The huge protected area of Danube Delta Biosphere Reserve (DDBR) (5800 km²) shelters 137 fish species from Animal Kingdom, Phyllum Chordata, Subphyllum Vertebrata, belonging to 3 Classes (Actinopterygii bony-fish which are mostly present, Petromyzontida-lampreys species accidentally present in the area and Chondrichthyes - marine sharks species) and 20 Orders, 45 Families, representing about 75 % Romanian fish fauna, clasified in:
- 129 native fish species:
1. 53 freshwater fish (including Carassius gibelio and Cyprinus carpio considered to be native to Danube delta after long debates).
2. 21 euryhaline species divided into: 12 euryhalin half-migratory or expansion species like gobiids, aterina, Syngnathus abaster and 9 long distance migratory fish species: anadromous like shads and sturgeons, but also catadromous like eel).
3. 55 exclusively marine fish species (probably much more in number considering other species are around in Black Sea basin waiting to enter in Danube Delta Biosphere Reserve limits). Some of them enter in freshwater gulfs for feeding like young mullets.
- 8 exotic non-native fish species: 7 freshwater fish species (Hypophthalmichtys molitrix, H. nobilis, Ctenopharyngodon idella, Pseudorasbora parva, Percarina demidoffi, Perccottus glenii) and 1 marine fish species (Mugil soiuy).
But the species number is not fixed in DDBR; being an open system, it is variable due to continuous dynamics of fish movements, so new species appear regularly in the system (like Perccottus glenii in 2007 another 69 species are around DDBR and can increase the number of fish species).
Totally 56 exclusively marine, 21 euryhaline and 60 freshwater fish species are present in Danube Delta Biosphere Reserve but sometimes in freshwaters gulfs or paramarine lakes enter some marine species or some freshwater fish species can penetrate Black Sea brackishwater.