The salinity gradient in the Baltic Sea means that the numbers of species living in the Baltic Sea decrease from the Kattegat and Skagerrak to the Gulf of Bothnia. The former two areas are dominated by marine species, while mainly freshwater species of flora and fauna are found in the Gulf of Bothnia. The low salinity makes the Baltic Sea a species-poor marine area.

Around the Åland Islands, the salt content of the seawater is about 5 or 6 PSU and is home to a mixture of marine and freshwater species. In Finland, the number of fish species found in Åland is high because not only does it have many freshwater species, it also has more marine species than elsewhere in the country. There are also many migratory species in Åland, which have nursery areas around Åland, and spawning grounds in both Sweden and Finland.

By 2018, a total of 58 species of fish had been found in Åland. Of these, 16 are freshwater species, 28 are marine, and 8 are migratory. In addition, six invasive species have also been found here.

High salinity favours marine fish species

Compared to the coastline of the Finnish mainland, the seawater around Åland has a relatively high salt content. Similar salinities are also found in the southern Archipelago Sea and at the mouth of the Gulf of Finland. Marine species are more common in these areas.

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Fish species of the Finnish marine areas

Commonly occurring marine fish species of the Åland Islands include herring, sprat, three-spined stickleback, cod, and bullheads. In general, the Åland Islands are also rich in small-sized species, such as common- and sand gobies, the viviparous blenny, straight- and broad-nosed pipefish, as well as sand eels.

It has recently been discovered that there are two different species of flounder in the Baltic Sea: the newly-discovered Baltic flounder, i.e. Platichtys solemdali, and the more common European flounder, i.e. P. flesus. The Baltic flounder is more common around Åland, while its more common cousin occurs in numerous parts of the southern Baltic Sea.

There are many strange and often quite unusual fish species in Åland. Such species are usually so small that they are rarely caught in regular fishing gear and therefore, are only rarely encountered.

Many species live solely on the bottom and of these, some are found more often. These include; turbot, black goby, long-spined bullhead, short-horned sculpin, lumpfish, two-spotted goby, and the fifteen-spined stickleback. Some species, such as the sea-snail, plaice, snake-blenny, and butterfish, have been observed once or only on a few occasions. Species living in the open sea include; garfish, greater sand eel, mackerel, and anchovy.