Common sole: life, behaviour and the long reads

Sole handling is straightforward and the fishery around it is where the risk actually is.

THE FISH. Small, flat, slippery, with a small mouth on the underside and no capacity to injure anybody seriously. There is a short sharp spine on the gill cover that will prick a hand gripping the head, and the skin is rough. Handle it flat with a wet hand, unhook with forceps because the mouth is small and the hook is often well in, and support the body.

THEY ARE DELICATE FOR THEIR SIZE. Sole are usually taken on small hooks and light gear, and a small flatfish dropped on shingle or dragged up a beach is damaged. Wet hands, and back in the water promptly.

THE FISHERY IS THE HAZARD. Sole are fished at night, from beaches, on a flooding tide, in the dark, often in cold conditions, frequently by anglers wading to reach the ground. Every element of that is a recognised risk factor: cold water immersion, a tide that cuts off ground behind you, and reduced visibility. Sole fishing has put more people in difficulty than sole have ever inconvenienced.

SO THE HANDLING INTELLIGENCE FOR THIS SPECIES IS MOSTLY ABOUT THE SESSION. Know the tide and the exit before dark, do not wade beyond what you would wade in daylight, tell somebody where you are, and carry a light and a means of calling for help.

Sole have a seasonal pattern that is straightforward and a daily pattern that is the whole of the fishery, and confusing the two is the commonest reason people fail to catch them.

THE SEASON IS AN INSHORE MOVEMENT. Sole move into shallow, clean sandy ground in the warmer months and retreat into deeper water as it cools, with spawning inshore in spring. That gives a shore fishery concentrated in the warm half of the year — but the fish are not arriving from far away, and the movement is a response to temperature rather than a migration in any strong sense.

THE DAILY PATTERN IS WHERE THE FISHING ACTUALLY IS, and it has two components that must both be right.

DARKNESS FIRST. Sole are nocturnal benthic feeders. They lie buried through the day and emerge after dark to work over sand for polychaete worms and small crustaceans, locating them by touch and by taste with the sensory papillae on the blind side of the head rather than by sight. This is a fish that finds its food in the dark by feel, and it does not do so in daylight. A sole beach in the afternoon is empty of catchable sole.

TIDE SECOND. As the flood covers ground that was dry, it opens fresh feeding — worm beds that were exposed, invertebrates becoming active in newly wetted sand — and sole move up the beach with the water to work it. The fishing therefore follows the tide up the beach rather than staying at a fixed distance, and casting a long way to where the fish were two hours ago is the standard error.

WHICH MAKES THE HAZARD PART OF THE SAME PATTERN. This is a fishery conducted at night, on a flooding tide, over featureless sand, frequently by anglers wading to reach ground — and ground that is flooding behind you in the dark is the specific circumstance that puts shore anglers in difficulty. The tide that makes the fishing is the tide to have worked out before dark.

Sole habitat is easy to summarise and worth explaining, because the parts that matter to the population are not the parts an angler encounters.

THE ADULT GROUND IS SOFT: FINE SAND, MUDDY SAND AND MUD. Sole are flatfish that bury, and they bury properly — a sole spends daylight under the sediment with only the eyes exposed, and it needs sediment fine enough to submerge in. That excludes coarse gravel and shingle ground, which is why sole and plaice, superficially similar fish, occupy noticeably different bottoms and are caught on different marks along the same coast.

DEPTH IS MODEST AND SEASONAL. Sole occupy shallow shelf water, moving into shallower inshore ground and estuaries as water warms and retreating to deeper water in winter. That movement is temperature-driven rather than calendar-driven, which is why the inshore sole season shifts between years.

THE FEEDING MODE EXPLAINS THE GROUND AND THE FISHING. Sole feed at night on polychaete worms, small bivalves and crustaceans in the sediment, locating them by touch and chemistry with the sensory papillae on the blind side of the head rather than by sight. A soft bottom rich in worms is sole habitat; a clean scoured bottom is not. It is also why sole are overwhelmingly a night fishery on small baits presented on the bottom.

THE NURSERY HABITAT IS THE PART THAT DECIDES THE POPULATION, and it is estuarine and coastal shallows. Juvenile sole settle into very shallow, warm, sheltered, fine-sediment ground — the inner reaches of estuaries and shallow bays — and spend their first years there before moving out. Those areas are also where dredging, reclamation, port development and poor water quality concentrate, so the constraint on many sole stocks is nursery ground rather than adult ground.

SALINITY TOLERANCE IS BROAD ENOUGH TO MAKE THAT POSSIBLE. Sole handle reduced salinity well and penetrate estuaries substantially, which is what gives them access to those warm productive nurseries in the first place.

AND THE HABITAT IS WHY THE FISH IS WHERE IT IS ON A BEACH. On a surf beach a sole will be in the fine sediment of a gutter or a trough rather than on the scoured bar, and on an estuary it will be on the mud rather than the channel gravel. The ground, not the location, is the thing to read.

Sole reproduction contains one of the most striking transformations in fish biology, and it explains why the species' recruitment depends on places adults never visit.

SPAWNING IS A BROADCAST EVENT IN SHALLOW COASTAL WATER. Sole move to shallow spawning grounds as water warms in spring, and eggs are released into the water column where they are fertilised and float. That is worth noticing: the adult is a bottom-dwelling, buried, asymmetric animal, and its eggs behave like those of an ordinary pelagic-spawning fish.

THE LARVA HATCHES SYMMETRICAL AND THEN REBUILDS ITSELF. A newly hatched sole is a normal bilaterally symmetrical fish larva swimming upright in the plankton with an eye on each side. During metamorphosis one eye migrates across the skull to join the other, the body flattens, pigment develops on one side only, and the animal settles onto the bottom to live on its side for the rest of its life. Very few vertebrates reorganise their skull during development; this one does it as a matter of course.

WHERE IT SETTLES IS WHAT DECIDES THE YEAR CLASS. The drifting larvae must be transported to shallow, warm, fine-sediment coastal and estuarine nursery ground, and settlement success depends on currents and wind delivering them there at the right stage. Good and bad year classes in sole are substantially a transport-and-nursery story, and the adult stock size explains less of the variation than people expect.

WHICH MAKES ESTUARINE NURSERY HABITAT THE LIMITING FACTOR FOR MANY STOCKS. Reclamation, dredging, port development and poor water quality in exactly those shallow inner areas reduce the amount of ground available to a settling sole, and the effect appears in the fishery several years later.

MATURITY IS REACHED AT A MODEST SIZE AND FEMALES GROW LARGER THAN MALES, with fecundity rising steeply with body size — so a large female contributes disproportionately to the spawning output.

AND THE MANAGEMENT PICTURE IS SEPARATE FROM ALL OF IT. Sole stocks are assessed and managed with quotas, gear rules, size limits and area measures that differ by sea area and change; nothing in the biology above indicates what any current recreational or commercial rule is.

Grows slowly and lives to around twenty-six years. Matures at three to five years. Spawns in shallow coastal water in spring, with pelagic eggs and larvae that undergo the flatfish metamorphosis, one eye migrating across the head as the fish settles. Juveniles use very shallow sandy nursery grounds, which links the offshore adult stock to inshore habitat condition.

Broadcast spawning in shallow coastal waters in spring, with pelagic eggs and larvae. Juveniles settle onto very shallow sandy grounds and remain there through their first years, so the loss or degradation of shallow nursery habitat affects recruitment independently of anything happening to the adults.

A nocturnal bottom feeder taking marine worms, small crustaceans and molluscs from and within sand and mud, working head-down with a small twisted mouth adapted to the task. The nocturnal habit is near-absolute and is the reason the fishery is a night fishery — a sole feeding is a sole that has emerged after dark.

Adapted to feeding in darkness in and on sediment, relying on touch and chemical detection through a fringe of sensory papillae on the blind side of the head rather than on sight. That sensory arrangement is why small, scent-rich baits fished hard on the bottom work and why visual presentation is irrelevant.

Moves inshore into shallow sandy ground in the warmer months and offshore into deeper water in winter, with spawning inshore in spring. The shore fishery is a summer and autumn night fishery over clean sand. Tide governs the session, with fish moving up the beach to feed as the water covers the ground.

A specialist night fishery over clean sand across northern Europe, taken on small hooks with ragworm or lugworm fished hard on the bottom, from beaches and estuaries. Highly valued as a table fish and heavily fished commercially, which makes recreational catches a small part of the picture but a well-loved one.