European Flounder

Fascinating facts about the European Flounder

• A single European flounder can lay about one million eggs that are lighter than water, letting them float to the surface for fertilisation; this strategy lets thousands of larvae drift back toward coasts on tidal streams, showing how a simple buoyancy trick fuels the species’ widespread success.
• During its spring migration, the fish can cover three to four miles (five to seven kilometres) each day without eating, a relentless trek that underscores the energy it sacrifices to reach spawning grounds before the summer heat arrives.
• Although most individuals grow to about 30 cm, the record length reaches 60 cm and a weight of 2.93 kg, proving that under the right conditions this modest flatfish can double the size most anglers expect to see.
• Unlike many flatfish, about 30 % of European flounders develop eyes on the left side, flipping their “upper” surface, a rare developmental twist that reminds us evolution can keep even eye placement flexible.
• In 2010, global catches topped roughly 19,000 tonnes, mainly from Baltic and Dutch-Danish waters; this volume highlights the species’ role in commercial fisheries despite being less prized than its relatives.

At a glance

About the European Flounder

The European flounder (Platichthys flesus) is an oval, right-eyed flatfish that drifts on sandy, shingle or muddy bottoms from the White Sea to the Mediterranean and Black Seas. Adults typically reach about 30 cm, though individuals up to 60 cm and 2.93 kg have been recorded. Its upper surface shimmers in dull brown, olive or fawn tones, dotted with reddish spots and darker blotches, while the ventral side is a pearly white that earned it the nickname “white fluke.” Rough tubercles line the lateral line and the bases of the dorsal and anal fins, and the dorsal fin bears 53–62 soft rays, the anal 37–46. About thirty percent of flounders are left-eyed, a rare reversal that adds to their cryptic charm. Nocturnal by nature, the flounder rests semi-buried in the substrate by day and emerges at night to hunt bivalves, shrimps, polychaete worms, gastropods and small fish. It tolerates low salinity, venturing into estuaries and even fresh rivers, unusual for a Pleuronectidae, before returning to sea to spawn. Spawning runs from January to July, with females releasing a million buoyant eggs that hatch in six to eleven days. The planktonic larvae use tidal streams to drift back toward coastal estuaries, where juveniles grow in shallow, low-salinity waters. A surprising twist is the species’ occasional hybridisation with European plaice, especially in the western Baltic, where hybrids are common.

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