World production of the Yesso scallop is almost entirely accounted for by wild seed collection and grow-out of the spat in suspended (hanging) culture or on ground lays. Off-bottom culture methods were developed in Japan and have spread to the other North Pacific Asian countries where culture is practiced. Similar technology based on Japanese experience is utilized worldwide for a wide range of commercially valuable scallop species.
Scallops spawn in spring at 7-12 °C. The density of settlement depends on the concentration of larvae in the water column; this is closely monitored to predict both the timing and intensity of settlement. Spat collectors are suspended in the water column when monitoring programmes determine that >50 percent of larvae exceed 200 µm in length. Two types of spat collector are used:
Plastic monofilament packed ‘onion’ bags attached 10 to a rope of 5 m length.
Conical plates of perforated plastic are strung together in batches, usually of 25, by rope to form a string of 2.5 m length, which is then covered by fine mesh.
The use of both types of collectors is similar. Larvae settle on the mesh contained in the bags 30-40 days after spawning. Settlement size larvae can pass through the outer mesh of either type of collector and set on the monofilament or conical plates, metamorphose and grow. Larger predators cannot access the growing spat, nor can the growing spat escape. Collectors of both types are strung from submerged, buoyed, horizontal longlines, and hang from 5-10 m below the water surface to 5 m above the substrate. The collector units are generally kept in place until just before the spat are ready to detach, which is when they exceed 8-10 mm shell height. The optimum temperature for larval development is 15+2 °C and optimum salinity 30+2‰. At densities of 20-30 larvae/m3, 100-400 seed can be harvested per collector. Spat harvest increases to 500-1500 per collector bag unit when larval density is within the range of 50-100/m3. Exceptionally, the yield can be greater.
Spat are harvested from the collectors about 3 months after settlement when they measure about 10 mm shell height. They are then transferred to intermediate (nursery) culture in pearl nets; this occurs in the autumn. Care is taken to remove predators, which may have settled as larvae within the units, and also organisms that will compete for food and space (e.g. mussels). Each pearl net has a bottom surface area of 0.12 m² and can be stocked with up to 50-60 of 10 mm seed. Pearl nets are strung together to form vertical ‘cage’ units of variable length, depending on water depth. There may be from 5 to 30 nets per unit, and units are suspended from submerged longlines set 5-10 m below the water surface. After 10 weeks, the seed will have grown to 20-30 mm shell height and occupy about 60 percent of the volume of the net. A survival rate of 90 percent is common during this period. At this time, in October, density is reduced to 15-20 scallops per net. Intermediate culture then continues through winter until the following spring, by which time the scallops have grown to ~50 mm. They are then ready to be transferred to the grow-out stage.
Grow-out to market size is either by sowing year-old seed on bottom lays or in various forms of hanging culture. Scallop culture is mainly a cooperative activity in Asian countries and maybe a part of a polyculture system.
This employs the same basic methods as nursery culture. However, multi-level lantern nets suspended 5 m below water surface are frequently used in water depths from 10-15 m. Strings of pearl nets are preferred in deeper water because they are less prone to swinging in pendulum motion in heavy sea conditions, which can result in mortality among the contained scallops. Stock density is reduced as the scallops grow. One-year-old scallops of 20-30 mm are stocked at 15-20 per pearl net and the number is further reduced to 5-7 per net one year later when the scallops are 50-70 mm shell height. Scallops of marketable size (100 mm) are available for harvest in years 2-3 (earlier in more favorable conditions of food supply and temperature). Scallops are often ear-hung in pairs from either horizontal lines in shallow water or vertical lines in deeper conditions when they are approaching 10 cm in size. In this method, a hole is drilled in the ear of the shell and a loop of nylon thread is passed through the hole and attached to vertical or horizontal lines in shallow water lease areas.
When seed quantity is surplus to suspension culture requirements, the excess from nursery culture at 20-30 mm shell height is sown on bottom lays in shallow water at 10-20/m2. However, bottom lays are usually sown with ~50 mm seed in March at densities of 5-6/m2. Bottom sown scallops take a year longer to reach market size than those grown in suspension.
Scallops are harvested at about 100 mm shell height after 2-3 years of culture. Harvesting from bottom culture is by SCUBA diver or by dredging. Harvesting from suspended culture uses the craft of various types, often fitted with mechanical winches. The time of harvesting is sensitive to the presence of harmful paralytic shellfish toxins (PSP, DSP, etc.) in waters; this requires careful monitoring.
Handling and processing
Scallops lack the ability to hold mantle cavity water and thus will rapidly desiccate and die when out of water. Care needs to be taken to avoid undue exposure to air and sun. Handling methods must therefore ensure that the scallops are removed from the growing units and transported to packing/processing plants swiftly. Processing other than washing and shucking is usually minimal. Whole scallops are transported chilled to local markets and shucked meats are frozen or canned.
Information on production costs is difficult to obtain, not only because the information is proprietary but also because of site specific factors, the diversity of methods used, and the widely varying levels of technology employed. The landed value of scallops is USD 6-7/kg in Japan (2004). There are no feed costs; this is a free resource throughout the culture cycle. Labour is a major recurring cost and scallop culture is very labour intensive. Culture is usually undertaken by Fisheries Cooperatives.
Diseases and control measures
No specific diseases of the Yesso scallop are reported in the various databases such as AAPQIS. Neither have any that have been implicated in unusual mortalities been reported in the literature. Like most bivalves, the shells are often bored into by the polychaetes, Polydora sp. and Dodecaceria concharum, the parasitic sponge, Cliona sp., and the Myxosporidian, Myxosporidia. The sporozoan parasite, Perkinsus sp. is endemic in most populations.
Market and trade
Yesso scallop production is mainly absorbed by local markets in the countries in which it is cultured. The short shelf life of live scallops dictates that chilled live product is only available close to cultivation sites. Otherwise, the market is for frozen meats. Quantities amounting to a few thousand tonnes are exported, mainly as frozen meats. The US and France are the principal importers of such products.
Status and trends
Production in Japan showed a steady increase from 1970 until it reached 200 000 tonnes in 1992, a level has been exceeded thereafter with annual fluctuations. Peak production was in 2002 at almost 272 000 tonnes. Scope for future increases is limited by the availability of suitable lease areas and by concern for sustainability, where the carrying capacity of the areas utilized is an issue. Market saturation may also be a factor in the flattening trend of production in the past 10 years.
Chinese production showed a dramatic increase from around 147 000 tonnes in 1990 to 916 000 tonnes in 1995 and to over 1 million tonnes by 1997. Production from 1998-2003 has shown wide variability (from 629 000-960 000 tonnes), which may be related to the availability of seed.
Potential exists for development in the Republic of Korea and the Russian Federation.
Please contact with us if any questions, our sales team will always be happy to provide you with the most professional info.