There are about 60 indigenous and permanent fish species in Finland. There are fewer fish species in Finland than in countries bordered by oceans. The small number of fish species in Finland is due to the cold climate, low salinity in the Baltic Sea, and slow process of natural propagation.

Fish migrated to the area of Finland as we know it today during the end of the last Ice Age approx. 10 000 years ago or after that. The early phases of the Baltic Sea have had a vital impact on the migration routes of Finnish fish. The first inland fish species to arrive were ones which endure cold water: Atlantic salmon, trout, whitefish, smelt, arctic charr and fourhorned sculpin. Perch, burbot, minnow and pike, which also prefer cold water, were also possibly among the first species.

Once the ice coat had melted completely, the land rose, and Finland lost its connection to the sea. For a while, the Baltic Sea was called the Ancylus Lake, which was larger than the current Baltic Sea. At that point, Finland obtained fish species which require warm water, such as the stone loach, bream, bleak, pikeperch, ruffe and sheatfish.

The opening of the Straits of Denmark approx. 7 500 years ago created the Litorina Sea, which gradually evolved into the current Baltic sea as the land rose. The salty water brought sea water fish such as the seasnail, lumpsucker and snake blenny to the Baltic Sea. It was only at this point, when there was a connection to the Atlantic Ocean, that the eel rose up Finnish rivers.

During the past 100-150 years some changes have been observed in the distribution of Finnish fish. These changes are related to the period of cold climate which prevailed in the 19th century and slightly before that. The sheatfish arrived Finland during the period of warm climate but has since become extinct. The last sheatfish was encounted in a Finnish lake in the 1860s and in the Gulf of Finland in the 1960s.

In order to obtain new valuable fish species in Finland, attempts have been made to naturalise several foreign fish species. The first attempts took place more than 100 years ago. There are fish populations in Finland which originate from foreign planted fish and reproduce naturally: brown bullhead and lake trout from North America as well as peled whitefish from Siberia.

The rainbow trout originating from North America has turned out to be an excellent species suitable for fish farming. Its farming as a food fish commenced in Finland in the 1950´s. However, the rainbow trout has not managed to reproduce in natural waterways in Finland.

Attempts to naturialise some other species, such as the Chinook salmon and starlet, have failed.

Visits by rarely seen species are affected by factors such as variations in the salinity of the Baltic Sea. Fish species which visit the sea area quite often include the kaife and mackerel. Rarer visitors include stellate sturgeon, anchovy, armed bullhead and plaice. The porbeagle, picked dogfish, allis shad and Atlantic bonito have been encountered in Finland only once, and the porbeagle ever seen was dead.

The number of fish species in the waterways in Northern Finland is much smaller than in Southern Finland due to the short growing season, summer. Lake Inarijärvi in Lapland has only 10 indigenous fish species while the corresponding number in Lake Saimaa is 30. The occurrence of a fish species in certain waterways requires that the fish species has had an opportunity to propagate there, that the waterway has suitable depth rations, water quality and temperature, and that the waterway offers suitable spawning places and sufficient nutrition. Propagation can be obstructed by factors such as a watershed divides, waterfalls and power plant dams.

A total of 104 fish species have been encountered in Finland.