Kouei Suisan: An Oyster Farm on Japan’s Inland Sea

Japan harvests much of its food from the sea: finfish, shellfish and seaweed. In addition, they have developed a diversity of mariculture (ocean farming). Oysters have been cultured for at least 400 years.

Mr. Koichi Isobe comes from generations of fishermen. Five years ago, however, he stopped fishing and began growing oysters and other shellfish on his farm Kouei Suisan (Public Prosperity Fishery) in the town of Murotsu, on the shores of Seto Naikai – Japan’s Inland Sea (see photo 1). Seto Naikai is protected by the central islands of Japan: Honshu, Shikoku and Kyushu. Its shallow, nutrient-rich waters grow a thick ‘soup’ of plankton. The shallow waters warm in the summer to 33oC (81oF) and drop to a chilly 7oC (45oF) in the winter; conditions ideal for raising oysters. They grow fast in the warm summer waters feeding on plankton. In the winter, growth slows and they fatten up.

Photo 1: Look out over Murotsu town and bay, including the rafts of Kouei Suisan.

Japanese oysters, Crassostrea gigas, (aka Pacific oysters, Miyagi oysters) spawn in the summer. They convert the energy stored in their tissues to make gametes and release them in dense, milky ‘clouds’ that consist of hundreds of millions of eggs and hundreds of billions of sperm. Oyster lovers do not eat them at this time – their flesh is soft and gooey – hence the rule to eat oysters only in the ‘R’ months (months that contain the letter ‘r’, i.e. September – April). Similarly in Japan, oysters are eaten from October to March.

Isobe-san buys oyster ‘seed’ from other farms that collect oyster larvae. Collector farms monitor the spawning oysters in July and August. As the oysters spawn, strings of scallop shells are placed into the water to collect the settling ‘spat’. These microscopic juveniles grow somewhat in the late summer and fall to become ‘seed’, and then mostly stop growing over the winter. Oyster farmers buy seed and begin to grow them at their own farms the following spring. This type of farming is called yōshoku, ‘grow from a seed’ (see photo 2).

Photo 2: Isobe-san holding a rope of scallop shell ‘collectors’ covered in oyster seed ready for growing (from May, 2009).

Isobe-san needs to place his order for oyster seed two months in advance of spawning, approximately one year before he actually takes possession of the seed. We visited his farm on June 1. He had just finished his ‘planting’ for the year. Each seed was 2-3 mm in length. They had already grown to 10 mm after a few weeks at his farm (see photos 3 and 4). They will grow to 150-200 mm by harvest time (see photo 5).

Photo 3: Isobe-san pulls up a rope of scallop shells and growing oysters (from May 2009).


Photo 4: Young oysters, June 1, 2011. They have grown from 2-3 mm in May to about 10 mm.

Photo 5: The remaining oysters from the last growing season, 150-200 mm, June 1, 2011.

This year, Isobe-san put out 10,000 ropes of seeded scallop shells, each rope had about 23 scallops, and each scallop was seeded with a few dozen oysters – about 7 million oysters in all! The ropes are arranged on 10 floating rafts. As the oysters grow through summer, the rafts will slowly sink, though not entirely below the surface. He begins the harvest in late Fall, working from the outside of each raft – oysters grow faster here where there is more plankton – until he completes his harvest next Spring. He expects to harvest 100 metric tons (220,500 lb) of oysters in the shell, equal to 40 metric tons (88,200 lb) of actual oyster meat. Isobe-san likes to eat oysters at the start of each season, but he does not eat so many by spring.

Seto Naikai was well protected from the tsunami of March 11. However, the most famous oyster growing region of Japan, Miyagi Prefecture, was at the center of the disaster. During our visit, Isobe-san received two phone calls from another oyster farmer, asking for his source of seeds. These farmers normally buy from farms in Miyagi, but these farms were completely destroyed. It may be a decade or more before the Japanese oyster industry recovers.

Isobe-san likes to experiment. He grows clams and scallops as well as oysters. He buys wild-caught asari (Venerupis philippinarum, Japanese littleneck clams, Manila clams) and torigai (Fulvia mutica, Japanese cockle, heart clam). He grows these clams in trays of sand hung from his rafts (photos 6 and 7). They are being harvested now and through the summer. When dug up and transferred to the farm, they stop growing. As they start to grow again on his farm, a distinct ‘check mark’ is left on the shell (see photo 8). Cultured clams are not as valuable as wild clams, but they are a reliable crop. This process of raising wild-caught young is called chikuyō (vs. the raising of ‘seed’ described above – yōshoku). Two years ago Isobe-san aged sake alongside his oysters (see photo 9). He had a party with friends to eat the new oysters with the special ‘oyster’ sake.

Photo 6: Baskets of clams are also suspended from the oyster rafts. Baby clams are dug up from the wild and grown to market size in these baskets.

Photo 7: Asari clams in basket.

Photo 8: Torigai clam (cockle) showing the ‘check mark’ where it stopped growing when first brought to the farm.

Photo 9: A case of sake and wine being placed next to the oysters for aging (May 2009). These bottles were consumed with oysters the following winter.

Isobe-san enjoys his work; he described it as almost a hobby. One reason he changed to oyster farming from fishing was the reliability of the harvest. Also, he can be insured for his oysters, in case anything goes wrong. His three children are not much interested in oyster farming, and that is okay with him. He likes his work and he wants his kids to find work that will fulfill them.

We will visit Isobe-san again throughout the year to watch his oysters grow and to enjoy his harvest next spring (see photo 10).

Photo 10: Isobe-san in his office with his Glendale Community College, Biology Department shirt.

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4 thoughts on “Kouei Suisan: An Oyster Farm on Japan’s Inland Sea

  1. Pingback: Oyster Harvest at Kouei Suisan | Glendale Community College Blog

  2. Pingback: Season Opens for Lobsters in Japan: Ama Dive | Glendale Community College Blog

  3. Pingback: Japan farm | Michelvainfilm

  4. Oh how clever to age the sake and oysters together! I’ve seen similar oyster floats here in the PNW. I’ve always wondered why farmers (and not just oyster farmers) tend to prefer buying seed or young from others rather than create their own. At least in the case of oysters, the seed is wild stock that is then farmed to maturity, if I understand what I’ve read. Glad to see you’re having fun over there! ❤

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