WWW.WAS.ORG • WORLD AQUACULTURE • JUNE 2019 51 (CONTINUED ON PAGE 52) use cylindrical cone-bottom fiberglass tanks for larvae culture. Planktonic swimming larvae pass through different stages over 20-25 days before becoming competent for metamorphosis. Competent larvae are then moved into downwellers for settlement (Fig. 6). Metamorphosis and settlement are critical events in the life cycle, marking a fundamental change of lifestyle from a planktonic existence in the water column to an increasingly sessile life on the bottom. Massive mortality of larvae can occur at this stage. Larvae become post-larvae or spat within 4-5 weeks and then are capable of active crawling along the bottom of culture vessels. In natural environments, post-larvae crawl along the sandy seabed and can dig into the substrate with its foot. They are also capable of attaching themselves to the substrate. During this stage, geoduck juveniles need clean substrates and good water flow. Hatchery operators in North America use various sandy substrates for post-larvae culture and development (Fig. 7). The duration of post-larval stages is 4-5 weeks under hatchery conditions (Goodwin and Pease 1989). Nursery Culture When siphon formation is complete and shell length (SL) is around 1 mm, operators transfer juveniles from land-based hatcheries to natural, sea-based secondary nursery systems. The requirement for large quantities of microalgae or to minimize the higher labor cost for a planting-size juvenile (8-10 mm SL) resulted in a shift from land-based to natural sea-based nurseries. Growing spat in raft-based floating upwelling systems (e.g. FLUPSY) is one of the most popular techniques for geoduck seed production in natural seawater environments. Some hatchery operators use double-layer bag nets in the sub-tidal seabed called “bags in the bottom” (BIBs) to produce planting-size juveniles. BIBs or FLUPSY systems for seed production are relatively successful but poor survival rates limit the supply of large seed. Protection of geoduck seed from predators is always a challenge in the operation of juvenile production systems. Juvenile geoduck clams are extremely vulnerable to epibenthic predators until they attain a spatial (depth) refuge. Unlike other bivalves, the siphon of geoducks is too long to retract into the shells (Fig. 8), thus they are more vulnerable to predation in a natural seabed environment (Liu et. al 2017). Bottom-feeding fish, crabs, sea stars and flat worms are the most common predators of juvenile geoduck smaller than 20 mm SL. Grow-out The planting of hatchery-raised seed into a farming area was first piloted by the Washington Department of Fish and Wildlife (WDFW) in the late-1970s. Planting practices initially consisted of broadcasting seed (8 mm SL) from the stern of a slow-moving vessel in shallow water adjacent to selected public beaches. The WDFW has since incorporated intertidal geoduck culture into shellfish enhancement programs. As unprotected juveniles are vulnerable to predation, WDFW gradually changed the planting technique to mesh-covered PVC tubes that proved to be successful in reducing predation. Washington State is now the world’s largest producer of farmed geoduck, with nearly 673 t having a total value of US$28 million in 2013 (Seafood Watch 2016). In addition, the US National Oceanic and Atmospheric Administration is working with local industries, including Taylor Shellfish and other state agencies through the Sea Grant program to develop a more efficient network for shellfish aquaculture development, including for geoduck. British Columbia began experiments in geoduck farming in subtidal environment in the 1990s. In 1996, a DFO/provincial pilot program for geoduck aquaculture research and development was approved in BC with the establishment of five subtidal aquaculture sites (DFO 2014, DFO 2017). The Underwater Harvesters Association, in conjunction with Island Scallop, Manatee Holdings and FAN Seafoods, undertook initial hatchery and seeding efforts in BC. Sections of the Strait of Georgia have been seeded with geoduck using an underwater planting device with varying success. BC is a comparatively small but growing producer of farmed geoduck, with total production about 75 t having a value of $2.4 million in farm gate sales (GSGislason and Associates 2012, Seafood Watch 2016). More recently, geoduck farming in coastal shoreline areas has been expanded in Alaska and Baja California with modern techniques. Farmers are using PVC tubes that are inserted into the substrate to protect out-planted geoduck seed. The PVC tubes are FIGURE 7. Trays with fine sand substrate for post-settled juveniles. FIGURE 8. Juvenile geoduck clams ready for out-planting.
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