Hard clams are the most commonly cultured of the bivalve species. Clam culture in the U.S. began in the early 1920s.
Broodstock management is a vital step in fulfilling the potential of hard clam aquaculture. Efforts to produce genetically improved bivalve broodstock are probably as old as bivalve mariculture itself.
The hard clam production process consists of three consecutive stages: hatchery, nursery and grow out. Each stage is designed to produce a specific size clam. The ultimate objective is to produce market size clams.
Farmers may develop any or all the production stages into viable businesses.
Hatchery:
The hatchery is where broodstock clams spawn and larval clams are raised through the post-set stage to 1 mm juveniles or seed clams. Broodstock are held in conditioning tanks at 19 ºC and fed a diet of cultured algae.
Hard clams can be stimulated into spawning in colder months by conditioning them gradually to increasing temperatures and by providing adequate food.
Broodstock are manipulated by alternately exposing them to chilled (18 to 24 ºC) and warmed (28 to 30 ºC) seawater containing a suspension of hard clam sperm.
After several cycles, the clams will spawn, with the male usually spawning first. The eggs are sieved, collected and placed in growing tanks where they develop into larvae. Females average 1 million eggs per spawn.
The next phase of the hatchery process is larval culture, which lasts through day 7. Larvae are raised in various sizes and types of containers. The larval tanks are supplied with filtered seawater (20 to 30 ppt) at a temperature of 20 to 30 ºC. The concentration of larvae in the tanks varies, but 20 to 30 larvae per ml is recommended.
During this stage, they are fed a diet of cultured algae. A major requirement of hatchery production is producing algae to feed young clams. For the first 7 to 10 days clams are fed small flagellates (Isochrysis), followed by diatoms such as Chaetoceros and Skeletonema. The initi
l feeding rate is 25,000 algal cells per ml or 1,000 cells per clam larva. The larval stocking density and algal feeding rate fluctuate through the larval grow-out cycle. Generally, the clam larval density is reduced and the feeding rate per clam larva is increased as the clams grow larger.
Between day 8 and day 12, the larval clams develop into the pediveliger stage. They are kept in post-set tanks and fed cultured algae. Filtered seawater at 26 ºC circulates through the system to ensure optimum survival and growth.
Post-set clam production is the next phase of production; it generally lasts 13 to 35 days. As in earlier stages, water temperature is maintained at 26 ºC, water is filtered, and the post-set clams are fed cultured algae. Newly set clams are placed in shallow raceways or in cylinders with up- or down-welling water flows. The clam seeds are kept in the hatchery until they reach about 1 mm.
At this point, the seed is graded and separated by size and maintained in the nursery until ready for planting. One of the long-standing problems in bivalve aquaculture is the difficulty of culturing massive quantities of suitable algal species economically.
Algae are needed to grow seed to the proper size for field planting. The cost of producing this algal biomass is relatively high compared to the cost of seed clam production or the projected annual gross revenue of this aquaculture venture.
Permitting:
The nursery and growout phases of clam culture require permits from state regulatory agencies if clams are grown and harvested from state-owned waters or bottoms. In addition, any hard structures will require permits from the state regulatory agencies.
Hatcheries may need permits for water intakes, docks and outfalls if structures are in public waters. Recent EPA regulations do not require National Pollutant Discharge Elimination System Permits for shellfish aquaculture.
A prospective grower should begin site selection and permitting as early as possible so that good sites are selected and regulatory holdups are avoided.
Authors:
Jack M. Whetstone, Leslie N. Sturmer and Michael J. Oesterling
