WWW.WAS.ORG • WORLD AQUACULTURE • DECEMBER 2017 51 Reusable polypropylene mesh (6 or 10 mm square openings) predator nets (90-460 m long × 1.2 m wide) are installed in strips, buried along the edges and fixed to the sediment with large steel staples (Fig. 4). Clam seed (3.2-3.5 mm shell length) is planted through the net mesh within strips at about 750 individuals per m2. During a growing cycle of 2-3 years, nets are swept approximately monthly from spring to fall (net fouling is minimal in winter) to remove fouling (Fig. 5). Fouling occurs mainly through attachment and growth of macroalgae (Ulva sp. and Sargassum sp.) on predator nets. Before clams grow too large to fit through the mesh, biofouling can result in clams moving to the surface on top of the net but under the algae in an attempt to escape apparent suffocation, but this results in seed loss when nets are swept. Thus, it is critical to keep predator nets clean until clams are too large to move through the mesh. When clams are too large to pass through the predator net, sweeping is necessary to remove macroalgae to avoid suffocation of clams and improve water flow (Fig. 6). Additional farm maintenance involves inspecting and re-burying edges of the predator net. The principal predators excluded are Dungeness crab (Metacarcinus magister (formerly Cancer magister), the red rock crab (Cancer productus), and the graceful crab (Cancer gracilis). Surf scoters (Melanitta perspicillata) and white-winged scoters (M. fusca) are also seasonally important predators (Lewis et al. 2007). In the Pacific Northwest, Manila clams are rarely found in significant densities in sand, presumably because they are easily detected and consumed by predators. This was painfully evident to Chuckanut Shellfish as they pioneered farming Manila clams in sand. When nets are dislodged by storms, predation can be 100 percent in as soon as 24 hours. Clams are harvested during accessible low tides between early spring and late fall, with some harvests continuing through the winter. After removing the predator net, the harvest machine is driven to the end of a harvest row. Harvest proceeds by driving the machine along the length of the harvest row, with the harvest crew picking out broken clams, other bivalves and shell fragments, and filling and palletizing packing bins (Figs. 7-9). During harvest, the machine loosens or softens the upper 8-10 cm of substrate and exposes scattered polychaete worms and smaller clams. However, after the following tidal inundation, the softened layer compacts to a consistency similar to adjacent undisturbed substrate. (CONTINUED ON PAGE 52) FIGURE 6. A modified tractor-powered “street sweeper” to remove macroalgae biofouling from predator nets. FIGURE 7. A tulip-bulb harvester modified to harvest Manila clams after predator netting has been removed. FIGURE 8 (TOP) AND 9 (BOTTOM). Close-up of the mechanical harvester and clams. A combination of a vibrating rake and rotating brush dig clams buried 2-4 inches deep onto a conveyor. The vibratory action also shakes most of the sand off the clams, so by the time they reach the bins, they are fairly clean. Each bin holds about 50 pounds of clams.
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