Flounder – fish of fresh and sea water

Flounder – fish of fresh and sea water

Many anglers still confuse flounders, dabs and plaice – but though they are all members of the ‘right-eyed’ flatfish family, they do have distinguishing features that set them apart. Plaice are smooth-bodied with a series of bony bumps between the eyes, and bright red/orange spots, while the dabs feel rough all over when rubbed from tail to head. The flounder is easily identified by the prickly region that runs along its lateral line and round the base of its dorsal and anal fins. It also has a square-cut tail, while in most other flatfish the tail is rounded.

Chameleon colouring

Like all flatfish, the flounder can adapt its mottled brown colouring to match that of the sea bed – each cell changes shape to vary the amount of pigment visible, so changing the colour of its skin.

Spending most of its day buried in the sand or mud of estuaries and coastal waters, the flounder becomes active at night, moving nearer the shore to feed at high tide, then retreating as it falls.

Fresh/sea water homes

The flounder is unique among British flatfishes in that it is able to live in both fresh and sea water. Although some venture upstream into completely fresh water, most are found in the brackish water of estuaries, attracted by the vast worm populations on which the mature fish feeds.

However, the fish cannot move very quickly between these two contrasting environments; it needs time to adjust physically to the point where its blood is neither diluted by too much freshwater nor dehydrated by salt water.

Large volumes of inland rainfall and melting snow reduce the salinity and temperature of the water, providing problems for the slow-changing flounder. The angler can take advantage of this as the fish tends to be most abundant – and vulnerable – in late autumn/winter when these problems are most likely to occur.

During this period the mature flounder feeds heavily to prepare itself for reproduction and to help see it through any cold snaps – and is therefore more likely to take an angler’s bait.

All change

In spring, the flounder moves out to the open sea to spawn; the female lays between half and two million eggs in depths up to 50m (165ft). These float on the sea’s surface, before hatching one week later as tiny, round fish.

The young flounder then moves to shallow coastal waters, feeding on microscopic plants and animals before moving on to molluscs, polychaete worms and soft-shelled crabs. (Unlike the plaice, it does not have the ability to crush tough shellfish.)

Anglers usually find that peeler crab is one of the best baits for catching this fish.

As with plaice and dabs, a dramatic change takes place when the young flounder reaches a length of about 1.5-3cm (/2-lin) – the fish’s body flattens and the left eye moves to sit alongside the right. The dorsal fin grows forwards along the edge of the head, and the young flounder now swims with its (eyeless) left side downwards. Sexual maturity is reached when the male is only 12cm (5in) long and the female 18cm (7in).

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