In 1969, shellfish production (including pearl oysters) in Japan was 561,132 tons worth 58,444 million yen (Table 1). Types of shellfish products landed during the past decade have changed because of certain effects on the environment and changes in commercial demands. Shellfish, like Meretrix, which live along the coastal area, have decreased remarkably in production because of the loss of their habitat from industrialization. In 1969, the oyster industry in the Seto Inland Sea area was seriously affected because the oysters were heavily fouled by the calcareous tube worm, Hydroides norvegica. For the past several years, production in the pearl industry has declined because of reduced demand for pearls. On the other hand, landings of scallops, abalone, and top shell, so-called high grade products, have been increasing yearly both in tonnage and value because of the demand by the consumer for fresh marine products. Production through aquaculture has remained commercially stable in Japan.
In the past, shellfish aquaculture in Japan was limited, strictly speaking, to oysters and pearls. Recently, both scallops and abalone are being grown using aquaculture techniques. In general, aquaculture requires three important techniques: 1) to catch the juveniles in desirable places with collectors, 2) to produce seed for culture using hatchery (artificial) techniques, and 3) to cultivate the seed to market size in the field. Aquaculture techniques are being utilized to grow oysters, pearls, and scallops, and it is expected that abalone will also be produced in the near future through aquaculture.
In my presentation at the first UJNR (United States-Japan Natural Resources) Aquaculture Panel meeting, I would like to explain briefly about the present status of shellfish aquaculture in Japan and its prospects for the future.
Production of oysters (Crassostrea gigas) has rapidly increased since the 1950's because of the development of hanging culture. Strings of oysters are hung from racks, rafts, or longlines, depending on water depth. Hanging culture makes good use of the water column and is not limited by depth or nature of bottom. In 1969, production was 245,458 tons (including shell) valued at 8,149 million yen (Table 1).
The first operation in oyster culture is to catch the juveniles (seed). Strings of shells are suspended during the summer spawning season from racks placed in coastal areas such as bays and inlets. After setting, the seed is hardened, a process of draping the strings over racks so that they are out of water for a considerable period of time during each tidal cycle. Hardening results in an increased percentage of survival, better growth rates, and less fouling. In northern Japan, almost all commercial oyster seed production takes place in Miyagi Prefecture. At present, oyster seed production supplies not only domestic uses, but also supplies some 30,000 to 50,000 cases of seed per year to the Pacific coast of the United States and to parts of Europe, principally France.
Two methods of hanging oyster culture are usually practiced in southern Japan. In the first method, seed oysters are transplanted to the rafts about 1 mo after setting to grow; no hardening is done. These oysters are harvested at the end of 1 yr. This method is characterized by a short growing period and low labor costs. In the second method, seed oysters are hardened until autumn or early winter. They are then transferred to the rafts to grow and are not harvested until the following year (known as 2-yr culture).
The best oyster producing area in Japan is in the Seto Inland Sea where calm conditions exist. Here there are wide areas which enable the growers to use large size rafts, 20 m long by 10 m wide. The Tohoku region of northern Japan is also favorable for oyster culture and longlines are used in some of the rough water areas. It takes 2 yr for the oysters to grow to market size.
Oyster production is expected to remain stable over the next few years. New species of oysters such as the European flat oyster, Ostrea edulis, and the Portuguese oyster, Crassostrea angulata, may enter commercial production in the future if the problem of pollution along the coastal areas can be overcome.
The aquaculture of scallops, Patinopecten yessoensis, has recently been developed in Japan. The production of scallops on natural grounds has remained poor during the past decade with annual landings fluctuating between 5,000 and 15,000 tons. Since the development of the hanging method for catching seed and culturing scallops, production has increased rapidly during the past few years and will continue to do so. Because considerable quantities of seed are being caught in Aomori and Hokkaido areas of northern Japan, the scallop fishery is currently expanding. The seed is not only being cultured in nets to market size but also released on the bottom where good returns are being obtained 2 to 3 yr later.
In 1969, 14,644 tons worth 1,833 million yen were harvested from natural grounds and approximately 5,000 tons were produced from hanging culture.
Seed scallops are caught in the spring. Several kinds of cultch materials are used: vinyl fibre (used gill net), vinyl film, branches of trees, etc. These are suspended from rafts or longlines. Three months after setting, the young scallops are transferred to nets where they grow to 3-4 cm by autumn. The culture of scallops can then go one of two ways: Either the seed can continue to grow in nets for 1 1/2 yr to market size or they can be released on the natural grounds. As high as an 80% return has been obtained from released seed on the coastal area of Hokkaido.
In the near future, a large-scale bottom farming of scallops can be expected along the coast of Okhotsk Sea, Hokkaido. This project will be made possible because large amounts of scallop seed are available. The site once produced 60,000 tons of scallops annually. To make the program successful, systematic engineering and newly developed harvesting machines such as suction dredges and underwater bulldozers will be utilized.
Considerable interest has developed in the culturing of abalone (Haliotis discus hanai in cold waters and H. discus in warmer waters). These are very important commercial species along the rocky sea bottom both in quantity and quality. Production has remained relatively stable over the past years as a result of good management practices such as the transplanting of natural sets to areas of poor setting. In 1969, abalone landings totaled 6,463 tons valued at 5,692 million yen (Table 1).
Two approaches are planned in the aquaculture of abalone-mass production of seeds in hatcheries and the artificial production of food in the field by seaweed afforestation. Artificial seed production was developed in the 1960's. Seed production is expected to expand as studies are completed on the conditioning of abalone for spawning and as commercial hatcheries are developed. Good returns can be expected from released seeds, 2-3 cm in shell length, when they are harvested 3-4 yr later.
There is a lack of knowledge about the food conditions in the abalone's natural habitat. For this reason, research was initiated in afforestation in the field. Afforestation could produce about 40 tons of food that could feed up to 4 tons of abalone. The seeds for this program will come from hatcheries.
The pearl industry first began in 1906. The industry has developed rapidly after World War II and reached its peak of production in 1967 when 130 tons of pearls were produced by 4,701 culturist (called management units). The increase was related to techniques in artificially collecting seeds and rearing the mother shells (Pinctada sp.). The mother shell industry has grown rapidly since 1952. In 1965, 11,000 tons of mother shells were harvested from 69,000 rafts operated by 7,859 management units.
A rapid increase in the number of management units occurred from the postwar era to 1964. These were Mostly small-scale units using less than 30 rafts. In recent years, the number of units has not been increasing and many units are using more rafts, indicating an enlargement of the individual industry units.
The everlasting aim of the industry is to improve the quality of the cultured pearl. In the 1920's, pearls were small but, after developing successful techniques of transplanting mantle pieces in the gonad, bigger pearls were produced. Between 1952 and 1969, the production of small pearls (less than 6 mm) decreased from 67% of the total production to 24%; those in the medium size range (6.0-7.6 mm) and large pearls (7.6 mm) increased in total production from 24 to 58% and from 9 to 18%, respectively.
As the industry developed, culture grounds spread rapidly over the middle and southeastern parts of Japan. These areas were used mainly for the production of mother shells. In 1956, 82% of all pearls produced in Japan came from the middle Pacific region; 73% of these came from rafts. Recently, however, production in this area has declined because of overcrowding and deterioration of culture grounds. In 1964, 20% of the total pearl production came from the Seto Inland region but, recently, production dropped to 10%. On the other hand, production in the southern Pacific and East China regions has been on the increase, and over half of the total Japanese pearl landings now comes from these two areas.
Until 1963, yields from pearl rafts were stable but, thereafter, production per raft has gone down. Reasons for this decline have been related to crowded conditions and to the use of unsuitable culture grounds. In the Seto Inland region, the deterioration of the waters caused by man-made pollution has caused a drop in yields from certain culture grounds. Special provincial laws have been undertaken to regulate production, recover the balance of supply and demand, and effectively utilize suitable culture grounds. The success of these regulations will insure the steady foundation of the industry and the production of better quality pearls.
1 Tohoku Regional Fisheries Research Laboratory, Shiogama, Miyagi-ken, Japan.
2 National Pearl Research Laboratory. Kashikojima. Mie-ken, Japan.