Hard clam production systems range from intensive (hatchery, nursery and some grow-out) to extensive (some grow-out).
Seed supply
All hard clam seed are produced in hatcheries. The production cycle begins with adult organisms with well-developed gonads. The conditioning of these adults relies on a number of techniques. For early spawning, adults may be conditioned in the hatchery facility by increasing water temperature while providing sufficient food in the form of unicellular algae. Conditioning takes 4 to 10 weeks depending on the starting point. Clams may also be placed in high growth areas and allowed to condition naturally for spawning later in the season.
Typically, conditioned adults are mass spawned by placing them in a seawater bath in which temperature can be adjusted. Sperm, microalgae, or serotonin may be used to provide a stimulus for gamete release. Fertilized eggs are retained on a screen, then counted and distributed into tanks for development. After two days the larvae have reached the straight hinge stage and are removed from the tank, redistributed into another tank, and supplied with unicellular algal food. This process is repeated for 7 to 14 days until settling (setting), when clams are typically held at about 1/ml. Clams are removed from the larval tanks and placed in containers with mesh bottoms in which water with unicellular algae enters from the top (downwellers) or placed directly on raceways and supplied with very low flow. The clams remain in downwellers or with limited flow for one to two weeks. Clams in downwellers are then placed in mesh bottom containers with the water flowing up through the seed (upwellers).
Nursery
There is no definitive moment at which the hatchery system ceases and the nursery ensues. However, most people consider the point at which unicellular algae are no longer supplied as the end of the hatchery part of the cycle. At this point the clams feed exclusively on materials in the natural water being pumped from a nearby estuary. Clams usually remain in upwellers until they reach from 2-5 mm SL and then they are distributed into raceways, mesh bags in the field, or kept in upwellers where they grow to planting size (8-15 mm SL).
Ongrowing techniques
Clams are planted in plots in the intertidal or shallow subtidal zones. In most areas these are covered with a plastic mesh (6.4 or 12.7 mm square mesh) to prevent predation losses. In some locations clams are planted in mesh bags which are staked to the bottom. Plot maintenance involves regular (at least weekly) monitoring to check the mesh for damage and to remove fouling. Clams reach market size in 1-1.5 years in southern waters and 2-4 years in more northern locations.
Harvesting techniques
Clams are harvested from the bags by picking up the bag, or removed from the plots by digging with a rake or by hand. All clams are size-graded, either by hand or with mechanical grader/counter machines, and those individuals that are too small to market are returned to a separate plot for additional growth.
Handling and processing
Almost all farmed clams are sold live in the shell.
Production costs
Hard clam seed (10-12 mm size) typically cost from USD 25-30/1 000. The supply can be variable and many growers buy from multiple sources to hedge delivery times and supplies. Others buy smaller seed and operate their own nursery systems to grow the seed to planting size. Other costs are nominal, but include the cost of the mesh, any hold-down devices and ancillary field gear. The main costs other than the seed are the boat, its engine, and the labour required to tend the plots. Typically, survival to market in the plots averages 50-70 percent. Below 50 percent it becomes unprofitable to grow clams. Occasionally, survival exceeds 70 percent, but that cannot be expected as routine.
Vibriosis |
Vibrio anguillarum; V. alginolyticus |
Bacteria |
Systemic infection of larval soft-tissues, resulting in tissue necrosis (due to production of exotoxin by the bacteria) & death |
Good husbandry; sterilization of water used in algal and batch culture |
Quahog Parasite Unknown Disease (QPX Disease); Chytrid-like Disease |
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Swellings and round yellow-tan nodules (1-5 mm in diameter) in mantle, often at edge & close to or directly adjacent to the siphon or adductor muscle; gills may also be infected; decrease in new shell growth; swollen, retracted, tan-coloured mantle edges; mucus & sand granules caught between swollen mantle & shell edges & high degree of chipping of the shell edge; can cause severe mortality |
No control measure known, except planting resistant stocks |
Suppliers of pathology expertise
Expertise in clam pathology is similar to that for oysters, and exists primarily in academic institutions or governmental laboratories.