Common prawn

Prawn sizes are quoted as TOTAL LENGTH, and the disagreement between sources comes from where the measurement starts: the rostrum — the serrated spike projecting forward between the eyes — is a substantial fraction of the animal, and not everybody includes it.

ORDINARY ANIMALS netted from harbours and rock pools run six to eight centimetres.

A GOOD ONE is over nine centimetres, which in a net full of prawns is immediately obvious.

THE EXCEPTIONAL END reaches around eleven centimetres, recorded for large females. Females grow larger than males in this species, so the big ones in any catch are predominantly female and frequently berried.

GROWTH IS BY MOULTING and is rapid in warm conditions, with animals surviving around three years. That is a short life and a fast turnover, and it means abundance in a given harbour can change markedly between years without any obvious cause — a good breeding season and a mild winter show up in the nets within months.

THERE IS USUALLY NO STATUTORY MINIMUM SIZE for recreational prawning, which puts the judgement entirely on the person holding the net. The sensible working rule is the practical one: small prawns are fiddly, they yield almost nothing, and returning them costs nothing. A net worked over the same ground repeatedly through a season will show you whether you are taking too many, and the answer arrives faster than it does with any fish.

AND BERRIED FEMALES — carrying eggs attached beneath the abdomen, easily seen against the translucent body — are worth returning even where no rule requires it. They are the visible part of next year's catch, and in a small harbour worked by a few people it is a decision that genuinely shows.

Prawns cannot hurt you in any meaningful way and they are the easiest catch in this catalogue to ruin. Almost everything about handling them is about time and temperature.

THE ONLY THING THAT PRICKS is the rostrum, the serrated spike between the eyes, which will mark a palm if an animal is gripped hard. Pick them up rather than grabbing them and it does not arise.

NOW THE PART THAT MATTERS. Decapod tissue breaks down after death far faster than fish does, through the animal's own enzymes, and in prawns it is fast enough to be measured in hours rather than days. A prawn left in a warm bucket goes soft, the flesh separates from the shell, and the head darkens. None of that is spoilage in the bacterial sense and all of it makes the catch unpleasant.

SO THERE ARE TWO ACCEPTABLE STATES: alive and cool, or dead and cold. Keep them in cool aerated seawater changed regularly, or kill and chill them immediately — a short immersion in iced seawater does both at once and is what commercial handling does for the same reason. Cook within hours rather than overnight.

DO NOT CROWD THEM. A bucket packed with prawns in warm water deoxygenates within minutes, not hours, and the catch dies before you have finished netting. This is the same failure that kills crabbing catches and it happens faster with prawns because they are smaller and more active.

RETURNING THEM. Put unwanted prawns back into cover — weed, under an overhang, into the shadow of a wall — rather than into open water. A prawn dropped into the open in daylight is taken immediately, which turns a release into a feeding of the local wrasse.

AND THE GROUND IS THE ACTUAL HAZARD IN THIS FISHERY. Prawning happens on weed-covered harbour steps, wet rock and pier ladders, usually at dusk, usually while looking down into water rather than at your feet. The prawn is harmless; the surface you are standing on is not.

Common prawns occupy the three-dimensional space inside coastal structure rather than a surface, which is why they are found by putting a net into something rather than by looking at the bottom.

THE HABITAT IS SHALLOW, SHELTERED, STRUCTURALLY COMPLEX INSHORE WATER. Prawns use rock pools, gullies, weed beds, eelgrass, kelp stipes, harbour walls, pier and jetty pilings, pontoons, breakwaters and any other submerged structure that carries growth. What they need is cover to hide in and surfaces to forage over, and they are as content on an artificial wall as on natural reef — which is why harbours are among the most reliable prawn ground anywhere.

TRANSPARENCY IS THE HABITAT ADAPTATION. A common prawn is almost invisible against weed and shadow, and its concealment strategy depends on being in among something rather than on running away. That is why they hold tight to structure, why they are far more numerous than they appear, and why the way to establish whether a wall holds prawns is to work a net through the weed on it rather than to look.

THEY MOVE UP AND DOWN THE SHORE WITH THE TIDE AND WITH THE SEASON. Prawns work into the intertidal on the flood and retreat with the ebb, using pools as refuges when the tide leaves them behind. Over the year they tend to move offshore into deeper water in winter and return to the shallows as water warms, so shore ground that is thick with prawns in late summer may be empty in February — a seasonal absence rather than a decline.

SALINITY TOLERANCE EXTENDS THE RANGE INTO ESTUARIES, where prawns occupy the lower and middle reaches wherever there is structure, sharing that ground with the more strongly estuarine shrimps.

AND THE PRAWN'S HABITAT IS ALSO ITS ECOLOGICAL ROLE. Prawns are both scavenger-grazers cleaning surfaces of detritus and small organisms, and prey for almost every inshore predator — bass, pollack, wrasse, cod, cuttlefish. Their concentration in structure is precisely why those predators hunt structure. Anybody thinking about where a bass sits around a harbour wall at night is, in effect, thinking about prawn habitat.

Prawn seasonality is one of the most reliable inshore patterns on a temperate coast, and it is driven by temperature acting on a short-lived animal.

THE MOVEMENT IS SHALLOW-IN-SUMMER, DEEP-IN-WINTER. Common prawns work into the intertidal and shallow inshore structure as water warms through spring, reach peak abundance in the shallows through late summer and autumn, and retreat to deeper water as the shallows cool. A harbour wall that is thick with prawns in September holds almost none in February, and the prawns have not died — they have moved down.

WHICH MAKES LATE SUMMER AND AUTUMN THE PRODUCTIVE WINDOW for anybody working shore structure, and it is also when the animals are largest, because a season's growth has accumulated.

THE BREEDING CYCLE RUNS THROUGH THE COLDER MONTHS in most populations, which is the opposite of what people assume. Females carry eggs attached beneath the abdomen — the 'berried' condition — through winter and into spring, releasing larvae into the plankton as the water warms and the food supply arrives. So the animals in deeper water in the cold months are the reproducing ones, and the shallow summer population is a feeding and growing population.

GROWTH IS FAST AND LIFE IS SHORT, which makes the seasonal signal unusually sharp. A prawn population turns over quickly, so abundance in any given autumn reflects that year's recruitment rather than a long accumulation, and prawn numbers swing between years accordingly.

MOULTING IS THE OTHER SEASONAL PROCESS AND IT IS TEMPERATURE-DRIVEN. Prawns grow by moulting, moulting frequency rises with temperature, and a freshly moulted prawn is soft and hides until the shell hardens. Warm-water periods therefore produce both faster growth and periodic dips in how many animals are out in the open.

AND FOR THE PREDATORS THAT MATTERS AS MUCH AS FOR THE PRAWNS. The late-summer concentration of prawns in shallow structure is a substantial food pulse for bass, pollack, wrasse and cuttlefish, and a good deal of inshore autumn fishing is really an expression of prawn seasonality. Where prawns may be gathered, in what quantity and by what method is a separate question with a local answer.

Prawn reproduction is a compact and efficient cycle in a short-lived animal, and its timing is the reverse of what most people assume.

THE FEMALE CARRIES THE EGGS AND SHE CARRIES THEM THROUGH THE COLD. After mating — which, as in most decapods, occurs when the female has recently moulted and her shell is soft — the eggs are attached to appendages beneath the abdomen and brooded there. In common prawns this brooding takes place largely through the autumn and winter months, so the berried females are in deeper water in the cold season rather than in the shallow summer population.

LARVAE ARE RELEASED INTO THE PLANKTON AS THE WATER WARMS, timed to coincide with the spring production that will feed them. They pass through several planktonic larval stages before settling into the shallow structured habitat the adults use. That timing is the whole reproductive strategy: brood through the unproductive months, release into the productive ones.

WHICH MEANS THE ANIMAL A GATHERER MEETS IN LATE SUMMER IS THE GROWING GENERATION, not the breeding one. The large prawns of autumn are that year's recruits fattened up; the breeding stock is elsewhere and deeper.

GROWTH AND REPRODUCTION BOTH DEPEND ON MOULTING, and a prawn cannot grow without shedding its shell. A female cannot moult while berried, so reproductive investment directly costs growth, and heavily reproducing individuals stay smaller.

LIFE IS SHORT AND TURNOVER IS FAST, so the population is rebuilt over a small number of years and abundance responds quickly to conditions. A poor larval year shows up in the shore population the following autumn rather than several years later, which makes prawn numbers unusually volatile and unusually responsive.

AND THE ECOLOGICAL CONSEQUENCE IS LARGER THAN THE ANIMAL. The spring release of prawn larvae and the summer growth of juveniles is a substantial contribution to inshore productivity, feeding everything from small fish to cuttlefish. Prawn reproduction is, in effect, part of the food supply of the whole inshore system.

WHERE PRAWNS MAY BE GATHERED, BY WHAT METHOD AND IN WHAT QUANTITY is a separate question with a local answer, and it is not derivable from the cycle above.