Along the west coast of Sweden, from its border with Norway down roughly 100 kilometres to the island of Tjörn near Stenungssund, the indigenous oyster has been growing in its wild state for centuries, if not millennia. The coldness of the water has helped to preserve it from parasites and viruses, which can only survive in warmer waters. On the other hand, the oyster here takes longer time to grow, is sometimes unable to reproduce and so a steady supply of oysters is difficult to guarantee. Since dredging has been forbidden since the late 19th century, although certain limited exceptions were granted, oysters have basically been left to their own devices.
The west coast was ceded to Sweden in 1658, as part of the treaty of Roskilde, up and to when it had been part of the Danish-Norwegian realm and oysters were regarded as public property and anyone had the right to fish for them. Since the coast was only sparsely populated and considered poor in natural resources, there was never any need to legislate ownership rights of the sea or shore. However, in 1686 sole rights of fishing oysters (and lobsters) were granted to an alderman (rådman) and merchant in Uddevalla, Anders Kock, whose only obligation was in turn to deliver fresh oysters and lobster, at a set price, every fortnight to the royal court in Stockholm from autumn to spring every year, not an easy task by any means given the harshness of Swedish winters and travel conditions. This he apparently managed to accomplish with varying success until the spring of 1698, when the then king Karl XII issued a decree that fishing for these species would once again become a common right. In the 18th century, according to one contemporary source, oysters were divided into three categories, depending on the kind of surface where they were found, mud, sand and rock (bärg) oysters, of which the latter were tastiest, having a “yellow, fat and firm flesh”. From recipes in old Swedish cookery books, it is apparent that oysters were used in soups, paté, bread, sauce and in a flan or pie dish. However, herring became the fish of the 18th and 19th centuries and boat building became an important trade and industry in the region. It appears that the natural beds of oysters were often overfished and during the 19th century occasional attempts were made to farm oysters at different spots along the coast, invariably petering out into failure. In 1888 apparently 15 tons of oysters were harvested. Dredging was practised with a so-called ostronskrapa (oyster dredge), which in this area was also known as an engelsman (Englishman), probably because it was brought over from Britain, or ulk (a Danish word for a sea scorpion). In shallower waters, tongs were also used. Even French, English and American oysters were on occasion implanted into beds in the hope of reinvigorating the depleted stocks. Some sought to imitate the French by experimenting with collectors or cultch made from bunches of birch branches and limed roof tiles and chicken-net. But any kind of commercial farming proved to be far too capricious and difficult. The cold winters have always been one great impediment, and it has been calculated that the reproduction cycle only really flourished once every 5 or 6 years. However, the stocks of wild oysters have been quite stable through the years and nowadays, since dredging was completedly stopped in the 1960’s, the only form of farming has been diving and manual gathering.
Although fishing is considered a common right, oysters are exempted from this general rule: firstly, they are regarded as the property of the landowner, and secondly, oysters cannot be taken up anyway without a license. Because Swedish law grants ownership rights to land-owners up to 300 metres out from the shore and fishing rights up to 200 metres, divers need to gain permission from the property owners in order to dive and pick oysters in the offshore areas, which has in some cases proved quite time-consuming, especially when several owners of the land are involved. The geography of the Bohuslän shoreline can be further complicated by the myriad of islands, rocks and skerries that splatter “the sea’s greystone gates” (Tomas Tranströmer’s memorable phrase of “havets gråstensgrindar”  from his poem Kväll – morgon [Evening – morning]), some of which are privately owned, whilst others belong to the State. The sea bottom tends to be granite-hard, quite flat and often covered with a not-so-thick layer of sand. Where the few streams flow out into the sea, the floor in these inlets is often carpeted with sediment and organic material, where the oyster surprisingly can thrive. The salinity of the water, especially in deeper parts, is quite high, around 22-30 ppt. Along the coast, surface water is usually more fresh, acting almost as a lid, whilst the saltier water is often quite deep and can be churned up by easterly winds, that drive the surface water out to sea and allow the saltier and often warmer water below to well up from the west.
These waters have been a safe haven for the flat oysters as they have had them all to themselves, but recently, in the last five years or so the cupped Pacific oyster (gigas) has become an invasive species, due mainly to the warm summers and mild winters of 2006 and 2007, and much to the annoyance of many of the local fishermen. They have gradually moved up along the German and Danish coasts from farms established in the North Sea. Initially, as with most forms of xenophobia, the gigas was treated as an unwelcome intruder and dangerous enough to swamp and ostracize the indigenous and prized edulis. As attitudes have been reality-tested, so they have softened, at least in some quarters. The cupped oysters are much more of a threat to the blue mussel beds; for instance, in the Wadden Sea during the 1980’s the native mussel beds were turned into oyster reefs, as both species prefer the shallower water, which on the other hand means that they are liable to freeze to death if ice is formed. And given the gradual rise in water temperature, the species is likely to establish itself and form quite extensive reefs, that can alter the bottom fauna over time.
Generally there is a good flow of nutrients in the water where the oysters prefer to settle, in clusters, often on southwest slopes behind islands that protect them from strong, incoming currents from the west. The flat oysters are found in deeper water, between 3 and 12 meters, whilst the trespassing cupped oysters prefer the shallower and warmer water, and have managed to survive some recent cold winters. But the decisive factor is whether reproduction can be prolific or not and that really depends on a good, long summer, and one without any serious, harmful algal blooms (HABs). It has been noted that the flat oyster has become more apt to spawn more than once during the spawning season, due to the rising water temperatures in recent summers.
Bengt Klemming is the younger of two brothers who run a diving company that not only caters for leisure divers and tourists but whose main occupation is managing oyster beds that lie on the privately owned sea floor along a 50 km stretch of coast between Hamburgsund and Strömstad. Now in the business for almost 20 years, as the only company licensed to operate as such, in which they supply about 80% of the Swedish oysters, he and Peter possess a local knowledge of the oyster banks that is quite unique. Their whole philosophy and business is infused by a total compliance to the tenets of sustainability. The oyster beds need to be sorted out and stewarded so that the oysters on the very bottom do not sink into the sand or be suffocated by the younger oysters that tend to settle on their shells. Another problem they notice is the amount of fecal material that can accumulate and that can stunt the filtering potential and therefore the growth of the oyster. Otherwise the reefs can perish. The flat oyster is a sensitive and fastidious creature that wants everything in moderation and cannot take too much stress and abrupt changes in salinity, temperature or even handling at all. They have noted that in certain cases the oyster will just close up and not eat. It grows quite slowly, especially in these colder waters, and cannot be harvested until it has attained a size of at least 6 cms. Most of the oysters that are gathered by hand lie at a depth of between 3 and 6 metres. Often the colour of the shells of oysters, lying in shallower water, have lighter shades, whilst the shells of deeper oysters are much stronger and darker, and seem almost heavier. According to Bengt, this is due to the ultra-violet rays that can more easily penetrate the surface of the shallow water. The brothers have observed the various cycles of growth and stagnation that have always typified the Swedish oyster population, and have recently witnessed the benefits of the warm summers of 2004 and 2005 when reproduction conditions led to a strong oyster community. Over the years, there has been a steady increase in the amount of brown algae, which suggests the rise in the presence of nitrogen, most likely from the land, and especially from over-fertilisation in agriculture. At the same time nitrogen is one of the building blocks for amino acids and in turn protein, which filter-feeders can thrive on.
During the cold winters, the oysters can increase their levels of glycogen to protect themselves from the cold, and it is something that can be perceived almost with the naked eye, as fat globules are formed on the mantle; but whether this has also to do with the decrease in the intake of food which appears to happen during these months is difficult to ascertain. That is why, for example, many connoisseurs are prone to rate oysters picked in January and February as the most succulent and tasty. Diving for oysters take time and the oxygen tanks allow for an hour under water. On a good day they can pick as many as 600 oysters (60 kilos) each. And this is done three times a week, so that they take up about 15 baskets every week with roughly 100-150 oysters in each. The annual harvest is around 6-7 tons, hardly sufficient for satisfying the growing domestic market, though the price commanded gives them a reasonable margin. The oysters tend to be covered with detritus and sand on the bottom, so they are moved to special spots for “steeping”, where they are kept for a few days in shallow and fresher water, in corves (sumpbassäng), to filter themselves clean. Swedish oysters do not need by law to undergo any depuration, as the water is classed as grade A, but this is considered as a traditional method of “purifying” the oyster before it is sent onto the wholesaler that also has a virtual monopoly for the distribution of the native oyster. However, as the coastal population increases during the summer, some of the water can become polluted from household wastewater that can affect the oysters in the autumn.
Grebbestad is the shellfish capital of Sweden, a small, sheltered harbour village, near the famous rock-carvings at Tanum, and one that not only hosts an annual oyster festival during the first weekend of September, and is home of the famous seaweed crispbread (Tångknäcke) but also has a thriving Oyster Academy (Ostronakademien). This association, with about 600 members, has as its expressed goal to protect and utilise the full potential of the Swedish native oyster. It organises events and oyster opening competitions and takes part even in commercial and academic meetings which try and promote awareness and various business possibilities in the cultivation of oysters. It has close connections with the local institute for marine sciences, the Sven Lovén centre, part of the University of Gothenburg’s Department of Biological and Environmental Sciences, one of whose many activities has been spearheading research and development of new technologies and methods for the farming of oysters, especially of the indigenous species. A special 3-year project called Nord-Ostron was started in 2009, pooling know-how from the three Scandinavian countries.
Ostrea is the name of a Swedish company, which started in 2004 to plan a large state-of-the-art hatchery to produce flat oyster spat, which would then be grown in the pristine waters off the coast of the Koster islands near the border with Norway. The hatchery was completed in 2008, although the facility is constantly being refined. A total of €4m has already been invested with the aim of producing 300 tons (3m oysters) annually from 2013, destined primarily for the French market. It is a hugely ambitious programme, both in a commercial and scientific sense, in which its very essence hinges on the crucial question of achieving a regular supply of spat, and that has been the main problem blighting the Swedish oyster industry as a source of regular income and employment in the area. The hatchery has been granted long-term support from the University of Gothenburg and its Sven Lovén Centre for Marine Sciences nearby and from regional governmental agencies as well as private investors, although little assistance has been forthcoming from the State. The staff are all well-trained academics with roots in marine biology. Kent Berntsson has been employed from the beginning as head of R & D. Not only has he completed a doctorate but has also worked as a mechanic and thus has a hands-on approach to the scientific and practical problems facing him and his colleagues at the hatchery. And there is evidence of pride in plenty as he shows off the ultra-modern facility, housed unobtrusively in one of the picturesque harbours of South Koster. It also cultivates its own microalgae.
The parent oysters were originally wild ones growing in different parts of the area and number 200 which are capable of producing up to 100m fertilised eggs, although a rule of the thumb is that only 0.25% survive to settle and grow into an adult oyster. Preselected for genetic variation and brought into the tanks as broodstock in January from cold water, they are gradually conditioned during a period of about 6 weeks to an increase in temperature that is raised 1°C each day to 20°C. The lighting is also incrementally raised to get the oysters to start developing their gonads more quickly. The conditioning process also involved feeding the broodstock large quantities of special microalgae that the hatchery itself produces. At about 13°C oysters have developed sexual organs and at 16°C they can start spawning. They seem to spawn only twice and then stop in the summer. Although oysters develop both sets of gonads, they seem to use one set, usually the male pair initially, and they seem to need more nourishment and energy before they are able to develop and spawn eggs, and so are far less common than active males. After the fertilised eggs have been incubated within the female oysters, they are released and automatically “swim” towards the light and surface. Here in the hatchery, the surface of these breeding tanks are skimmed off into containers where the minute larvae can be fed with specially prepared microalgae, drained, sorted and cleaned every two days with seawater purified by ultraviolet radiation; the larvae need to be thinned out to avoid too much density in the tanks and when they are seen to have developed an “eye spot” and a “foot”, after they have grown to about 0,3mm in size, this means they are ready to settle. Unfortunately it seems that only 5% of the larvae survive this ordeal. They are then transferred to the setting tanks, in which the floor is covered with a microcultch of sand, made from crushed oyster shells, no larger than 0,35mm in diameter. Settlement is another critical stage in the life of the oyster larva and those that have failed die and have to be removed, so that they are filtered through a half millimetre screen. Once this has been achieved the oyster spat will be put in trays, according to size basically, in rectangular nursery silos or growing tanks with an inbuilt upwelling system where they are fed with enrichened algae and salt water. Growth is dependent on such factors as spat density, water flow and the supply of natural phytoplankton in the water, which is supplemented by the introduction of microalgae. The oyster spat, once it has attained a size of 5mm are then placed out into suspended crates in the sea. The hatchery is continually experimenting with algal feed, its composition, concentration and amount, so as to optimise growth at various larval stages.
The company is in the process of installing a system for raft culture with a Canadian Flupsy (floating upweller system) about 200m from the hatchery, where the spat can be well-fed and grown out to marketable size. The first oyster spat grown from the hatchery were placed in the Flupsy in August 2010, and some have already reached a size of 5 cms, more quickly than anticipated. Ostrea has decided to work with a shellfish farmer, Keith Reid, from Vancouver Island in Canada, who has devised an off-bottom raft system of culture, incorporating an elaborate Flupsy unit, whose principal aim is to cut labour costs and raise productivity and yield. Tumbling will also be incorporated into the process. Proponents of the system stress not only its economic benefits but also its potential for rapid early growth of oyster spat and for conditioning the oyster to be more active. On the other hand, there are critics who voice concerns about stressing the juvenile oyster and about the quality of its flesh and shell structure.
The whole production process is meticulously monitored for presence of any bacterial flora that may accompany the inflow of seawater that can harm the oysters. Hygiene precautions are strictly observed, and the company refuses to resort to any supplementary antibiotic in the culture of algae or spat cultivation. They have recently installed a large, photobioreactor, which lines a glass wall along the southern side of the building, to cultivate their own mix of microalgae for use in the various growth stages. As production gathers pace and reaches the levels projected, there are strong hopes of collaborating with 50 or so contacts growers along the coast, who will buy oyster spat from the hatchery for cultivation, which then will be bought back for distribution and sale by Ostrea, thereby providing a viable or subsidiary means of employment for local fishermen.
There are also plans to build a packing plant on the mainland with depuration facilities, which at the moment according to Swedish law is not needed; as the water has been declared grade A by the authorities for years and since the area has been awarded the country’s first marine national park in 2009 (Kosterhavet National Park), the water quality has improved even more. Furthermore, it lies adjacent to the bordering Ytre Hvaler National Park that was established at the same time as Norway’s sole marine conservation area. The Koster fjord is a narrow, deep fissure, running like a long dagger from north to south and in parts is over 200m deep. Its biodiversity is quite unique. The water temperature stays a constant 5-7°C and has a high salinity of about 35 ppt. It is this water that is pumped up into the hatchery. The one worry about the salinity of the water is posed during the early summer when the Glomma river, the longest in Scandinavia, which runs down through the forests of Norway’s eastern flank, discharges its melted snow into the sea just north of the islands, especially if there is a southerly current. With the advent of the national park, an increased awareness of the need to protect the marine ecology has stimulated many fishermen to employ more environmentally friendly methods and thereby helped them market their produce as organic. Just in this very special location, nature is on their side, but will the technological advances within hatchery culture be on the side of nature in the years to come?
Victoria Hanson said:
Tack, for your beautiful and clear explanation of oyster cultivation in Sweden. One hopes that the industry continues in the wise and conscientious path of using the best of nature and science for intelligent improvements to luxury food production.
My mother’s father’s family still lives on Tjorn [sorry, no proper letter on my keyboard]. Perhaps you know of the old, wild, boom/bust industry of syll and lutefisk, which I have heard brought both prosperity and devastation.
I live now on the west coast of northern California, along Tomales Bay – formed by the famous San Andreas Fault. Our community has been driven to insane rage over oysters!
The beauty and ‘legendary romance’ of salt air and fresh seafood seduce an epicurean elite into ignoring that, from the beginning, oyster cultivation here has been an ecologically reckless race for profit – totally dependent on imported exotic species. Along with aquatic pathogens and parasites a tiny, elite clique promotes grossly deceptive PR to justify their pseudo-sustainable, biotech-industry-driven exploitation of the abundant, natural, local heritage. Perhaps they flatter themselves that the responsible practices you describe in Sweden, can rub off the stains of foisting genetically engineered organisms on innocent victims in the water and on shore.
Thanks again Victoria! As you so rightly point out, aquaculture and oyster cultivation have used the romantic picture of the sea and unspoilt nature to try and con consumers that it is all and always sustainable. Most fishing, like hunting, has been prone to the boom and bust mentality ever since the colonial white man took over. The Native Americans lived and understood nature in a completely different way. If you still can understand Swedish you can google nordostron and see how science and industry are trying to muscle their way into oyster farming. Again, it’s not so simple as they seem to think and no environmental organizations are being consulted. Anyway, you live in a nice part of the world!
i am looking for flat edulis oysters i would like to purchase 100 kg and i need to order 4 kg to try them. size 0.2 to 0.8 mm
Hi Francesco! Are you looking for edulis spat? Can’t help you there but I know there are some hatcheries trying to grow oyster seed.