Arno Gasteiger

The cultured oyster

Written by       Photographed by Arno Gasteiger, Kennedy Warne and Roger Grace

Oysters have been flourish­ing in New Zealand’s coastal waters for millions of years. Fossilised oyster shells are a common feature of the exposed out­crops of mudstone, sandstone and conglomerate found along the north­east coast — some date back 20 mil­lion years.

Just a few kilometres from the Mahurangi Estuary (one of New Zealand’s prime oyster cultur­ing areas), fossiliferous rocks at Mathesons Bay are packed with oys­ters. Some of these ancient speci­mens would make an oyster glutton drool and even call for a knife and fork, for the shells are the size of bread-and-butter plates.

Neither cutlery, nor even humans, were around in those Miocene times, but when humans did eventually arrive on the scene it is clear that they were partial to oysters from their earliest days. In midden material from all around the world ­Denmark, Ireland, east and west coasts of America, Japan and Austra­lia — oysters feature prominently and are often accompanied by the primitive tools needed to open the shellfish. Many of these middens are huge; one in Ireland measured 300m square and 15m high — a vast rub­bish dump containing oysters, cockles, mussels and periwinkles, and showing that shoreline food has long been a staple of humanity.

Oysters would have been a most valuable food resource for primitive hunters and gatherers. In addition to the essential proteins in the meat, fat oysters have stores of glycogen; a very good energy food for people living before the days of cereal crops and other cultivated carbohydrate sources.

Oysters have been cultured for at least 2000 years, with the earliest re­cords of culturing techniques being those of the Romans. They had an especially high regard for the oyster and sought out naturally bountiful growing areas and different local strains or species as they expanded the Empire.

The Romans cultured the shellfish on sticks and ropes suspended from elevated walkways and gantries. Elsewhere around the world cultur­ing methods have been as varied as fashion and local materials have allowed.

Generally, culturing has been in quiet waters where wave action is too weak to damage the supporting structures or carry away the develop­ing crop. While protected from storms, these sites invariably have soft bottoms of fine silt which can clog the feeding and breathing appa­ratus of sedentary animals.

Creating suitable culturing condi­tions has therefore depended on pro­viding hard surfaces for the oysters to attach to and keeping the shellfish above bottom sediments.

Simple methods for cultivating mud oysters involved no more than the spreading of large amounts of cu­lch for the spat to settle on. The culch was usually just empty oyster shell from previous harvests, and there were various penalties and bad omens associated with failure to re­turn this most suitable of all sub­strates to the growing beds. Periodi­cally the beds were raked to lift the developing oysters out of the ever-settling fine sediments. This was a full-time activity for the bed workers in the lowland coastal areas of such countries as England and Belgium where these culturing methods were extensive right up to the last century.

In Japan oysters are grown on old shells that have been perforated and strung on ropes suspended from floating rafts or buoyed lines. Else­where they are cultured on recycled roof tiles and slates, custom-made trays, bamboo fences and frames, as well as on sticks set out on racks like those seen in New Zealand today.

Oyster farming is still a young in­dustry in New Zealand. It had its beginnings in the late 1960s with the then Marine Department promoting the culture of the New Zealand rock oyster (Saccostrea glomerata). There was considerable appeal in the pros­pect of growing a luxury food for which the affluent might be prepared to pay high prices, just by putting sticks out in the tide.

Oyster leases were applied for in harbours and inlets all around north­ern New Zealand, and the now famil­iar geometric arrays of growing racks sprang up as keen culturers rammed tanalised timber and money into the mud. However, the culturing of the New Zealand rock oyster was never a great financial success. The growing time from freshly caught spat to mature oysters of marketable size ­three to four years — was too long for growers to recoup an adequate return on their investment. The long cultur­ing periods also brought other prob­lems, particularly fouling of the oys­ters and competition from other marine organisms. A further problem was erratic and often inadequate set­tlement of spat (‘spatfall’).

The chance arrival of the Pacific oyster (Crassostrea gigas) in the early 1970s proved to be the saviour of the oyster culturing industry, though at first some growers were reluctant to farm it, and systematically removed young Pacific oysters from their growing sticks. Opposition, however, was futile. The Pacific soon became dominant in the farmers’ spat-catching areas and on natural shores. The new species was here to stay.

The main problem for farmers in changing over to the Pacific was that optimum growth for these oysters oc­curs about half a metre lower on the shore. Existing rack systems had to be lowered or replaced — a time-consuming and costly process. The benefits, though, were unequivocal: an oyster that settled heavily and consistently, grew three times as fast (reaching marketable size in 10-14 months) and was in good condition over much of the year. New methods to grow and handle the Pacific oyster evolved rapidly, and by the end of the decade the Pacific had replaced the New Zealand rock oyster as the main cultivated species.

The change-over to the Pacifc oys­ter caused its share of problems for distributors. Neil Harrington, man­ager of Auckland export packhouse Kia Ora Seafoods, recalls the diffi­culty of having to contend with two rock oysters in a relatively small mar­ket. “We immediately got some con­noisseurs of the native rock oyster who refused point black to eat Pacific oysters, but we also attracted some people who found the mellow flavour of the Pacific more to their taste and possibly for the first time in their lives became oyster eaters.”

There were plusses and minuses on the export front as well. The Pa­cific oyster had the advantage of a well-established international repu­tation, but the New Zealand rock oys­ter had the advantage of being ex­tremely long-lived out of water (staying fresh for three weeks or more) and of travelling well.

Neil Harrington looks nos­talgically on the New Zealand rock oyster as a product which never got the chance it deserved, but for growers like Jon Nicholson of Bio­Marine, a large oyster farm in the Mahurangi Estuary, there is no ques­tion that the arrival of the Pacific was the best thing that could have happened.

Pacific oysters are now farmed from Parengarenga in the far north to Ohiwa, near Whakatane, in the Bay of Plenty. Of the current 146 leases the majority are single-lease farms of only a few hectares. There are only half a dozen large ventures (with five or more leases) covering areas of some tens of hectares. Between them they produce an estimated 4000 tonnes per annum (approximately 32 million oysters). In 1988 50% of this harvest was exported, bringing in a revenue of $4.6 million.

The main production areas are the Bay of Islands, Whangaroa, Ma­hurangi and Coromandel. In west coast harbours such as the Kaipara growers have the problem of their oysters being heavily overspatted in their second summer. Some growers have overcome this problem by mov­ing oysters to the east coast during the spawning season, while others are using the prolific spatfall to ad­vantage by producing single seed oysters for other growers .

The positioning of oyster farms is a compromise between water quality and wave exposure. Pacific oysters grow best in open water, where tem­peratures are less variable, oxygen levels are higher and amounts of gill-clogging sediment are less. Oyster farmers look for areas where strong water flows bring a plentiful supply of food and where wave action stops mud from building up around the shells. However, greater exposure puts racks and crop at risk, so grow­ing sites just inside estuaries are sought as the prime areas for oyster culture.

The height of the growing racks is set at about the level of neap low tide (the highest of the low tides). At these times of the lunar cycle, when there is least difference between high and low tide, the oysters are barely ex­posed in air at low water.

The description of oyster farming which follows is based largely on the methods used at Bio-Marine in the Mahurangi Estuary, one of the first farms set out specifically for growing Pacific oysters.

To grow oysters, first you must catch the spat. Essentially this is very simple, as oysters will grow on al­most any hard, cleanish surface. In harbours, estuaries and sheltered bays where sediments accumulate, suitable natural substrates are fairly scarce. The spat will therefore settle very readily on any artificial hard surfaces provided. Sticks put out for this purpose are usually nailed to­gether in bundles and wired to ‘catching racks’. Spacers are placed between the sticks so that water car­rying the oysters’ swimming veliger larvae is able to circulate freely, en­suring an even settlement. Before use, new sticks are dipped in a thin slurry of sand and cement which dries to give a rock-like film over the surface of the wood, ideal for the settlement of oyster spat. Locally de­veloped and manufactured PVC sticks are also used.

Pacific oysters breed by the sepa­rate males and females releasing their spawn of sperm and eggs into the water simultaneously. Spawning is initially stimulated by a rise in sea­water temperature to about 22°C, but spawning by one individual also serves as a strong stimulus to neigh­bouring oysters to join in too.

Spawning may occur at any time from December through to March, but the peak is generally around new year. Getting the right number of spat on a stick is tricky. An ideal density is between six and eight dozen, but it is possible for 10,000 spat to settle on a single stick. To prevent such over-settlement (which results in mal­formed, undersized oysters) some farmers delay putting out their spat-collecting bundles until the peak of spawning has passed. This approach has its risks because all spawning may suddenly cease with a sharp change in weather conditions, leav­ing the farmer with insufficient spat on the sticks, and hence wasted space.

To a certain extent variations in settled spat densities can be accom­modated by good management. Sticks with a lower density of oysters can be placed closer together on the growing racks. Alternatively, the catching sticks can be kept in their bundles to limit growth. There is in­sufficient water flow through the stick bundles to support active growth and juveniles soon reach a small size plateau. At this size (about 20mm in diameter) they are, so to speak, kept ‘on hold’ — healthy, but unable to grow further.

This arrangement is very conven­ient for harvest management because active growth only commences after the catching sticks have been broken out of their bundles and laid out on the growing racks. By setting out sticks at regular intervals from the reservoir of bundled sticks, the oys­ter farmer is able to spread his har­vest of marketable-sized oysters through the year. Moreover, forecast peaks in demand can be accommo­dated by increasing the number of sticks set out in advance to corres­pond with an expected growth rate.

As with most farming ventures, growing conditions vary from year to year, affecting the crop quality and time to reach maturity. With the warm temperatures that persisted through the late summer and autumn this year, oysters have grown much faster than normal. As a conse­quence, some oysters set out to be ready for the 1989 Christmas market will reach optimum size too soon. These will have to be harvested at a time when there is less demand, and careful management of remaining stocks will be required to ensure suf­ficient numbers for the festive season.

At Bio-Marine the process of set­ting out the thousands of collecting sticks is speeded up by the use of a nailing gun. The thin steel nails or staples slowly rust away, but hold well enough to secure the sticks until the oysters are ready to harvest.

During harvesting, sticks with mature oysters are lifted from the racks and loaded into cradles or ‘frames’ on a flat-decked barge for transport back to a landing. There the cradles are hoisted on to a truck to be taken to the processing factory.

To release all the oysters from each stick requires only a few stout blows along its length with a piece of iron waterpipe. The oysters are given a crude wash to remove the worst of the mud that accumulates over and between the tightly packed, frilly shells. Any clumped oysters are bro­ken apart by hand as they pass along a conveyor belt to a rotating tumble-washer with high pressure water jets. The cleaned oysters are then size sorted. About half are of standard marketable size, with a quarter being of premium size and the remainder undersized. The latter are packed loosely into plastic mesh bags which are returned to the growing racks for about four months, until the oysters reach marketable size.

Bio-Marine markets its oysters in three ways.

Some, in good-shaped shells, are sold alive and unopened. Many are exported live to New Caledonia, where restaurant diners are prepared to pay premium prices to have the living animals opened in front of them at the table.

A large proportion of the oyster harvest is sold in the ‘half shell’. The flat upper shell valve is removed after cutting its connection to the large, central shell-closing muscle, leaving the succulent mollusc lying in a deli­cate serving dish that it grew itself.

As food presentation is all-important, it is critical that oysters in the half shell are not damaged or spoilt with broken fragments of shell, and look truly appetising. The skill of the oyster opener is therefore an important factor in the reputation of the processing house.

Any oysters that are slightly damaged during opening, or specimens that do not look fully fat, or are over­sized, are taken out of the shell and sold to commercial and domestic ca­terers and fish shops as ‘meat’ oysters.

Present-day oyster prices are as follows:

At an ordinary restaurant half a dozen oysters in the half shell with a twist of lemon and little other prepa­ration cost about $10 (= $20 per dozen).

The same dozen oysters would re­tail for $5 in a city shop which had purchased them from the processor/ packer for $2.60. Some packers are also growers, but many handle product from small operations whose proprietors receive only $1.65 per dozen for unopened oysters suit­able for marketing in the shell. Meat oysters are worth very little in com­parison: a grower is lucky to get 15 cents a dozen.

There is considerable fluctuation in local demand for rock oysters throughout the year. Demand drops from March to August, when Bluff oysters are available, and is also af­fected by a period of low restaurant activity during winter. Low season demand is about 5000 dozen per week, but this increases to 25,000 dozen per week during the high sea­son from October to January. There­after supply drops off as spawning oysters lose condition.

The domestic market is supplied mainly with oysters in the half shell and ‘meat’ out of the shell, chilled in plastic tubes or pots for the restau­rant trade and for private consumers inexpert at opening shelled oysters. The whole of the domestic market is worth a little over $4 million per annum.

The export market commands slightly higher prices (about $4.50 per dozen from the packhouse) with three quarters being sent as oysters in the half shell. The live export of unopened oysters, a moderate part of this market, is available to grower-marketers who can ensure that their product reaches its overseas destina­tion within about 24 hours of har­vesting. As unopened oysters require much less processing they are a pro­fitable commodity at about $3 per dozen.

[Chapter break]

Most People Enjoy a good oyster, but to some they are nothing short of ambrosia. The Romans went to great trouble to ship the succulent shellfish from England to Italy packed in ice. (Though that is probably no more bizarre than us air-freighting consid­erable tonnages of oyster shell, with small amount of oyster meat inside, all the way to Japan, the United States and other distant countries.) In America a good many people have even been killed in disputes over rights to the prized beds of the Potomac River.

The oyster is one of the few ani­mals that we eat live. Open the shell, drink the liquor and then let the slip­pery mollusc slide out of its shell straight into your mouth. Some like to chew them slowly while others just roll them around the palate for a while before swallowing the crea­tures whole.

Oysters seem to have a special ap­peal that is found in few other foods, for they have frequently been assoc­iated with amazing exhibitions of gluttony and excess. In ancient Rome thousands were consumed in lavish Bacchanalian feasts. It was appar­ently normal etiquette for the re­vellers to gorge themselves until they could eat no more, and then retire to a vomitarium with peacock feathers or other throat ticklers. Thus relieved, they could return to the party and start all over again.

Oysters are molluscs, like cockles, mussels, grazing snails, whelks, slugs and octopus, but they are much simpler than many of their relatives. Because they do not move, they have no need for eyes or tentacles to sense where they are going — in fact, no need for a head at all. They possess neither teeth nor jaws, for they eat only microscopic plankton filtered by their gills and swallowed in a mucus stream.

A common feature of nearly all molluscs is a muscular foot. Slugs and snails crawl along on it, clams use it for burrowing into mud and sand and mussels use it to attach elastic anchor ropes called byssus threads to rocks. As the headless adult oyster does none of these things, it is not surprising to find that it also has no foot.

The free-swimming larval oyster, however, does have an active foot that allows it to crawl about in search of the ideal site to settle. As soon as the shell is cemented it has no further use for the foot, which withers and is lost.

This lack of a foot is such a diag­nostic feature that it can be used to distinguish oysters from other shell­fish even after they have been smoked and canned in oil. (Such diagnosis is required from time to time by importing authorities to verify that canned smoked oysters are true to label.) By looking for the foot — and not finding it— a biologist is easily able to complete the verification.

Lewis Carroll appears to have known about this detail of the oys­ter’s anatomy. In his poem “The Walrus and the Carpenter” from Through the Looking Glass we read:

“But four young oysters hurried up,

All eager for the treat,

Their coats were brushed, their faces washed,

Their shoes were clean and neat ­And this was odd, because, you know,

They hadn’t any feet.”

On the surface of it, oysters lead an idyllic life. They stay in their beds all day long; the only work they do is opening and shutting their shells; their meals are delivered to them, and they spend most of their lives drinking, putting away on average 180 litres a day.

Of course, the problem with cementing your bed to the substrate and dispensing with the means to get out of it is that it is impossible to escape from stressful environmental changes, competition for a limited food supply, or from animals that try to eat you. Oysters can only defend themselves by adopting the siege strategies of medieval people who sought shelter in thick-walled castles. Such strategies often failed because the invaders eventually found a way in or penetrated the walls, or else the inhabitants died of starvation or disease.

Life for the oyster contains similar difficulties.

Sediment can choke them, hot summer days coinciding with low tides can cook them, run-off from the land can bathe them in fresh water (causing internal stress) and they are a tasty target for predators.

Tooth marks all over the bare sur­faces of oyster racks and sticks show that they are routinely grazed very thoroughly by fish. This would be sufficient to remove a large propor­tion of freshly settled oyster spat if the catching sticks were not kept wired up in bundles until the juve­niles had grown sufficiently to no longer be vulnerable to this inciden­tal predation.

The oyster-borer Lepsiella scobina is a frequent killer of wild rock oysters on sheltered shores, but rarely attacks rack-cultured speci­mens. Also known as oyster-drills, these whelks are beautifully equipped for penetrating the oyster’s armour. The rasping tongue with rows of hardened teeth (called the radula, and found in all snails) is modified in the whelks for drilling holes and mincing flesh. Each re­placeable row has just three multi-cusped teeth, and on the terminal row these are arrayed around the working tip like the three cutting wheels of a geological drill.

The radula is held within a pro­boscis tube that extends down to the oyster surface. The proboscis holds the drill tip in position as it is rotated alternately clockwise and anticlock­wise to wear away a perfectly round hole. The whelk lubricates the abrad­ing tip with a mucus secretion that may contain an acid to dissolve or soften the limestone matrix of the oyster’s shell.

It may take from one to three days to bore right through, depending on the size of the whelk and the thick­ness of the oyster shell, but the preda­tor is rewarded with an enormous feast at the end. The proboscis and radula are squeezed through the hole to macerate the live oyster’s soft tis­sues into a pulp which is sucked up through the proboscis into the whelk’s mouth.

If you were to put yourself in the position of the oyster, this would surely be one of the worst ways to die. Imagine listening to that inces­sant grinding for a couple of days, knowing that when the drilling was complete you would feel the toothed proboscis snaking across your body. It would then start mincing up only the less vital parts of the body to keep you, the victim, alive for as long as possible so that the shell did not gape to allow scavengers in to share the feast.

Other predators include crabs, which break off the shell margins to gain access to the flesh inside, large starfish, which pull the shells open before smothering the whole animal and digesting it, and rays, which crush the shells with their powerful flattened teeth.

The `mudworm’ Polydora can be a serious problem on oyster farms, par­ticularly if oysters are too tightly clumped and mud accumulates be­tween the shells. Polydora is a small marine worm that lives in a tunnel that it bores chemically in chalky mollusc shells. The filter-feeding worm only seeks shelter from its host, but as it grows larger it must enlarge its living space, and narrow tunnels can become sizeable caverns partly filled with mud and mucus.

Sometimes the worms bore right through the shell and the oyster re­sponds by secreting a thin patch of nacreous shell over the hole (the same process by which pearls are formed in the pearl oyster). The prob­lem with these internal shell blisters is that they are thin and easily broken. There is a danger that using a sharp blade to free an oyster from its shell may result in a blister being broken, and unsightly anaerobic mucus oozing out from the mud-worm’s tunnel to taint the oyster.

This year some farms have seen up to 80 per cent of their harvest spoiled by mudworm.

Infections by fungi, bacteria and viruses are as common in the sea as they are to us and other organisms on land. Many infections are common in all marine shellfish, but are at a low level and generally go unnoticed. However, they can become serious in farmed populations where huge numbers are concentrated in small areas in conditions that allow very easy transmission between individ­uals. Fortunately for New Zealand oyster farmers there is no record of Bonamia, the scourge of the Bluff beds, affecting Pacific oysters.

The oyster industry in New Zea­land faces other kinds of hazards too. With world oyster production in ex­cess of one million tonnes per year, our 4000 tonnes of exports relegates us to being a minor player competing against suppliers who are closer to the major markets (Hong Kong, USA,Singapore and Australia) and for whom labour costs are often much lower.

Strong international price compe­tition and a depressed local economy have given New Zealand growers a hammering. For many growers profits have not been commensurate with the capital investment and sheer hard work of operating an oyster farm. To make matters worse, the influx of a whole range of new seafood products on to the market has provided stiff competition for the oyster. Once the pieces de resistance of any smorgasbord, oysters have be­come passé, just one more item in an increasingly cramped section of the food market.

After a decade of strong expansion in the industry, production levels and prices have been static for the last few years. All is not gloom, however. With renewed interest in all aspects of shellfish cultivation (and govern­ment predictions that the country has the potential to earn $250 million from aquaculture) oysters are likely to be on the menu for a long time to come.