European Flounder

The European flounder (Platichthys flesus) is a right-eyed flatfish in the family Pleuronectidae, notable among flatfish for its unusually broad tolerance of salinity. While most flatfish remain in fully marine waters, European flounder regularly move into brackish estuaries and even far upstream into freshwater rivers, making them one of the most euryhaline flatfish species. They are widespread along the coasts of Europe, from the Mediterranean and Black Sea north to the White Sea and Baltic. The species supports recreational and commercial fisheries across its range and, while broadly common, some local populations face pressure from habitat degradation in estuarine nursery areas.

European flounder can be distinguished from similar European flatfish by a combination of features.

• Body: oval to diamond-shaped, moderately flattened
• Eyes: typically right-sided, though left-eyed individuals occur more often than in most flatfish
• Coloration: mottled brown to greenish upper side, sometimes with faint orange spots, contrasting white underside
• Skin: rows of small bony tubercles along the base of the dorsal and anal fins and along the lateral line, a key diagnostic feature
• Size: modest, usually 20-30 cm The bony tubercle rows separate it from the similarly shaped European plaice, which instead has smooth skin with prominent orange spots.

European flounder occupy a wide range of environments from fully marine coastal waters to brackish estuaries and, uniquely among common flatfish, freshwater rivers and lakes well upstream of tidal influence. They favor soft sandy or muddy substrates in shallow coastal zones, harbors, and estuarine channels. Their range extends along the eastern Atlantic from the Mediterranean and Black Sea northward through the North Sea, English Channel, and Baltic Sea to the White Sea off northern Russia. Juveniles commonly use estuaries and river mouths as nursery habitat before some individuals migrate further upstream, while adults return to coastal or offshore waters to spawn.

European flounder are bottom-dwelling ambush feeders, lying camouflaged on soft substrates and preying on worms, small crustaceans, and mollusks detected largely by scent and touch. They are notably tolerant of low-oxygen and low-salinity conditions, allowing extended residence in estuaries and rivers where few other marine flatfish can survive. Individuals are largely solitary and not strongly territorial, moving opportunistically to follow prey and favorable conditions. Spawning takes place in coastal marine waters, typically in late winter to spring, with pelagic eggs and larvae drifting before settling as juveniles move into estuarine nurseries. Their use of freshwater habitats makes them an important link between marine and inland aquatic ecosystems.