dalmatian pelicans.
The ecosystem also includes a series of mammal species such as: the otter (Lutra lutra), the mink (Musteola nutreola), the stoat (Mustela ermina aestiva), the wild boar (Sus scrofa), the wild cat (Felix silvestris), the fox (Vulpes melanogaster), the hare (Lepus europaeus). Other mammals have recently adapted: the raccoon dog (Nyctereutes procynoides), the muskrat (Ondrata zibethica), nutrie (Myocaster coypus). Also, the pheasant (Phasianus colchicus) has recently spread over a large area. The key elements of the ecosystem of the ridges are the ridges along the river which at first were covered with willows - which later on, because of the deforestation, were replaced with Canadian poplars.
The fauna of the bank ridges on the seashore is also represented by the racerunner (Eramis argenta desertis), by birds such as the white-tailed eagle (Haliaetus albicella), the great owl (Bubo bubo) and others.
The great number and variety of mammals inhabiting the Danube Delta find here upper places, mainly those avoided by water. Preponderant are here: the otter, the mink, the muskrat, the wild boar, the fox, the wolf, the polecat and the wild cat. The sandy areas give shelter to turtles, adders and colonies of snakes.