Order: Pleuronectiformes Family: Right-eyed flatfishes Pleuronectidae
Foto: Lauri Urho
Description: Three of the flatfish species in the Baltic Sea belong to the family of right-eyed flatfishes, and of these the most common and abundant is the flounder. Good criteria for distinguishing the flounder from the plaice (Pleuronectes platessa) and the dab (Limanda limanda), all of which are flat, oval-shaped fishes, are the bony prickles on both sides of the lateral line and at the base of both the dorsal and caudal fins in the flounder. The eyes of a flounder are usually, though not necessarily, on the right side. Although a genuine marine species, the flounder can tolerate low salinity levels, and even fresh water.
Distribution: The flounder is found in all European coastal waters from the Black Sea to south of Novaya Zemlya, but not in the waters of Iceland, Greenland or Spitsbergen. In Finland, it is scarce or occur only occasionally in the northernmost Gulf of Bothnia, in coastal waters north of the city of Pori and in the eastern Gulf of Finland. It has also been observed in Pulmankinjärvi, a lake in the far north of the country, into which it is known to migrate from the Barents Sea along the river Tenojoki (Tana).
Reproduction: Flounder spawn for the first time at 2−3 years of age, males usually somewhat earlier than females. In the southern Baltic Sea, spawning takes place from March to April and the spawning grounds are in deep water on sand or mud bottoms and at the edge of deeps. The floating eggs are small and transparent and are incubated in the intermediate water layer. In northern areas, the spawning behaviour of flounder changes, and the eggs are released in shallow water off the coast. Owing to the low salinity, the eggs do not float but sink to the bottom. The hatched flounder larvae are 3−4 mm long and symmetrical. Reproduction rarely succeeds at a salinity of less than 6‰,. As the salinity of Finnish coastal waters does not exceed 5−7‰, variations in salinity probably have a marked influence on the spawning success of flounder.
Diet, growth and migrations: Flounder larvae hatched in the summer metamorphose from symmetrical juveniles to adult-like young, and their eyes move to the darker upper-side of the body. Before the end of the summer, young flounder, about 2 cm long, settle on sandy bottoms, where they spend the first two years of their life. In the autumn, the one-summer old juveniles on the Finnish coast are usually 2.5 –5.0 cm long. In the western Gulf of Finland, the female usually attains a weight of 600 g in 10 years. The male grows somewhat more slowly. The growth rate varies considerably, however, as in the southern Baltic Sea, for instance, a five-year-old flounder is already about 30 cm long. The reason for the slow growth in Finnish coastal waters is the low salinity, requiring flounder to expend much energy to maintain water balance. The mouth of flounder is noticeably smaller than that of turbot (Psetta maxima), which affects food choices. The main food consists of Baltic macoma (Macoma baltica) and blue mussel (Mytilus edulis). Other food items are aquatic sowbug (Saduria entomon) and mysids. When the surface sea waters warm in early summer, flounder move to sand and mud bottoms at depths of 5 to 30 metres; in winter they may move to depths as great as 20−50 metres. Although a relatively local species, in autumn flounder may extend their migrations over quite extensive areas.
Fishing and catches: Flounder are delicious fish though not widely eaten in Finland. They are important for commercial fishery in the archipelago and Åland Sea, especially in the autumn. That is when they are at their best, having had time to gain weight after the exhausting spawning event. The annual commercial flounder catch was 25 tonnes in 2006 but the catch from recreational fishery was as high as 244 tonnes in 2001, but only 76 tonnes in 2006, most of which came from Finland's southwestern sea area.
Vulnerability, threats and management: Relatively little is known about the population dynamics of Finland's flounder stocks. In the Archipelago Sea, stocks were weak in the 1920s and again in the 1960s, possibly as a result of salinity variation, because the weak stocks coincided with periods of low salinity. The potential future decline in salinity of Finnish coastal waters may have an adverse effect on stock sizes.