Baltic Herring
The Baltic herring (Clupea harengus membras) is the dominant fish species in the Baltic Sea. They occur in large shoals throughout the Baltic except the Curonian lagoon.
Project's main site - www.balticseaportal.net
Sprat
Sprat (Sprattus sprattus balticus) is abundant species all over the Baltic Seat except brackish bay areas. It is forming large shoals in the areas where waters of different salinity are mixing.
Cod
This is one of the 5 subspecies of the Atlantic cod (Gadus morhua callarias) adapted to the brackish Baltic water.
It is common throughout the Baltic, however the distribution depends on the stock values. 
Flounder
It is Baltic subspecies of the European flounder (Platichthys flesus), abundant throughout the Baltic, except depths below 150 m, the northern part of the Gulf of Bothnia, eastern part of the Gulf of Finland...
Whitefish
Whitefish (Coregonus lavaretus) inhabiting the Baltic Sea has two forms or subspecies: river-spawning and sea-spawning.
Twaite Shad
Twaite shad (Alosa fallax) is one of the three species of the herring family inhabiting the eastern Baltic Sea. 
Viviparous Blenny
One of most interesting demersal species is eelpout or viviparous blenny (Zoarces viviparus). It is the only viviparous species in the Baltic, so-called arctic relict.
Garpike
Garpike (Belone belone) is not well-known to larger public. However, it is easy to distinguish by its long beak-shaped jaws.
Sturgeon
Sturgeon (Acipenser sturio) is anadromous species, wich goes for spawning in rivers. Formerly distributed in the coastal waters all over the European seas and the Atlantic, now it is the most threatened species in this part of the world.