The brackish nature of the Baltic Sea and the rapid abiotic changes during its geologically young existence are among the factors that have shaped the fish community of our marine areas. Especially important have been the changes in salinity, stratification and current systems.
The fish species living in the Baltic originate roughly from four different sources:
1) Arctic species
2) Boreal marine species
3) Wandering species (anadromous-catadromous)
4) Freshwater species
Most of the marine species present in the Baltic are euryhaline, tolerant to changes in salinity, originating from marine estuarine areas. Most of the marine species living in the Baltic have migrated from the Atlantic and North Sea but some may have spread through a White Sea connection from the Arctic Ocean.
There are 120 marine species living in the North Sea, 69 in the Kiel Bay, 41 in the Baltic Proper, 20 in the Bothnian Sea and 6 in the Bay of Bothnia.
Marine species tend to grow slower and to be generally dwarfed due to metabolic stress caused by decreased salinity. The size of marine fish in the Baltic decreases and growth slows down from the Danish straits in SW to Bay of Bothnia in NE as salinity decreases and climate gets colder. Marine species tend to have higher fecundity and bigger eggs to compensate for the decreased density in brackish water. Freshwater species tend to grow bigger due to the higher productivity in the Baltic compared to many freshwater environments.
The biogeoraphy of Baltic fish species have been formed by mainly abiotic factors. One example is bottom anoxia caused partly by strong stratification and eliminating demersal and bottom living fishes.
Baltic forms of Atlantic planktivorous fishes. Baltic herring (Clupea harengus membras L.) is a form of atlantic herring (Clupea harengus harengus L.) whereas european sprat (Sprattus sprattus L.) is the same species as in the Atlantic. In the Baltic the populations differ more from the Atlantic stock the further one goes from the Danish straits. Baltic herring has an annual migratory cycle between feeding and spawning grounds. There is reported a "homing effect" among baltic herring; fish tend to spawn in the same coastal areas where it hatched. Baltic herring is traditionally divided into spring- and autumn spawning stocks, but in later years this division has been questioned. European sprat keeps offshore all year as it spawns pelagic eggs.
The atlantic cod is a true demersal fish, feeding on benthic macrofauna and practicing cannibalism depending on age and size. The fish is also known as "the vacuum cleaner of the seas". There exists two quite distinct forms of cod in the Baltic; one is the atlantic cod (Gadus morhua morhua L.) and the other is the baltic cod, also known as Bornholm cod (Gadus morhua callaris L.). Baltic cod migrates rarely west but atlantic cod can traver far to the NE during salinity pulses. Cod is a pelagial spawner but its eggs float only in salinities above 10-12 PSU, spawning is thus possible in southern parts of the Baltic only. Cod eggs also need oxygen levels above 2 mL/L to survive, long periods of deepwater stagnation cause dramatic decreases in cod stocks. Presently the stocks of Baltic cod have crashed and have not recovered to stabile densities.
Sand eels (Ammodytes, Hyperoplus)
Shorthorn sculpin (Myoxocephalus scorpius L.)
Longspined bullhead (Taurulus bubalis Euphrasen)
Fourhorn sculpin (Myoxocephalus quadricornis L.)
Viviparous blenny (Zoarces viviparus L.)
Rock gunnel (Pholis gunellus L.)
Snake blenny (Lumpenus lamprataeformis Walbaum)
Sand goby (Gobius minutus Gmelin / Potamoschistus minutus Pallas)
Black goby (Gobius niger L.)
Two-spotted goby (Gobiusculus flavescens Fabricius)
Lumpsucker (Cyclopterus lumpus L.)
Striped seasnail (Liparis liparis L.)
These can be divided into two grops which behave in opposite manner during spawning:
Anadromous species which move from sea to freshwater for spawning, mostly salmon and related species:
Atlantic salmon (Salmo salar L.) spawns during fall with different periods of freshwater life for subadults. Young salmon migrate to the sea and return to the same river for spawning as adults. Sea trout (Salmo trutta L.) has a similar life history. Other anadromous species include Grayling (Thymallus thymallus L.), Common whitefish (Coregonus lavaretus L.), Vimba bream (Vimba vimba L.) and Lamprey (Lampetra fluviatilis L.).
Catadromous fishes move from freshwater to the sea for spawning, European eel (Anguilla anguilla L.) is an catadromous fish from the Baltic.
35 freshwater species live in the Baltic, five species are latecomers or invasive species.
Vendace (Coregonus albula L.)
Common whitefish (Coregonus lavaretus L.)
Broad whitefish (Coregonus nasus Pallas)
Charr (Salvelinus alpinus L.)
Grayling (Thymallus thymallus L.)
Northern pike (Esox lucius L.)
Carp bream (Abramis brama L.)
Zope (Abramis ballerus L.)
Bleak (Alburnus alburnus L.)
Asp (Aspius aspius L.)
White bream (Blicca bjoerkna L.)
Crucian carp (Carassius carassius)
Gudgeon (Gobio gobio L.)
Ide (Leuciscus idus L.)
European chub (Leuciscus cephalus L.)
Ziege (Pelecus cultratus L.)
Eurasian minnow (Phoxinus phoxinus L.)
Roach (Rutilus rutilus L.)
Rudd (Scardinius erythrophthalamus L.)
Tench (Tinca tinca L.)
Burbot (Lota lota L.)
Spined loach (Cobitis taenia L.)
Stone loach (Noemacheilus barbatulus L.)
Wels catfish (Silurus glanis L.)
Three-spined stickleback (Gasterosteus aculeatus L.)
Ninespine stickleback (Pungitius pungitius L.)
European perch (Perca fluviatilis L.)
Ruffe (Gymnocephalus cernua L.)
Zander (Stizostedion lucioperca L.)
Bullhead (Cottus gobio L.)
Alpine bullhead (Cottus poecilopus Heckel)
