European flounder: life, behaviour and the long reads

The flounder's habitat is the reason it exists as a separate story rather than as a footnote to plaice.

IT IS AN ESTUARINE FISH, NOT A MARINE ONE THAT VISITS. Flounders occupy the whole salinity gradient from open coast through brackish estuary into genuinely fresh water, and adults are found well above the tidal limit of rivers, sometimes many miles inland. No other British flatfish does this. A flatfish caught in fresh water is a flounder until proved otherwise.

THE GROUND IS MUD AND FINE SAND. Where a dab wants clean sand and a plaice wants clean sand or mixed gravel, a flounder is at home on the soft intertidal mud of an estuary — the ground everything else avoids. That is why flounder fishing happens in places that look unpromising: harbour mud, creek mouths, the inside of a sandbar, the tidal reach of a river running through a town.

DEPTH IS SHALLOW AND SOMETIMES ABSURDLY SO. Flounders work up over the flats on a flooding tide into water that barely covers them, feeding on the invertebrates the tide has reached. Fishing very close in, and fishing the flood rather than waiting for depth, is the whole approach on many estuaries.

THE SEASONAL PATTERN RUNS THE OPPOSITE WAY TO MOST SEA FISHING. Fish move up the estuary and into fresh water through spring and summer, and drop back seaward in late autumn and winter to spawn at sea. Autumn is the season for the estuary angler, because fish that have fed all summer concentrate on the way down and are in their best condition. A fish caught far up a river in high summer is often thin.

SUMMER FISH IN THE RIVER, WINTER FISH IN THE SEA. That single sentence organises most of what an angler needs, and it also explains the species' vulnerability: it depends on open sea to spawn and on estuarine mud as a nursery, so damage at either end reaches the whole population.

WATER QUALITY IS PART OF THE FISHING HERE IN A WAY IT IS NOT ELSEWHERE. Estuaries receive what the catchment sends down, and a flounder spends its life in that water. Whether fish from a particular estuary should be eaten is a question for the local authority and food agency, not for FishingHQ, and it is a real question rather than a formality.

NO ACCESS, PERMISSION, CLOSE SEASON OR SIZE LIMIT IS STATED HERE. Tidal reaches frequently sit at the boundary between sea and freshwater regimes, which is exactly where rules get complicated; ask the relevant fisheries authority before fishing one.

Flounders are bigger than dabs and smaller than good plaice, and they are caught in water where you often cannot see them at all.

THE NUMBERS. Most rod-caught flounders are 25 to 35 centimetres and 300 to 700 grams. Over 40 centimetres and a kilogram is a good fish; the reported upper end is around 50 centimetres and two to three kilograms, and fish at that size generally come from rich, productive estuaries rather than from open coast.

THE DIET IS WHATEVER THE ESTUARY HAS. Ragworm above all, then small molluscs, shrimps, small crabs and small fish, with the balance shifting between systems and seasons. This is an opportunist working intertidal mud, not a specialist, and that is why ragworm is the near-universal flounder bait: it is the thing the fish is already eating in quantity.

SMELL LEADS, BECAUSE SIGHT CANNOT. Estuary water is usually coloured and often genuinely opaque, so the visual approach that works for plaice over clean sand is unavailable. Flounders find food by scent and by detecting disturbance in the sediment, and their behaviour follows: practitioners hold that a fresh, leaking worm bait does considerably better than a tidy one, and a bait that has been in the water an hour has stopped working.

WHICH IS THE WHOLE EXPLANATION OF THE BAITED SPOON. A small spoon or flashing blade set above a baited hook and drawn slowly across the bottom does two things at once: it stirs the sediment, which is the signal a feeding flounder is tuned to, and it flashes, which draws attention in whatever visibility there is. Then the fish arrives to find a worm. It is one of the oldest specialist rigs in British sea angling, it is genuinely effective rather than traditional for its own sake, and it works because it matches how the fish hunts rather than what it eats.

MOVEMENT IS THE ACTIVE INGREDIENT. A static bait catches flounders, and the practitioner view is that a slowly moved one does better. Retrieving a few turns, pausing, and retrieving again covers ground and repeatedly restarts the disturbance signal, and on a big flat where fish are thinly spread that is worth more than any bait refinement.

The flounder's life cycle runs between two habitats and it needs both.

SPAWNING IS AT SEA, in relatively deep water in late winter and spring, with floating eggs. Adults that have spent months far up an estuary or river must therefore make a seaward migration to breed, and that movement in late autumn and winter is predictable enough to fish.

THE NURSERY IS THE INTERTIDAL. After the flatfish metamorphosis — the eye migrating across the skull, the body flattening, the fish tipping onto its side — tiny flounders settle into very shallow estuarine sand and mud, often in water measured in centimetres. Those flats are the species' nursery and they are precisely the habitat most affected by dredging, land claim, smothering and poor water quality.

THEN THE DIRECTION REVERSES. Juveniles move progressively further up the estuary and into fresh water as they grow, which is the opposite of the usual arrangement where young fish stay in brackish nurseries and adults go to sea. A flounder is genuinely a two-habitat animal, and its population can be limited by either end.

LEFT-EYED FISH ARE COMMONER HERE THAN ELSEWHERE. Reversed individuals — both eyes on the left — occur in most flatfish and occur in flounders at a noticeably higher rate. A reversed flatfish from an estuary is usually this species rather than a rarity.

HANDLING: THE FISH IS TOUGHER THAN IT LOOKS AND SO IS ITS SKIN. Flounders tolerate handling and brief air exposure better than most flatfish, which is genuinely useful when unhooking in mud. But the bony tubercles along the dorsal and anal fin bases are abrasive, and gripping a thrashing fish firmly will take skin off a hand — hold it flat across the body with wet hands, not by the edges.

UNHOOK OVER WATER OR OVER SOMETHING WET. A flounder dropped on soft estuary mud is very hard to return in decent condition, and one dropped on shingle is worse. Deep hooking on ragworm is common because the bites are gentle and easy to ignore; strike them rather than let them develop, and if a fish is hooked out of sight cut the trace close rather than digging for the hook.

Flounder fishing is estuary fishing, and almost every effective method follows from that one fact.

THE FISH IS IN THE MUD, NOT ON THE SAND. Flounder tolerate fresh and brackish water better than any other British flatfish, run far up estuaries and into rivers, and feed over soft mud and muddy sand where plaice and dab will not be. An angler fishing 'flatfish ground' on a clean surf beach is usually fishing for the wrong flatfish.

LEDGERING WITH A LIGHT FLOWING TRACE is the foundation. A single or two-hook trace on a light lead, fished so the tide moves it slowly across the mud, covers ground and imitates a disturbed bottom. A rig pinned rigidly in place is the commonest mistake in flounder fishing.

BAITED SPOONS ARE A GENUINE FLOUNDER SPECIALITY and one of the few pieces of British sea tackle designed for a single species. A small metal spoon above the hook, drawn slowly across the bottom, kicks up sediment and flashes; the flounder investigates the disturbance and finds the worm. It works because it exploits exactly the curiosity that makes flounder follow a moving bait.

SLOW RETRIEVE IS THE COMMON THREAD. Whether the rig is a spoon, a trace or a plain lead, drawing it back a foot at a time with pauses is held by estuary flounder anglers to out-fish leaving it still. This is the opposite of the discipline most beach anglers have learned.

AND THE TIDE DECIDES EVERYTHING ELSE. Flounder follow the flood up onto mud flats to feed and drop back on the ebb, so the productive window is a specific state of tide on a specific bank rather than a time of day. On many estuaries the last two hours of the flood and the first of the ebb are the whole session.

FLOUNDER ARE ALSO CAUGHT ON LIGHT LURE TACKLE — small soft plastics worked slowly along the bottom — which is a recent development and a natural extension of why the baited spoon works.

Flounders hatch from pelagic eggs at sea as symmetrical larvae, undergo the flatfish metamorphosis, and settle into very shallow water — commonly the intertidal sand and mud of estuaries, which are the species' nursery. Juveniles then move progressively further up the estuary and into fresh water as they grow, returning seaward to spawn as adults. That reversal of the usual pattern, with the young in salt and the adults ranging into fresh, is unusual and is what makes the flounder a genuinely estuarine fish rather than a marine one that tolerates estuaries.

Spawning takes place at sea in late winter and spring, in relatively deep water, with pelagic eggs. Adults that have spent the summer and autumn far up estuaries and rivers must therefore drop back down to salt water to breed, and that seaward migration in late autumn and winter is a predictable movement an angler can fish. The dependence on both open sea for spawning and estuarine mud for nursery habitat makes the species vulnerable at two separate points in its cycle.

An opportunistic bottom feeder taking ragworm, small molluscs, shrimps, small crabs and small fish, with the diet shifting according to what the estuary offers. Flounders feed heavily on the invertebrate life of intertidal mud and sand, and they follow the tide up and across the flats to do it — a flounder will work into water barely deep enough to cover it. They are also markedly attracted to movement and to disturbance in the sediment.

Sight and smell together, with smell dominant in the coloured, sediment-laden water flounders usually inhabit. Estuary water is frequently too murky for vision to lead, which is why scented baits and slow movement work so well and why bright presentation matters far less than it does for plaice on clean sand. Flounders also respond strongly to vibration and disturbance in the substrate, which is the mechanism behind the traditional baited spoon: a spoon dragged slowly across the bottom stirs sediment and flashes, and the fish comes to investigate.

Flounders move up estuaries and into fresh water through spring and summer, feed there through the warmer months, and drop back down towards the sea in late autumn and winter to spawn. The best estuary fishing is generally autumn, as fish that have fed all summer concentrate on their way seaward. Fish caught well up a river in high summer are usually in poor condition compared with autumn fish that have begun to move down.

The estuary flatfish, and one of very few species that gives shore anglers good sport in the depth of winter in water most people write off. Flounder fishing is a distinct tradition with its own tackle — light gear, small hooks, ragworm, and the baited spoon, which is one of the oldest and most effective specialist rigs in British sea angling. It is also the flatfish most often caught by freshwater anglers, in tidal rivers where the two worlds overlap.