Japan’s oyster growers scramble for seed

Miyagi Prefecture supplied 70 percent of Japan’s seed oysters before suffering extensive damage in the mid-March tsunami. Now other Japanese prefectures are now scrambling to secure juvenile oysters, as summer is the season for suspending them from floats in the ocean.

In Japanese mariculture, oysters are attached to lines suspended from floating bamboo or wooden rafts. The rafts allow three-dimensional use of ocean space for dense cultivation and can be moved to avoid red tides and parasite concentrations. However, they are susceptible to damage from typhoons. In Miyagi Prefecture, most rafts were sunk or carried to sea by the March 11 tsunami.

Empty oyster shells contained by wires attached to racks are used as a hard substrate or “clutch” to which floating oyster spat can attach. Oyster spat, after hardening on the racks for three months, is brought ashore. The shells (spatted clutch) are removed, placed in baskets and washed. The shells are then culled into single pieces and examined for predators. Shells with little spat are discarded.

While some seed is cultivated in hatchery operations, most of the Japanese industry is dependent on Miyagi and Hiroshima Prefectures for production of spatted clutch. When the juveniles reach a certain size, they are set out on suspended lines to mature.

Mie Prefecture, the country’s No. 2 producer of mature oysters, depended on Miyagi for 7 million juveniles yearly. To help meet the shortfall, the prefecture has allocated JPY 30 million toward raising oysters from spat at its fisheries research center. But only 200,000 juveniles will be available by July, according to the prefecture’s marine resources section. Nagasaki Prefecture is also trying to raise its own young oysters on a limited scale.

Okayama and Niigata prefectures have tried instead to procure seed from other sources, such as Hiroshima Prefecture, the second-largest producer of young oysters in Japan, but production there can only be increased incrementally.

Japanese aquaculture usually produces 200,000 metric tons in fresh meat weight of oysters annually, but the harvest of fall 2012 is likely to be off sharply due to inadequate juveniles in the coming season.

Miyagi Prefecture used to export seed oysters to the U.S. West Coast. But when France experienced a decade of oyster die-off from 1970 due to disease it turned to Japan for Pacific oysters. In response to the higher prices resulting from this increased demand, the oyster industry of Washington state, with help from Washington Sea Grant at the University of Washington, developed its own spatted clutch production. France soon also became independent of Japanese clutch. Now, little Japanese oyster spat is exported, so the shortage should affect only the domestic market.


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